UNIT IV How ThingsWork AIM To study the dependence of the potential difference across a resistor on the current through it and to determine its resistance and to verify the Ohm’s law. THEORY According to the Ohm’s law, the potential difference (V) across the ends of a resistor is directly proportional to the current (I) through it provided its temperature remains the same. That is V ∝ I V or = constant = R I or V = RI. Here R is a constant for the given resistor at a given temperature and is called its resistance. The SI unit of resistance is ohm (Ω). A graph between the potential difference across the two ends of a resistor and the current through it is a straight line pasing through the origin. The slope of this graph gives the resistance R of the resistor. To verify the Ohm’s law, we measure the potential difference across the two ends of a resistor at different currents through it in an electric circuit. The current through the resistor is measured by connecting an ammeter in series with it. The potential difference across the two ends of the resistor is measured by connecting the voltmeter in parallel with it. A straight line graph obtained between V and I verifies the ohm’s law. MATERIALS REQUIRED A resistor of about 5 Ω, an ammeter ( 0 - 3 A), a voltmeter (0 - 10 V), four dry cells of 1.5 V each with a cell holder (or a battery eliminator), a plug key, connecting wires, and a piece of sand paper. PROCEDURE 1. Note the range and least count of the given ammeter and the voltmeter. 2. Fresh connecting wires have an insulating layer on it. Similarly the connecting wires lying unused for some time may also develop an insulating layer. (How?) It is therefore important to clean the ends of connecting wires using a sand paper. 3. Draw a circuit diagram for studying the Ohm’s law as shown in Fig. 48.1 in your notebook. Observe how different components like the ammeter, voltmeter, resistor, and the plug key are connected with the cells (or battery eliminator). 4. Set up the circuit by connecting different components with the help of connecting wires. Initially connect only one cell in the circuit (that is make cell connections between points A and B). In case a battery eleminator is used, keep the rating of the eliminator at the minimum (say at 2 V). 5. Make sure that the positive and negative terminals of the ammeter and voltmeter are correctly connected in the circuit as shown in Fig. 48.1. Get the circuit set up by you checked by the teacher, before inserting the key into the plug. 6. Insert the key in the plug to let the current establish in the circuit. Note the readings of the ammeter and voltmeter and record them. The voltmeter measures the potential difference (V) across the two ends X and Y of the resistor, and the ammeter measures the current I through it. Remove the key from the plug to avoid unnecessary heating of wires. (How does it happen? Think it in accordance with the Joule’s law of heating.) 7. Now instead of using one cell in the circuit, connect two cells in the circuit (that is make cell connections between points A and C, in case RESULTS AND DISCUSSION • Compare the value of resistance R of the resistor obtained from the calculations (as given in the observation table) and obtained from the graph. • The value of resistance R of resistor for all values of current through it remains the same (or almost same). The graph between V and I is a straight line and passes through the origin. This verifies the Ohm’s law. PRECAUTIONS AND SOURCES OF ERROR • The connecting wires should be thick copper wires and the insulation of their ends should be removed using the sand paper. • Connections should be tight otherwise some external resistance may introduce in the circuit. • The ammeter should be connected in series with the resistor such that the current enters at the positve terminal and leaves at the negative terminal of the ammeter. • Voltmeter should always be connected in parallel to resistor. • The pointers of the ammeter and voltmeter should be at zero mark when no current through the circuit. If not, then ask your teacher to correct it. • Current should be passed through the circuit for a short time while taking observations; otherwise current would cause unnecessary heating in the circuit. Heating may change the resistance of resistors. AIM To study the factors that affect the resistance of a resistor. THEORY On applying the Ohm’s law, it is observed that the resistance of a resistor depends on its length, on its area of cross-section and on the nature of its material. Precise measurements have shown that the resistance (R) of a uniform metallic conductor is directly proportional to its length (l) and inversely proportional to the area of cross-section (A). That is, R ∝ l and 1Rα . ThusA l R = ρ (1)A Here is a constant of proportionality and is called the electrical resistivity of the material of the conductor. The SI unit of resistivity is ohm meter (Ω m). In this experiment, we study these factors in an electric circuit by employing different resistors (wires) of different lengths and areas of cross-section. Using Ohm’s law, the resistance of a conductor in an electric circuit can be determined by measuring the current through it and potential difference across its ends. An ammeter (connected in series with the resistor) measures the current through it and a voltmeter (connected parallely with the resistor) measures the potential difference across its two ends. All the wires must be attached to connectors such as crocodile clips, as shown in Fig. 49.1(b). This will ensure that the entire length of the wire will come in the circuit as a resitor. 6. Connect wire 1 between points A and B. Make sure that the positive and negative terminals of the ammeter and voltmeter are correctly connected in the circuit as shown in Fig. 49.1. Get the circuit checked by the teacher, before inserting the key into the plug. 7. Insert the key in plug to let the current establish in the circuit. Note the readings of the ammeter and voltmeter and record them. Ensure that the key is removed from the plug just after taking the ammeter and voltmeter readings to avoid unnecessary heating of wires. 8. Now replace the wire 1 by the wire 2. Insert the key in the plug and measure the current through wire 2 and measure the potential difference across the ends of the wire 2. Notice the difference in the values of current and potential difference. Remove the key. 9. Repeat step 8 for wires 3 and 4. OBSERVATIONS AND CALCULATIONS (i) Range of the ammeter = ___ - ___ A. (ii) Least count of the ammeter = ___ A. (iii) Range of the voltmeter = ___ − ___ V. (iv) Least count of the voltmeter = ___ V. Sl. Label of the Resistor Current through Potential difference Resistance No. connected between the resistor, I across the ends of of the resistor, points A and B resistor, V R = V/I (mA) (A) (V) (Ω) 1. Wire 1 2. Wire 2 3. Wire 3 4. Wire 4 RESULTS AND DISCUSSION Infer about the factors that affects the resistance of a resistor and answer the following: • How does it change with length? • How does it change with the area of cross-section? • How does it change with the resistivity of the material of the wire? (Get the resistivity of the materials from the textbook/Appendix - I). How Things Work PRECAUTIONS • The connecting wires should be thick copper wires and the insulation of their ends should be removed using the sand paper. • Connections should be tight otherwise some external resistance may introduce in the circuit. • The ammeter should be connected in series with the resistor such that the current enters at the positve terminal and leaves at the negative terminal of the ammeter. • Voltmeter should always be connected in parallel to resistor. • The pointers of the ammeter and voltmeter should be at zero mark when no current is flowing through the circuit. If not, then ask your teacher to correct it. • Current should be passed through the circuit for a short time while taking observations; otherwise current would cause unnecessary heating in the circuit. Heating may change the resistance of resistors. How Things Work AIM To determine the equivalent resistance of two resistors connected in series combination. THEORY When two resistors of resistance R1 and R2 respectively are connected in a series combination (Fig. 50.1), then their equivalent resistance R is given bys R = R + R. (1)s12Fig. 50.1: (a) Two resistors AB and CD are placed one after the other; (b) Two resistors AB and CD are connected in a series combination In order to determine the resistance of a combination of resistors in series, the current I flowing through the circuit is measured with an ammeter connected in series with the combination. The potential difference V across the combination of resistors is measured with a voltmeter connected in parallel (Fig. 50.2). MATERIALS REQUIRED Two resistors of (each of 2 W resistance), an ammeter (range 0 – 5 A), a voltmeter (range 0 – 5 V), three dry cells of 1.5 V each with a cell holder (or a battery eliminator), a plug key, connecting wires and a piece of sand paper. PROCEDURE 1. Note the range and least count of the given ammeter and the voltmeter. 2. Fresh connecting wires also have an insulating enamel layer at the top. Similarly the connecting wires lying unused for some time may also develop an insulating layer. (How?) It is therefore important to clean the ends of connecting wires using a sand paper. 3. Draw a circuit diagram for the series combination of resistors as shown in Fig. 50.2 in your notebook. Observe how different components like the ammeter, voltmeter, combination of resistors in series (of known resistances R1 and R2) and the plug key are connected with the cell(s) (or battery eliminator). 4. Place the given resistors one after the other and join the ends labelled B and C as shown in Fig. 50.1. Set up the circuit by connecting different components with the help of connecting wires as shown in the circuit diagram. 5. Make sure that the positive and negative terminals of the ammeter and voltmeter are correctly connected in the circuit as shown in Fig. 50.2. Get the circuit set up by you checked by the teacher, before inserting the key into the plug. 6. Insert the key in the plug to let the current establish in the circuit. Note the readings of the ammeter and voltmeter and record them. The voltmeter measures the potential difference (V) across the two ends A and D of the series combination of two resistors, and the ammeter measures the current I through the series combination. Remove the key from the plug to avoid unnecessary heating of wires (How does it happen? Think it in accordance with the Joule’s law of heating.) 7. Repeat the activity for three different values of current through the circuit and record the readings of the ammeter and voltmeter in each case. The current through the circuit may either be decreased or How Things Work increased by changing the number of cells in the circuit (or by changing the settings of the battery eliminator terminal). OBSERVATIONS AND CALCULATIONS Range of the ammeter = ___ - ___ A. Least count of the ammeter = ___ A. Range of the voltmeter = ___ -___ V. Least count of the voltmeter = ___ V. Resistance of first resistor R1 = ___ Ω. Resistance of second resistor, R= ___ Ω.2 Sl. Number Current Potential Equivalent Average No. of cells in through difference Resistance of value of the circuit the series across the series combination, RS RS combination, IS RS = Vs/Is VS 1. 2. 3. 4. R1 = ____ Ω, R2 = _____ Ω Equivalent resistance [from Eq. (1)] = R1 + R2 = ___ Ω RESULTS AND DISCUSSION Compare the observed value of the equivalent resistance of the series combination of the two given resistors (from observation table) with the calculated value of it using Eq. (1). PRECAUTIONS • The connecting wires should be thick copper wires and the insulation of their ends should be removed using the sand paper. • Connections should be tight otherwise some external resistance may introduce in the circuit. • The ammeter should be connected in series with the combination of resistors such that the current enters at the positve terminal and leaves at the negative terminal of the ammeter. • Voltmeter should always be connected in parallel to the combination of resistors. • The pointers of the ammeter and voltmeter should be at zero mark when no current flows through the circuit. If not, then ask your teacher to correct it. • Current should be passed through the circuit for a short time while taking observations; otherwise current would cause unnecessary heating in the circuit. Heating may change the resistance of resistors. AIM To determine the equivalent resistance of two resistors connected in parallel combination. THEORY When two resistors of resistance R and R12parallel combination (Fig. 51.1), given by 111 =+ R RRp 12 or R1R2R = p (1)R1 + R2 In order to determine the resistance of a combination of resistors connected in parallel, the current I flowing through the circuit is measured with an ammeter connected in series with the combination. The potential difference V across the combination of resistors is measured with a voltmeter connected in parallel (Fig. 51.2). MATERIALS REQUIRED Two resistors of (each of 2 Ω resistance), an ammeter (range 0 – 5 A), a voltmeter (range 0 – 5 V), three dry cells of 1.5 V each with a cell holder (or a battery eliminator), a plug key, connecting wires, and a piece of sand paper. PROCEDURE 1. Note the range and least count of the given ammeter and the voltemeter. 2. Fresh connecting wires also have an insulating enamel layer at the top. Similarly the connecting wires lying unused for some time may also develop an insulating layer. (How?) It is therefore important to clean the ends of connecting wires using a sand paper. 3. Draw a circuit diagram for the series combination of resistors as shown in Fig. 51.2 in your notebook. Observe how different components like the ammeter, voltmeter, combination of resistors in parallel (of resistances R1 and R2) and the plug key are connected with the cell(s) (or battery eliminator). 4. Place the given resistors side by side and join end A with the end C, and end B with end D (Fig. 51.1). Set up the circuit by connecting different components with the help of connecting wires as shown in the circuit diagram (Fig. 51.2). 5. Make sure that the positive and negative terminals of the ammeter and voltmeter are correctly connected in the circuit. Get the circuit set up checked by the teacher, before inserting the key into the plug. 6. Insert the key in the plug to let the current establish in the circuit. Note the readings of the ammeter and voltmeter and record them. The voltmeter measures the potential difference (V) across the two How Things Work ends A and D of the series combination of two resistors, and the ammeter measures the current I through the series combination. Remove the key from the plug to avoid unnecessary heating of wires (How does it happen? Think it in accordance with the Joule’s law of heating.) 7. Repeat the activity for three different values of current through the circuit and record the readings of the ammeter and voltmeter in each case. The current through the circuit may either be decreased or increased by changing the number of cells in the circuit (or by changing the settings of the battery eliminator terminal). OBSERVATIONS AND CALCULATIONS (i) Range of the ammeter = ____ - ____ A. (ii) Least count of the ammeter = ____ A. (iii) Range of the voltmeter = ____ -____ V. (iv) Least count of the voltmeter = ____ V. (v) Resistance of first resistor R1 = ____ Ω. (vi) Resistance of second resistor, R2 = ____ Ω. Sl. Number Current Potential difference Equivalent Average value of No. of cells in through across the parallel Resistance of Rp the circuit the parallel combination, Vp combination, Rp combination Rp = Vp/Ip Ip (A) (V) (Ω)(Ω) 1. 2. 3. 4. R1 = ____ Ω, R2 = _____ Ω R1R2 Equivalent resistance [from Eq. (1)] = = ___ ΩR1 + R2 RESULTS AND DISCUSSION Compare the observed value of the equivalent resistannce of the parallel combination of the two given resistors (from observation table) with the calculated value of it using Eq. (1). PRECAUTIONS AND SOURCES OF ERROR • The connecting wires should be thick copper wires and the insulation of their ends should be removed using the sand paper. • Connections should be tight otherwise some contact resistance may introduce in the circuit. • The ammeter should be connected in series with the combinations of resistors such that the current enters at the positve terminal and leaves at the negative terminal of the ammeter. • Voltmeter should always be connected in parallel to the combinations of resistors. • The pointers of the ammeter and voltmeter should be at zero mark when no current through the circuit. If not, then ask your teacher to correct it. • Current should be passed through the circuit for a short time while taking observations; otherwise current would cause unnecessary heating in the circuit. Heating may change the resistance of resistors. How Things Work A bar magnet of about 10 cm, a small compass needle, iron filings, drawing board, adhesive tape or drawing pins of brass, and white paper sheets. AIM To draw magnetic field lines of a bar magnet. THEORY A field of force that exists around a bar magnet is called its magnetic field. We see that when iron filings are sprinkled around a bar magnet, they arrange in a particular pattern as shown in Fig. 52.1. The lines along which the iron filings orient themselves represent magnetic field lines. These lines are closed curves and do not intersect each other. These field lines are crowded around the poles of the magnet. The degree of their closeness represent that the magnetic field is stronger at poles. The magnetic field lines around a bar magnet can be obtained using a compass needle. (A compass needle is a small magent. Its one end, which points towards north is called its north pole and the other end, which points towards south is called south pole.) MATERIALS REQUIRED Fig. 52.1 : Iron filings near the bar magnetalign themselves along the field lines PROCEDURE (a) To observe the pattern of iron filings around a bar magnet 1. Fix a sheet of white paper on a drawing board using adhesive tape or drawing pins of brass. 2. Place a bar magnet on this sheet in the middle of it. 3. Sprinkle iron filings around the bar magnet and gently tap the drawing board till a pattern, as shown in Fig. 52.1, is formed. 4. Observe the pattern. What does it show? Notice that the iron filings are crowded around the poles of the bar magnet. 5. Remove the iron filings from the paper. (b) To draw the magnetic field lines around a bar magnet 6. Identify the north and south poles of the bar magnet. Place the bar magnet in the middle of the paper. Mark the position of north and south poles and also draw the boundary of the bar magnet. 7. Place a small compass needle very near the north pole of the magnet. 8. You will observe that the south end of the compass needle aligns itself towards the north pole of the bar magnet. 9. Mark the positions of its two ends of the compass needle. 10. Move the compass to a new Fig. 52.2 : Drawing a magnetic field position such that its south line with the help of a end occupies the position compass needle previously occupied by its north pole. 11. In this way, proceed step-bystep till you reach the south pole of the magnet, as shown in Fig. 52.2. 12. Join the points marked on paper by a smooth curve. This curve represents a field line. 13. Repeat the above procedure and draw as many lines as you can. You will get a Fig. 52.3 : Magnetic field lines pattern like the one shown in around a bar magnet How Things Work Fig. 52.3. You might have noticed that the deflection in the compass needle is more when it is placed closer to one of the poles of the magnet. OBSERVATIONS AND CALCULATIONS The attached paper sheet shows the pattern of magnetic field lines drawn around the bar magnet. RESULTS AND DISCUSSION From the magnetic field lines around a bar magnet, it may be confirmed that: • the magnetic field lines are closed and continous; • the deflection in compass needle increases as it is moves towards the poles; • two magnetic field lines do not intersect; and • the magnetic field lines are crowded at poles of the bar magnet. PRECAUTIONS • There should not be any other magnetic material near the bar magnet except the compass needle while drawing the magnetic field lines. • Size of the compass needle should be small. • The bar magnet should be sufficiently strong to produce an appreciable deflection in the compass needle placed at a distance of 15 cm from the bar magnet. APPLICATIONS This method can be used to identify the magnetic materials. The strengths of two bar magnets can also be compared. QUESTIONS • You are provided an iron strip and a bar magnet. How will you distinguish them? • How does a compass needle work? • How will you make a compass using an iron needle, piece of thermo-cole and a magnet? • Do you think that the needle of a compass is a magnet? • Why does the needle of a compass point north and south? • Can an ordinary magnet be used as a compass? • What does degree of closeness of magnetic field lines indicate? • Why are more iron filings concentrated around the poles of the magnets? • In this experiment it is advised to use a small compass needle. size compass needle? How Things Work AIM To draw the magnetic field lines of a current-carrying straight wire. THEORY An electric current through a wire (conductor) produces a magnetic field around it. The existance of magnetic field can be observed using a magnetic compass needle. The direction of the magnetic field depends on the direction of the current through the wire. In this experiment we shall make an attempt to draw the magnetic field lines of a current carrying straight wire and find the effect of change of direction of current through the wire on the magnetic field lines. MATERIALS REQUIRED A battery (12 V) or a battery eliminator (12 V/2 A), a variable resistance (a rehostat), a plug key, a thick copper wire (prefereably SWG-12) of about 50 cm length, a rectangular wooden plank with a small hole at the centre through which the thick copper wire may comfortably pass, ammeter (0 - 3 A), a white sheet, a small compass needle, a wooden stand, adhesive tape, connecting wires, and a piece of sand paper. PROCEDURE 1. Note the range and least count of the given ammeter. 2. Fresh connecting wires also have an insulating enamel layer at the top. Similarly the connecting wires lying unused for some time may also develop an insulating layer. It is therefore important to clean the ends of connecting wires using a sand paper. 3. Draw a circuit diagram connecting a variable resistance (a rheostat), an ammeter, battery (or a battery eliminator), plug key with the thick copper wire XY as shown in Fig. 53.1. 4. Fix a sheet of white paper on a rectangular wooden plank using adhesive tape. Make a small hole O at the centre of the white sheet such that it overlaps the hole on the plank. Insert the thick copper wire XY through the hole O. Place the plank horizontally on the table as shown. 5. Set up the circuit, as shown in Fig. 53.1, by connecting different components. The position of the thick copper wire XY must be normal to the plane of the rectangular cardboard. The upper end X of the thick copper wire may be fixed on a laboratory (wooden) stand. This will ensure the position of the wire XY to be vertical. 6. Make sure that the positive and negative terminals of the battery (or battery eliminator) and ammeter are correctly connected in the circuit. Get the circuit set up by you checked by the teacher, before inserting the key into the plug. 7. Insert the key in the plug to let the current flow in the circuit. Change the position of the variable of the rheostat such that a current of about 2 A flows through the thick copper wire XY. Note and record the reading of the ammeter. How Things Work 8. Place a compass at a point (say P) over the white sheet placed on the rectangular cardboard. Observe the direction of the compass needle. By convention, the direction of the north pole of the compass needle would give the direction of the magnetic field produced by the electric current through the straight wire at point P. 9. Mark the position of the compass needle’s two ends and move the compass so that its south end occupies the position previously occupied by its noth end. Again mark the new positions indicated by the two ends of the compass needle. In this way proceed step-by-step till you get a complete circle as shown in Fig 53.1. Also mark the direction of the magnetic field line by an arrow. Thus the magnetic field line of a current carrying straight wire is a circle having the centre at position of the current carrying wire. 10. After drawing one magnetic field line remove the plug key and wait for few minutes so that the wire comes back to its normal temperature. 11. Repeat the above procedure and draw as many concentric circles (representing magnetic field lines) as you can. What happens to the deflection of the needle if the compass is moved away from the thick copper wire. Do you observe that the deflection in the compass needle decreases? What does it mean? It represents that the magnetic field produced by a given current in the conductor decreases as the distance from it increases. 12. Using this circuit, you can also see the effect of change in current through the conductor on the deflection in the compass needle placed at a given point. For this, place the compass needle at a fixed point Q (say). Now by changing the position of the variable contact of the rheostat, increase the current through the thick copper wire. What effect do you observe? The deflection in the compass needle also increases. Do you observe a decrease in the compass needle at a given point Q (say) with a decrease in current through the thick copper wire. It indicates that the magnitude of the magnetic field produced at a given point increases as the current through the thick copper wire increases or vice-versa. 13. Reverse the direction of current through the straight thick copper wire by interchanging the terminals of the battery (or battery eliminator) and observe the direction of the compass needle. Does it get reversed? OBSERVATIONS The attached sheet shows the pattern of magnetic field lines around a current-carying straight wire. RESULTS AND DISCUSSION On the basis of observations on the magnetic field lines, the following can be infered: • The magnetic field lines around a current-carrying straight wire do not intersect each other. • The deflection in the compass needle decreases as it is moved away from the current carrying straight wire. • The deflection in the compass needle changes as the current through the wire changes. • The direction of magnetic field lines gets reversed if the direction of current through the straight copper wire is reversed. PRECAUTIONS • Plug key should only be inserted in the circuit when you are recording the observations. • After drawing one magnetic field line remove the plug key and wait for few minutes so that the wire comes back to its normal temperature. • Please do not touch the hot wire during the experiment, lest you may hurt yourself as the wire may become very hot. • There should not be any magnetic material near the current carrying straight wire except the compass needle. • The thick copper wire XY must be positioned straight and vertical through out the experiment. In case a proper wooden stand is not available, then the wire must be passed through a cork that can be clamped in the laboratory stand. • A resistor of variable resistance (variable resistor) or rheostat should be connected in series with the thick straight copper wire to regulate the current through the straight copper wire. How Things Work may even melt. It is therefore advised to use a thick copper wire (preferably SWG-12) whose diameter is about 2 mm. In case if it is not available, other wires may also be used. Such thick copper wires are often used in the earthing of domestic electric circuits. • In this experiment, a relatively higher current flows through the circuit and for this reacson, it requires a high voltage battery or a high capacity battery eliminator. QUESTIONS • How magnetic field lines of a straight current carrying wire are different from the field lines of a bar magnet? • What affect you will notice on the pattern of field lines if you interchange a rheostat by the plug key in this experimental arrangement? • How can the direction of the magnetic field be determined? Suggest one method. • What will happen to the pattern of field lines if the thick copper wire is kept horizontally straight instead of vertically straight? Discuss in groups. AIM To study the magnetic field of an electromagnet. THEORY An electromagnet is a magnet consisting of a soft iron core with a coil of insulated wire wound round it. When a current flows through the wire the core becomes magnetized; when the current ceases to flow, the core loses its magnetization. In this experiment, using a compass needle we shall study the magnetic field produced by an electromagnet. (A compass needle is a small magent. Its one end, which points towards north is called its north pole and the other end, which points towards south is called south pole. A circular scale around the compass needle is used to measure its deflection. We will investigate the variation in the magnetic field produced by the electromagnet by changing the magnitude and direction of the current in the coil; and also the variation in the magnetic field as the compass is moved away from the electromagnet keeping the current through the coil constant. MATERIALS REQUIRED A small electromagnet, a compass needle with a circular scale, a battery of at least 6 V, a variable resistance (a rheostat), an ammeter (0 - 3 A), a plug key, measuring scale, connecting wires and a peice of sand paper. How Things Work PROCEDURE 1. Note the range and least count of the given ammeter. 2. Clean connecting wires using the sand paper. 3. Using connecting wires, set an electric circuit comprising of an elctromagnet, a battery, a variable resistance (a rheostat), plug, and an ammeter such that the axis of electromagnet lies closer and parallel to one of the side edge of the table, as shown in Fig. 54.1. Initally keep the sliding contact of the rheostat around its maximum. In such a situation, the magnitude of current through the coil will be minimum on inserting the key into the plug. 4. From one of the end of the electromagnet, draw a line PQ of more than 20 cm on the table along the axis of the electromagnet. Mark the divisons R, S, T, U at a regular inteval of 5 cm (say) on the line PQ. 5. Place the compass needle on line PQ at point R near an end of the electromagnet such that the centre of compass coincides with the mark of point R. Rotate the compass such that its needle lies at 0°. 6. Insert the key into the plug to allow the current flow through the coil wound round the soft iron core of the electromagnet. The core will become magnetized and will produce magnetic field around it. This will produce a deflection in the compass needle. By means of changing the position of variable connector on the rheostat, adjust the value of current through the coil such that a deflection of about 30° appears in the compass needle placed at point R. Note and record the value of current through the coil and deflection in the compass needle. 7. Increase the current through the coil by changing the position of varaible on the rheostat. What happens to the compass needle? Does it deflect more now? Note and record the ammeter and compass needle readings. Repeat this step for three more values of current through the coil. Keep the current in the coil such that it does not produce a deflection more than 60° in the compass needle. Remove the key from the plug. 8. To change the direction of the current through the coil, interchange the connections of battery terminals. Observe the change in the deflection of the compass needle. 9. Insert the key into the plug. Again note and record the current through the coil and deflection in the compass needle placed at point R along the axis PQ of the elctromagnet. Remove the key from the plug. 10. Move the compass needle to position S. Ensure that the compass needle lies at 0°. Insert the key into the plug and observe the deflection in the compass needle. Now the compass needle is placed at a distance of 10 cm from the end P of the electromagnet. Observe and record the deflection in the compass needle when placed at different distances from the end P of the electromagnet. That is when the compass needle is placed at points T, U etc. Remove the key from the plug. 11. In this experiment, you might have drawn a few lines on the table. As a courtsey to your friends who might perform this experiment later, please clean the table. OBSERVATIONS AND CALCULATIONS Range of the ammeter = ____ - ____ A. Least count of the ammeter = ____ A. (a) Variation in the deflection in the compass needle at a point with a change in current through the coil Position in compass needle from an end of the electromagnet = ____ cm. 1. 2. 3. 4. (b) Variation in the deflection in the compass needle as the compass is moved from the electromagnet but the current through the coil remains the same Current through the coil of the electromagnet = _____ A. How Things Work 1. Point R 5 2. Point S 10 3. Point T 15 4. Point U 20 RESULTS AND DISCUSSION From your observations, infer the following: • The change in the magnetic field strength of an electromagnet at a given point with the change in current through the coil of the elctromagnet. • The change in the magnetic field strength of an electromagnet with the increase in distance from the electromagnet. • The effect of change in the direction of electric current through the coil on the direction of magnetic field produced by the electromagnet. PRECAUTIONS AND SOURCES OF ERROR • The connecting wires should be thick copper wires and the insulation of their ends should be removed using the sand paper. • The ammeter should be connected in series with the electromagnet coil such that the current enters at the positve terminal and leaves at the negative terminal of the ammeter. • The pointer of the ammeter should be at zero mark when no current flows through the circuit. If not, then ask your teacher to correct it. • Plug key should only be inserted in the circuit when you are recording the observations. • There should not be any magnetic material near the electromagnet except the compass needle. Electromagnet should be kept on a wooden table. • Size of the compass needle should not be very large. • The amount of current through the coil of the electromagnet should be such that its magnetic field is sufficiently strong to produce an appreciable deflection in the compass needle placed at a distnce of 20 cm from an end of the electromagnet. MATERIALS REQUIRED A strong horse shoe magnet, a small aluminum rod (about 5 cm), an ammeter (0 - 3 A), two wooden stands, four dry cells (each of 1.5 V) with a cell holder (or a 6 V battery or a battery eliminator), a plug key, connecting wires, a piece of sand paper, a card boad, a graph paper, and adhesive tape, PROCEDURE 1. Using clean connecting wires connect a conductor (a small aluminium rod) in series with dry cell holder (or a battery or a battery eliminator), an ammeter and a plug key. Do not insert the key into the plug. 2. Suspend the aluminium rod on a wooden laboratory stand such that its length AB remains horizontal. Also place a strong horse shoe magnet on another wooden laboratory stand such that its north and south poles lie in the same vertical plane. 3. Align the two laboratory stands such that the aluminium rod lies between the two poles of the magnet with the magnetic field directed upwards, as shown in Fig. 55.1. Hold a cardboard just behind the magnet and aluminium rod assembly. Fix a graph paper on it to mark the deflection in aluminium rod. For this mark the initial position of the aluminium rod on the graph paper. 4. Connect the aluminum rod in series with one cell (or set the battery eliminator on 2 V terminals) and insert the key into the plug to allow the current flow through the conductor. (Let the current flow through point B to A in the conductor.) Note and record the ammeter reading. 5. Do you see any displacement in the conductor? In which direction it moves? Does it move towards the left? Examine that this displacement is in accordance with the fleming’s left-hand rule. For this stretch the thumb, fore finger and middle finger of your left hand such that they How Things Work are mutually perpenducular (Fig. 55.2). Adjust your fore finger in such a way that it points in the direction of magnetic field (upwards), and the middle finger points in the direction of current (B to A) then your thumb must point towards the left direction. Measure the displacement of the aluminium rod AB, using the graph paper placed behind (step 3). 6. Reverse the direction of electric current passing through the aluminum rod by interchanging the connections of cell (or battery or battery eliminator) and ammeter. (Always ensure that the poistive terminal of ammeter is connected with the positive terminal.) Observe the direction of displacement of conductor. Does it moves in opposite direction? Mark and record it. 7. Repeat steps 4 to 6 for different values of current through the conductor. Use two cell dry cells for 3 V supply, three cells for 4.5 V supply and four cells for 6 V supply. (Change the battery eliminator settings accordingly if a battery eliminator is used.) Note and record the current through the rod AB, its displacemnet (for both the directions of current). OBSERVATIONS AND CALCULATIONS Sl. No. of cells Voltage of Current Displacement of aluminium rod (mm) No. cells used supply (V) through when current flows through it along the rod AB BA (deflection AB (deflection towards left) towards right) 1. 1 1.5 2. 2 3.0 3. 3 4.5 4. 4 6.0 RESULTS AND DISCUSSION From your observations, infer the following: • On passing current through a straight conductor (aluminium rod) kept in a magnetic field, the conductor gets displaced. • The displacement of the conductor increases as the current increases in the conductor. • The direction of displacement of the conductor rod changes with the change in the direction of current through it. • The displacement of aluminium rod is in accordance with Fleming’s left-hand rule. PRECAUTIONS • Clean the ends of the connecting wire using a sand paper to remove the insulating layer from them. • Aluminium rod (AB) should be suspended in such a way that it should not touch the horse shoe magnet. • Horse shoe magnet should be fixed a using wooden stand. • There should not be any other magnetic material near the experimental set-up. • Plug key should be inserted in the circuit only at the time of recording the observations. that a current-carrying conductor also produces magnetic field that changes with a change in the current flowing through it. Thus if a coil is placed near to a current-carrying conductor, an induced current in the coil may set-up due to a change in the current through the current-carrying conductor. In this experiment we first see the effect of moving a magnet in a coil connected with a galvanometer (Fig. 56.1). We will then see the effect of varying current in a coil (coil-1) on another coil (coil-2) connected with a galvanometer (Fig. 56.2). How Things Work Fig. 56.2 : Current is induced in coil-2 when current in coil-1 is changed MATERIALS REQUIRED Two coils of copper wire each having about 50 turns, a rheostat (variable resistor), an ammeter (0 - 3 A), a galvanometer, a strong bar magnet, a plug key, connecting wires, and a piece of sand paper. PROCEDURE A. Existance of induced current in a coil due to the relative motion between a coil and magnet 1. Take a coil AB and connect it with a galvanometer as shown in Fig. 56.1. 2. Take a strong bar magnet and move its north pole (or south pole) towards an end (say B) of the coil. Observe the position of needle of galvanometer. Is there any deflection in it? There is a momentary deflection in the galvanometer needle in one direction (say in right direction). What does this indicate? This indicates the presence of a current in the coil AB. 3. Stop moving the bar magnet. What do you observe now? Does the galvanometer shows any reading? No. The deflection of the galvanometer becomes zero when the motion of the bar magnet stops. What does it mean? 4. Now withdraw the north pole of the magnet away from the coil and observe the deflection in galvanometer. The galvanometer needle now deflects in the opposite side (that is in left side), depicting that the current is now set up in the direction opposite to the first. 5. Move South Pole of the magnet towards the end B of the coil and observe the deflection in the galvanometer needle. 6. Place the magnet stationary at a point near to the coil retaining its north pole towards the end B of the coil and observe it. The galvanometer needle deflects towards the right when the coil is moved towards the north pole of the magnet. Similarly, the needle moves towards the left when the coil is moved away. Deflection of galvanometer drops to zero when coil is kept stationary. 7. Finally keep the coil and the magnet both stationery and observe the deflection. There is no deflection in the galvanometer. It is thus clear that motion of a magnet with respect to the coil produces an induced potential difference which sets up an induced electric current in the circuit as has been noticed by the deflection of galvanometer in above steps. Record your observations. B. Existance of induced current in a coil due to a change in the current through another coil placed close to it 8. Connect a coil (coil-1, say) of copper wire with a source (a cell or a battery), through a rheostat, an ammeter and a plug key, as shown in Fig. 56.2. Use clean connecting wires to make the circuit. 9. Bring another coil (coil-2) connected with a galvanometer close to the coil-1 circuit. 10. Plug the key and allow a current to flow through coil-1. What do you observe? Does the galvanometer needle connected with coil-2 deflects. Yes it momentarily deflects. It stops at the zero mark as soon as the current in coil-1 becomes steady. (Adjust the variable connector of the rheostat to allow an appreciable current to flow through the coil1. Note that a rheostat is basically a resistor of variable resistance. It is connected in series in a circuit to change the resistance in the circuit.) 11. Change the current through coil-1, by moving the variable slider on the rheostat coil. What do you observe? Do you see that the galvanometer connected with coil-2, shows deflection in one direction when current in coil-1 circuit increases while the deflection in galvanometer needle reverses when the current in coil-1 circuit decreases. 12. Record your observations in the observation table. How Things Work OBSERVATIONS AND CALCULATIONS (a) Existance of induced current in a coil due to the relative motion between a coil and magnet (Fig. 56.1) (b) Existance of induced current in a coil (coil-2) due to a change in the current through another coil (Coil-1) placed close to it (Fig. 56.2) Sl. No. Activity Deflection in the needle of the galvanometer (right/left/no deflection) Inference 1. Coil stationary; N-pole of the bar magnet magnet moving toawrds the coil. 2. Coil stationary; N-pole of the bar magnet magnet moving away the coil. 3. 4. Coil stationary; S-pole of the bar magnet magnet moving toawrds the coil. Coil stationary; S-pole of the bar magnet magnet moving away the coil. 5. Bar magnet stationary with its N-pole towards the coil; Coil moving towards the magnet. 6. Bar magnet stationary with its N-pole towards the coil; Coil moving away the magnet. 7. Bar magnet stationary with its S-pole towards the coil; Coil moving towards the magnet. 8. Bar magnet stationary with its S-pole towards the coil; Coil moving away the magnet. 9. Both coil and magnet stationary Sl. Activity Deflection in the needle Inference No. of the galvanometer (right/left/no deflection) 1. No current through the coil-1. 2. Current increasing in coil-1. 3. Current decreasing in coil-2. RESULTS AND DISCUSSION Write the inferences of the activities in observation table and conclude that the phenomenon of electromagnetic induction is the production of induced current in a coil placed in a region where the magnetic field changes with time. Also comment on the ways you can induce a current in a circuit that does not have any source of electricity. PRECAUTIONS • Clean the ends of the connecting wire using a sand paper to remove the insulating layer from them. • Bar magnet should move (inside or outside the coil) such that it does not touch the coil. • Wrapping of copper wire of coils should be uniform. • There should not be any other magnetic material near the experimental set-up.

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