Solution of Chapter 12. Electricity (NCERT - Science Book)

What does an electric circuit mean?

Define the unit of current.

Calculate the number of electrons constituting one coulomb of charge.

Name a device that helps to maintain a potential difference across a conductor.

What is meant by saying that the potential difference between two points is 1 V?

How much energy is given to each coulomb of charge passing through a 6 V battery?

On what factors does the resistance of a conductor depend?

Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source? Why?

Let the resistance of an electrical component remain constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?

Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal ?

Use the data in Table on page 23 of this book to answer the following:

(a) Which among iron and mercury is a better conduction?

(b) Which material is the best conductor?

Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 resistor, an 8 resistor, and a 12resistor, and a plug key, all connected in series.

Redraw the circuit of Question 1 putting an ammeter to measure the current through the resistors and a voltmeter to measure the potential difference across the 12 n resistor. What would be the reading in the ammeter and the voltmeter?

Judge the equivalent resistance when the following are connected in parallel:

(a) 1 and 10⁶

(b) 1,10³and 10⁶

An electric lamp of 100, a toaster of resistance 50 , and a water filter of resistance 500 are connected in parallel to a 220 V source. What is the resistance of an electric iron connected to the same source that takes as much current as all three appliances, and what is the current through it?

What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?

How can three resistors of resistances 2, 3, and 6 be connected to give a total resistance of

(a) 4, and (b) 1 ?

What is (a) the highest, and (b) the lowest, total resistance that can be secured by the combination of four coils of resistances 4, 8, 12 and 24?

Why does the cord of an electric heater not glow while the heating element does?

Compute the heat generated while transferring 96000 coulombs of charge in one hour through a potential difference of 50 V.

An electric iron of resistance20 takes a current of 5 A. Calculate the heat developed in 30 s.

What determines the rate at which energy is delivered by a current?

An electric motor takes 5A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h.

A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R', then the ratio R/R' is:

Which of the following terms does not represent electrical power in a circuit?

An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be:

Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then in parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combination would be:

How is a voltmeter connected in the circuit to measure the potential difference between two points ?

A copper wire has diameter 0.5 mm and resistivity of 1.6 x 10^-8 m. What will be the length of this wire to make its resistance 10? How much does the resistance change if the diameter is doubled?

The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below:

I (amperes): 0.5 1.0 2.0 3.0 4.0

V (volts): 1.6 3.4 6.7 10. 2 13.2

Plot a graph between V and I plotted by using the given data is shown below:

When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.

A battery of 9 V is connected in series with resistors of 0.2, 0.3, 0.4 , 0.5 and 12 respectively. How much current would flow through the 12 resistor?

How many 176 resistors in parallel are required to carry 5 A on a 220 V line?

Show how you would connect three resistors, each of resistance 6, so that the combination has a resistance of (i) 9, and (ii) 4 .

Several electric bulbs designed to be used on a 220 V electric supply line are rated 10 W each. How many bulbs can be connected in parallel with each other across the two wires of 220 V line if the maximum allowable current is 5 A ?

A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each of 24resistance, which may be used separately, in series or in parallel. What are the currents in the three cases?

Compare the power used in the 2resistor in each of the following circuits:

(i) a 6 V battery in series with 1 and 2 a resistors.

(ii) a 4 V battery in parallel with 12 and 2 resistors.

Two lamps, one rated 100 W at 220 V and the other 60 W at 220 V are connected in parallel to electron mains supply. What current is drawn from the line if the supply voltage is 220 V?

Which uses more energy, a 250 W TV set in 1hr or a 1200 W toaster in 10 minutes ?

An electric heater of resistance 8draws 15 A from the service mains for 2 hours. Calculate the rate at which heat is developed in the heater.

Explain the following:

(a) Why is the tungsten used almost exclusively for the filament of electric lamps?

(b) Why are the conductors of electric heating devices such as bread toasters and electric irons made of an alloy rather than a pure metal ?

(c) Why is the series arrangement not used for domestic circuits?

(d) How does the resistance of a wire vary with its area of cross-section?

(e) Why are copper and aluminium wires usually employed for electricity transmission?