Can you have an AC series circuit in which there is a phase difference of (a) 180° (b) 120° between the emf and the current?
(a) No. (b) No.
For an LCR circuit with angular frequency ω, the impedance is given by the formula 
where R = resistance, L = inductance, C = capacitance.
Now, the phase difference between the current and the voltage is given by the formula 
, where 
= phase difference, ω = angular frequency, L = inductance, C = capacitance, R = resistance.
Since there are no restrictions on the values of L, C or R, it is obvious that 
can take any value between -∞ to +∞, that is 
can take any value between -900 to +900 (since tan 900 = ∞)
Since both 1200 and 1800 fall beyond the permitted range of values, we cannot have an AC circuit with any of the given phase differences. (Ans)