CBSE Class 12 Physics Notes Chapter 7 Alternating Current

What is Alternating Current?

In a circuit consisting of Alternating Current, the change in the current and the electromotive force can be seen with respect to time.

What is wattless current?

When it comes to a purely inductive circuit ora purely capacitive circuit, no power is dissipated even if the current is seen to flow in the circuit. here, \(Cos\phi=0\) and the current in this circuit is referred to as wattless current.


A phasor is termed to the vector which rotates at an angular speed about the origin. The magnitude of a phasor represents the peak value of the current or the voltage. The phase difference between voltage and current in an ac circuit can be represented in terms of voltage and current by rotating the phasors.

Step Up Transformer

If the secondary coil has more numbers of turns than the primary, the voltage is seen to step up, that is how the name is given for this type of transformer. If the secondary coil has less number of coils, it is referred to as a step-down type of transformer.

Important Questions

  1. A 5000 V, 70 Hz ac supply s connected to a 500 Ω resistor
    1. Define the net power consumed over a full cycle(a)
    2. Find out the RMS value of the current

2.(a)What is the RMS voltage if the peak voltage of an ac supply is 600 V?

(b)What is the peak current if the RMS value of current in an ac circuit is 40 A.?

  1. 500 V, 60 Hz ac supply is connected to a 55 mH inductor. Determine the RMS value of the current in the circuit.

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Practise This Question

A bulb and a capacitor are connected in series to a source of alternating current. If its frequency is increased, while keeping the voltage of the source constant, then