Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Visualize Alternating Current


To visualize that current reverses its direction periodically in an AC circuit.


The electricity that we use in our houses flow in the form of alternating current (AC). The electricity we uses in torch is in the form of direct current (DC). In DC circuits, the current flows in one direction only. At any point, in the circuit, the current has a fixed direction. However, in AC, the current keeps changing its direction between two terminals. How many times does the current change its direction, that depends upon the source which generates the AC. The electricity that is generated in various power stations in India is at 50 Hz. So the AC that we use in our houses changes direction 100 times per second. This change is so rapid that we do not feel any discontinuity such as flickering of the bulb.


A low frequency oscillator, galvanometer


Connect a galvanometer to the oscillator through a resistance. Keep the frequency at low value, 1 Hz or so. Look at the needle of the galvanometer.


The needle goes towards right and then come back towards left. This process keeps on repeating. Thus, the current keeps changing direction in the circuit. If we increase the frequency of oscillator, the needle oscillate faster. At certain stage (may be 10 Hz or so), the change become so fast that the galvanometer needle is not able to follow the change and hence stay almost at zero.