Level 1: Spin a Generator Twice as Fast: Twice the Power?
Unplug the lamp and keep pedalling.
A bicycle dynamo lights a lamp. With the lamp disconnected (an open circuit), turning the dynamo at the same speed becomes:
With the lamp connected, the induced current flows in the dynamo's coil, and by Lenz's law the magnetic force on that current opposes the rotation. That opposing torque is how your pedalling energy becomes light and heat.
With the lamp disconnected there is still an EMF but no current, so there is no magnetic braking and the dynamo is easier to turn: only friction is left. A power station's turbines likewise work harder when more current is drawn.
Every Level 2 starts from a quick question, Level 1, and changes one thing. Seeing what that change does is the skill JEE Main tests. Try it here, free and with no signup, then read the worked answer.
