Question

in a generator,if the induced emf makes the current to flow in such a way that it creates a force to oppose the external mechanical force.why don't these forces cancel each other?
the generator action is such that it absorbs the mechanical energy in this opposition and converts it to electrical energy,my question is if the conductor needs more external force to overcome this opposing induced force,then proportionately the induced force also increases and balances the applied force,such that the motion stops after some time?....where did I go wrong?!!

Answer 1

@hartnn @mathslover @Opcode

Answer 2

@AravindG @Ashleyisakitty @charlotte123

Answer 3

Simply because emf is NOT a "mechanical force" measured in Newtons. Instead it is a potential, or energy per unit of charge, measured in volts.

Answer 4

no actually I meant the force due to the induced current ,which is in opposite direction to the applied force

Answer 5

A simple motor and a simple generator are one and the same thing.
When a current flows through the coil of a motor the coil rotates in a magnetic field and so the coil acts as a generator with the induced emf in the opposite direction to the applied emf.

So we have E I = I^2 R + Eb I which is power input from the power source making the motor rotate equal to the power lost due to the resistance of the coil I^2 R plus the external work done by the motor Eb I.
E is the supply emf, I the current in the circuit and Eb is the back emf generated by the coil rotating in the magnetic field.

Just to show this is reasonable suppose the coil had no resistance and the were no losses due to friction at the bearings etc and the motor was doing no work. You turn on the power supply the motor draws current and speeds up. As it speeds up the back emf increases until it reaches a value equal to the supply emf. No more current flows through the coil and the coil rotates at constant speed. No power is needed to make the coil continuing to rotate because it has reached a constant rotational speed and there are no losses.

With the coil having resistance and friction al the bearings the coil will speed up until E I = I^2 R + Eb I where I^2 R is the loss in the coil resistance and Eb I is the power dissipated doing work against the friction at the bearings.

Now let's make the motor do some work by getting it to lift a load. How does this work in terms of our equation. Easy. The rotational speed of the coil decreases. So Eb decreases because for a generator the emf generated if proportional to the speed of rotation of the coil.

This lowering of Eb increases the current I in the circuit by a larger fraction than the decrease in Eb so that Eb I increases - power to do work against the bearing friction and lifting a load and more power from the power source E I because I has increased.

How do you make this motor a generator and make power go into the power supply. Easy by making the coil rotate faster by applying an external torque the back emf can be made to increase to exceed the power supply voltage. Electric vehicles use this idea when going downhill to recharge the batteries and produce a breaking effect as well.

And finally, when is a simple motor most likely to be destroyed? Answer when it is first connected to the power supply. Why? Because when the coil is not rotating no back emf is produced and the current in the circuit is very high. So in turn the heating effect due to the resistance of the coil is very large and the coil might burn out. E I = I^2 R
As the motor speeds up the current decreases. This is also the reason you should not stall a motor.

Answer 6

thanks :) one more question it's nagging in my mind, in the generator why do we say that there is a maximum rate of change of flux when the flux linkage through the coil is zero?