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## Class 12 Physics (India)

### Course: Class 12 Physics (India)>Unit 3

Lesson 5: Cell, EMF, terminal voltage, & internal resistance

# Cells, EMF, terminal voltage & internal resistance

EMF is the work done by the cell in moving a coulomb of charge across its terminals. It represents the energy transferred per coulomb to the charges. Some of this energy is lost as heat due to internal energy. Hence the net energy gained by the charge = the emf - heat lost. This net energy gained per coulomb is called the terminal voltage. This is the voltage across the ends of the battery. Created by Mahesh Shenoy.

## Want to join the conversation?

• Isn't the electric field on both ends of the battery constant? W=qE, when the voltage drops, why can the battery still successfully "push" the electrons to the other end? • What causes the current to run faster? The lack of resistance in the rest of the circuit? Why then does the internal resistance fail to impede the speed of the current, instead creating heat?
(1 vote) • The internal resistance does indeed impede the speed (wow, it sounds like assonance XD ) of the current carrying charge, but since the negative terminal does the work to PUSH the charge against such resistance as well as the repulsion by terminals, providing the charge enough energy, such that it restores its initial speed and no decrease in speed is visible. and hence due to all this hard work the heat is evolved. Its just like, when your car gets stuck between large rocks or in a swamp, you apply all the force you can probably by hitting that accelerator.... and as you persistently try to get out it, your engine has heated up a good deal. So its kind of analogous to our case, where we try to get over the obstacles and in the process we end up dissipating heat, which is an indication that work is being done against an external force.
• @, i thought +ve charges can't actually move, i.e. protons can't move. isn't it actually only the electrons can move from 1 atom to the next. ?
(1 vote) • There is this IDEA of current flow that the scientists and researchers have been following since the earliest of times and hence, we too have come to accept it as the CONVENTION. This convention created, is to assume the flow of positive charges from the positive terminal to the negative terminal. But what actually happens inside a circuit is the flow of electrons in the opposite orientation as that of our convention. Now one reason for such contradiction could be that by the time the concept of electrons as current carriers was discovered, a lot of research and results had already been plotted assuming the motion of positive charges and hence in order to not disturb the then equilibrium, we simply took it into consideration as a CONVENTIONAL ASSUMPTION.
• The intuition that I applied to understand why the energy lost would be more if the charge moves faster was: since the charge wants to travel faster, it will have to overcome the resistance offered quicker, that is, it should have more energy than before to get past the resistance. Thus, energy lost would be more and terminal voltage would be lesser. Is this a correct way to think?
(1 vote) • How comes a voltmeter manages to show you the emf when you connect it to the battery in parallel,however,when you connect a resistor,the shown voltage is influenced by the internal resistance
(1 vote) • In one round around the circuit the charge looses 5v out of 9, so remaining is 4v. Then the second time around the loop does it again loose another 5 to end up in -1??
(1 vote) • So my textbook goes that VOLTAGE DROP --> is the work done per unit charge in carrying the positive test charge through the electrolyte (because it is no longer available for use to us) but according to sir here the work done (in ideal condition ) on a couloumb charge == EMF
I am confused>>>?
(1 vote) • Please explain to me why electromotive force is independent of the following factors?
The factors are

1. The shape of electrodes
2. The distance between the electrodes
3. The amount of electrolyte in the cell
(1 vote) • Why emf is not force?From the formula of force F=m.a ,after charge move from negative to positive end does its acceleration change? 