3.5.1.6 Electromotive Force and Internal Resistance

Question 1

Where does resistance comes from?

Answer 1

Resistance comes from electrons colliding with atoms and losing energy.

Question 2

In a battery, what is used to make electrons move?

Answer 2

Chemical Energy,

Question 3

What happens as the electrons move through the battery?

Answer 3

They collide with the atoms inside the battery. This tells us that batteries and cells must have resistance.

Question 4

What is internal resistance?

Answer 4

The internal resistance of a source of electricity is due to the opposition to the flow of charge through the source.
This causes electrical energy produced by the source to be dissipated inside the source when charge flows through it.

Question 5

What causes cells and batteries to warm up?

Answer 5

Internal resistance.

Question 6

How can the internal resistance be described?

Answer 6

The loss of potential difference per unit current in the source when current passes through the source.

Question 7

How can a cell with internal resistance be drawn?

Answer 7

A cell next to a resistor, with a boxed around both.

Question 8

What is load resistance?

Answer 8

The total resistance of all the components in the external circuit.

Question 9

What is electromotive force?

Answer 9

The electromotive force of a source is the electrical energy per unit charge produced by the source.
It is the amount of electrical energy transferred to each coulomb of charge.

Question 10

How can the emf be described in terms of voltmeters?

Answer 10

The voltmeter reading across the terminals of a cell when no current is drawn from it.

Question 11

What is the equation for electromotive force?

Answer 11

Electromotive force = electrical energy / charge.

Question 12

What is the unit for emf?

Answer 12

Volts

Question 13

What is the potential difference across the terminals of the source?

Answer 13

The electrical energy per unit charge delivered by the source when it is in a complete circuit, and when current is being drawn from the cell.

Question 14

Why is the terminal p.d. always less than the emf when a current passes through the source?

Answer 14

Due to some energy being transferred to the charge carriers to do work to move through the power source.

Question 15

What is the terminal potential difference?

Answer 15

This is the potential difference across the load resistance (R) when energy is transferred from one coulomb of charge to the load resistance/external circuit.

Question 16

What would happen if a cell has no internal resistance?

Answer 16

The terminal p.d. would be equal to the emf of the cell.

Question 17

Explain internal resistance in terms of charge carriers?

Answer 17

The charge carriers have to travel through some material in the power source.

This requires the charge carriers to do work through the power source.

Therefore, the charge carriers have lost energy before they leave the cell.

Question 18

What is lost volts (v)?

Answer 18

The energy waster per coulomb overcoming the internal resistance.

Question 19

What does the law of conservation of energy tell us about any circuit?

Answer 19

Energy per coulomb supplied by the source = energy per coulomb transferred to the load resistance + energy per coulomb wasted in internal resistance.

EMF = Terminal Potential Difference + Lost Volts

Question 20

Give three equations which link:

emf
current
load resistance
internal resistance
terminal potential difference
lost volts

Answer 20

ε = V + v

ε = IR + Ir

ε = I(R + r)

Question 21

What is the equation for power of a cell with an internal resistance?

Answer 21

Iε = I²R + I²r

P = I²R + I²r

P = Iε