3.5.1.6 - Electromotive Force And Internal Resistance

Question 1

What is internal resistance?

Answer 1

When chemical energy in a battery causes electrons to move and collide with one another.

Question 2

What is the electromotive force?

Answer 2

The amount of electrical energy the battery produces for each coulomb of charge.

Question 3

What is emf measured in?

Answer 3

Volts

Question 4

E.m.f =

Answer 4

Energy / charge

Question 5

In real power supplies what always happens to some energy?

Answer 5

Some energy is lost overcoming the internal resistance.

Question 6

Energy per coulomb supplied to the source =

Answer 6

Energy transferred in load resistance + energy per coulomb wasted in internal resistance

Question 7

What are the five EMF equations?

Answer 7

Emf = V + v
Emf = I(R + r)
V = emf - v
V = emf - Ir

V = terminal pd
v = lost volts
R = load resistance 
r = internal resistance

Question 8

How can you work of the total emf in a series circuit?

Answer 8

By adding all the individual emf’s together.

Question 9

For cells in parallel how can you find the total emf?

Answer 9

Add all the individual values of emf together.

Question 10

When investigating internal resistance and e.m.f. how and what values do you need to take?

Answer 10

Vary the current in the circuit by changing the value of the load resistance, using the variable resistor. Measure the p.d. for several different values of current.

Question 11

What table and graph do you need to draw?

Answer 11

A table for varying values of V and I. Plot V on the y axis and current on the x.

Question 12

What will the graph look like?

Answer 12

Negative gradient crossing both axis.

Question 13

How do you find the emf and internal resistance from the graph?

Answer 13

Start with the equation v = emf - Ir. Then rearrange to get V = -rI + emf. Since r and emf are constants, it is like the equation y = mx + c. So the intercept on the y axis is emf and the gradient is -r.