Module 4 - EPR (Internal Resistance)

Question 1

Define Internal Resistance

Answer 1

The inherent property of sources of e.m.f to resist the flow of charge due to their material

Question 2

Define Lost Volts

Answer 2

The energy transferred per unit charge by electrons to overcome the internal resistance of the source of e.m.f

Question 3

Define Terminal pd

Answer 3

The remaining energy transferred per unit charge to electrons after the loss of energy from the e.m.f

Question 4

Use Kirchhoff’s laws to derive the Interrnal Resistance equation

Answer 4

Σε =Σpd therefore:
ε = V + Vl

Question 5

Put the Internal Resistance equation in the 3 forms

Answer 5

1. ε = V +Vl
2. ε = V + Ir
3. ε = I (R+r)

Question 6

Describe the relationship between current and terminal pd

Answer 6

I α 1/V , this is because “the larger the current the larger the lost volts”
and because ε = V + Ir and so for a constant ε if the I increases the terminal pd will decrease proportionally

Question 7

Describe how to find the e.m.f of a circuit

Answer 7

because we know ε = V + Ir, when I = 0, the ε is equal to V, therefore:
- disconnect the source of e.m.f from the circuit,
- put the voltmeter in parallel with the source of e.m.f, this potential diffference reading is equal to ε.

Question 8

Describe how to find the internal resistance of a circuit

Answer 8

because we know ε = V + Ir
we can rearrange into the form y = mx + c:
V = ε -Ir therefore:
- if a VI graph is plotted the gradient will be -r
- connect a source of emf in parallel with a voltmeter, and in series with an ammeter and a variable resistor
- By using the variable resistor to vary the current read off the ammeter and the voltmeter to get values for I and V and plot a VI graph usioing this data
- calculate the gradient of this graph and multiply by -1 to get the internal resistance