Module 4 EMF and Internal Resistance

Question 1

What is internal resistance?

Answer 1

the resistance within power supplies (or wires within a circuit)

Question 2

What is the effect of internal resistance?

Answer 2

Reduces PD available to circuit

Question 3

What causes internal resistance?

Answer 3

work being done by the charges as the move through the power source or wires

Question 4

What is terminal PD?

Answer 4

PD available to the external circuit

Question 5

When does internal resistance not apply?

Answer 5

when there is no current being drawn

Question 6

What is lost volts?

Answer 6

PD lost in the power source due to internal resistance

Question 7

What is the symbol for internal resistance?

Answer 7

r (lower case r)

Question 8

How can lost volts be calculated?

Answer 8

I x r (current x internal resistance)

Question 9

What happens with increasing internal resistance?

Answer 9

terminal p.d drops

Question 10

How to draw a battery with internal resistance?

Answer 10

Question 11

What is ε?

Answer 11

EMF of the battery

Question 12

What is V?

Answer 12

terminal PD in a circuit

Question 13

What is the EMF equation?

Answer 13

ε = V + Ir

ε is EMF
V is terminal PD
I is current
r is internal resistance

Question 14

Factorise ε=V+Ir

Answer 14

V = IR where R is the resistance of the circuit
ε = IR + Ir (current is same through circuit and power source)
ε=I(R+r)

Question 15

What is the effect of connecting identical cells in series

Answer 15

increases available emf (add EMFs)
also increases internal resistance (add in series),
therefore limiting the current that can be produced (less current)

Question 16

What is the effect of connecting identical cells in parallel

Answer 16

Produce the same emf as one cell

much smaller internal resistance (adding resistors in parallel decreases resistance)

Provides a greater current

Question 17

Describe (or draw) the circuit used to find the internal resistance of a battery

Answer 17

A battery (with internal resistance) in series with a variable resistor and an ammeter, with a voltmeter in parallel with the battery

Question 18

Describe the measurements which would be taken to find the internal resistance of a battery

Answer 18

Using a voltmeter, measure the pd across the battery whilst changing current using a variable resistor

Use the ammeter in series to measure the current

Use this information to plot a graph of V on the y-axis and I on the x
gradient = -r

Question 19

Explain, by using an internal resistance equation, how using a graph of V terminal PD on the Y-axis and I on the x, we can find the internal resistance of a battery and it’s EMF

Answer 19

rearrange ε=V+Ir to match y=mx+c

V=-rI + ε

We can see that the graph will give us a gradient of -r (r is positive of this) and the y intercept is the EMF of the source

Question 20

What is the effect on terminal PD and lost volts of increasing the current?

Answer 20

As the current through the cell/battery/power source increases the terminal pd decreases

Meaning lost volts increases with current

When current is zero the terminal pd=EMF of cell. Lost volts = 0

Question 21

When is the power output of a variable resistor highest for a circuit with internal resistance?

Answer 21

when R=r

R resistance of whole circuit
r internal resistance of power supply