Electromotive force + Internal Resistance

Question 1

Explain what is meant by a potential divider.

Answer 1

A combination of resistors in series connected across a voltage source (to produce a
required pd)

Question 2

State and explain what happens to the voltmeter reading when the resistance of R
is increased while the temperature is kept constant.

Answer 2

When R increases, pd across R increases ✓
Pd across R + pd across T = supply pd ✓
So pd across T / voltmeter reading decreases

Question 3

State and explain what happens to the ammeter reading when the temperature of
the thermistor increases.

Answer 3

At higher temp, resistance of T is lower ✓
1
So circuit resistance is lower, so current / ammeter reading increases

Question 4

Define the volt.

Answer 4

1 joule per coulomb

Question 5

State what is meant by electromotive force

Answer 5

emf is the work done / energy transferred by a voltage source / battery / cell ✓per
unit charge

Question 6

Describe how you would make a direct measurement of the emf ɛ of a cell,

Answer 6

Voltmeter across terminals with nothing else connected to battery / no additional
load.

Question 7

Explain why this meter must have a very high resistance

Answer 7

This will give zero / virtually no current

Question 8

The overhead cables used to transmit electrical power by the National Grid usually consist
of a central core of steel cables surrounded by a sheath of cables of low resistivity
material, such as aluminium.
What is the main purpose of the steel core?

Answer 8

To increase the mechanical strength of the cables.

Question 9

State the significance of the critical temperature of a material.

Answer 9

below the critical temperature / maximum temperature which resistivity /
resistance ✓
is zero / becomes superconductor

Question 10

Explain why the efficiency of electrical power transmission is improved when
conventional wires are replaced with superconducting wires.

Answer 10

(with no resistance there can be) no power loss

Question 11

State what is meant by a superconducting material.

Answer 11

no resistance
M1
(at or) below critical temperature

Question 12

State an application of a superconductor and explain why it is useful in this
application.

Answer 12

MRI scanner, strong magnetic field, no energy dissipation

Question 13

Define the term electromotive force (emf).

Answer 13

joules per coulomb (of charge)/work done per unit charge
(treat reference to force as neutral)
M1
where charge moved (whole way) round circuit

Question 14

State what happens to the resistance of the filament lamp as the current
increases.

Answer 14

resistance (of filament lamp) increases

Question 15

State and explain whether a filament lamp is an ohmic or non−ohmic
conductor up to its working power

Answer 15

filament lamp is a non-ohmic conductor as current is not (directly)
proportional to voltage / resistance is not constant

Question 16

A student decides to use two potato cells in series as a power supply for a light
emitting diode (LED). In order for the LED to work as required, it needs a voltage of
at least 1.6 V and a current of 20 mA.
Explain whether the LED will work as required.

Answer 16

total emf is above 1.6 V✓

but will not work as current not high enough / less than 20 mA

Question 17

Define resistance.

Answer 17

ratio of voltage (across component) to current (through

component) or R = V/I with terms defined and R as subject

Question 18

A semiconducting diode is an example of a non-ohmic component. State what
is meant by a non-ohmic component.

Answer 18

a non-ohmic conductor does not have a constant resistance