2.2 Resistance

Question 1

What is potential difference?

Answer 1

The work done moving a unit charge between 2 points in a circuit.
V= W/Q

Question 2

What is resistance?

Answer 2

A measure of how difficult it is for current to flow through an appliance.
A component has a resistance of 1Ω if 1A flows through it when a p.d of 1V is applied across it.
R=V/I measured in ohms = VA^(−1)

Question 3

What is meant by an ohmic conductor?

Answer 3

A conductor that obeys Ohm’s law, meaning that current is directly proportional to potential difference providing the temperature remains constant.

Question 4

What does the gradient of a current- potential difference graph represent?

Answer 4

1/R.
Because the gradient = rise in current/change in potential difference i.e. gradient = I/V.
As resistance= V/I, the gradient = 1/R

Question 5

Answer 5

Question 6

Answer 6

Question 7

Answer 7

Question 8

Why does the current increasing on a filament lamp cause an increase in the resistance?

Answer 8

As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude impeding the movement of electrons through the lamp as they collide with the ions (resistance has increased).

Question 9

What is resistivity?

Answer 9

The resistance of a 1m cylinder with a cross sectional area of 1m^(2)
p = RA/L
p = resistivity, Ωm
R = resistance, Ω
A = cross sectional area, m^(2)
L= length, m

Question 10

Describe an experiment to determine the resistivity of a metal.

Answer 10

Question 11

What is power?

Answer 11

The rate of energy transfer.

Question 12

What is a superconductor?

Answer 12

A material that, when cooled below its critical temperature, has a resistivity of zero.

Question 13

State 2 applications of superconductors.

Answer 13

1. Power cables, which would reduce energy loss through heating to zero during transmission.
2. Strong magnetic fields, which would not require a constant power source. These could be used in maglev trains, where there would be no friction between the train and rail. They could also be used in certain medical applications.

Question 14

What is resistivity?

Answer 14

The resistivity is a property of a metal which shows how easy or difficult it is for current to flow in the material (at a specific temperature).
⍴ = RA/L
Where:
⍴ = resistivity,
R = resistance,
A = cross-sectional area
L = length.

Question 15

Describe an experiment to determine the resistivity of a metal.

Answer 15

Question 16

True or false? The resistivity of both metals and semiconductors increases with temperature.

Answer 16

False.
The resistivity of a metal increases with temperature because as the metal ions heat up they vibrate more and the electrons bang into them and slow down.
Some semiconductors get less resistive as temperature increases, because supplying energy actually causes more charge carriers to be released, so current can flow more easily.

Question 17

What is superconductivity?

Answer 17

Superconductivity is when a material conducts with zero resistance. A material will begin to do this when it is cooled to below a certain temperature. Not all materials can do this however.

Question 18

What is the transition temperature?

Answer 18

The temperature below which a superconductor must be cooled in order to have zero resistance is called the transition temperature.

Question 19

When do metals show superconductivity?

Answer 19

Most metals do show superconductivity and will have transition temperatures a few degrees higher than absolute zero.
Initially, it was thought that superconductivity was not possible above 30K. In 1987, a Nobel prize was given out for the discovery of superconductivity of a material at a transition temperature of 35K. Higher transition temperatures were later found to exist. Some materials are described as high temperature superconductors which have transition temperatures above 77K.

Question 20

Describe some uses of superconductivity.

Answer 20

• Superconductors can be used as magnets to make vehicles (such as trains) float.
• MRI scanners use superconducting magnets to expose the human body to a strong magnetic field.
• Superconducting magnets are often used in particle accelerators to alter the paths of subatomic particles.