2.2 Resistance

Question 1

The potential difference between two points is

Answer 1

the work done, that is the loss of electrical potential energy per unit charge passing through the two points

Question 2

A voltmeter

Answer 2

measures PD, it must be connected in parallel

Question 3

Power means

Answer 3

rate of doing work or rate of transfer of energy

Question 4

Work =

Answer 4

V x I x change in t

Question 5

Power =

Answer 5

VI

Question 6

Ohm’s Law

Answer 6

The current through a conductor is proportional to the PD across it. This law applies to metals at constant temperature.

Question 7

Resistance =

Answer 7

PD across conductor/ current through conductor

Question 8

Ohm=

Answer 8

VA^-1

Question 9

Conductance, G =

Answer 9

current through conductor/ PD across conductor

Question 10

Unit of conductance

Answer 10

Siemens, S

Question 11

S=

Answer 11

AV^-1

Question 12

Two examples of non-ohmic conductors

Answer 12

• Filament lamp

- LED

Question 13

G=

Answer 13

1/R

Question 14

P=

Answer 14

IV = I^2R = V^2/R

Question 15

R=

Answer 15

(resistivity x length)/ CS area

Question 16

Unit of resistivity=

Answer 16

Ohm metre

Question 17

A wire’s resistance is proportional to

Answer 17

its length

Question 18

A wire’s resistance is inversely proportional to

Answer 18

its cross sectional area

Question 19

The conductivity is the

Answer 19

reciprocal of a material’s resistivity

Question 20

A superconductor is

Answer 20

a material that below a certain temperature loses all its electrical resistance

Question 21

Uses of superconductors

Answer 21

• Power transmission cables
• MRI scanners
• Electromagnets

Question 22

There is a limit to the current that a superconducting wire can carry

Answer 22

because it gives rise to a magnetic field, and too great a magnetic field makes a superconductor ‘go normal’ even at very low temperatures

Question 23

PRACTICAL: Investigation of the I-V relationship for a conductor

Answer 23

Starting from 0 increase the PD in steps then present the data on a graph (I vertical, V horizontal)

Question 24

PRACTICAL: Determining the resistivity of the metal of a wire

Answer 24

Change the length of the wire using crocodile clips
Measure the resistance with an ohm meter
Measure the diameter with a caliper or a micrometer
Plot a graph (R vertical L horizontal) and calculate using resistivity = [(pi x diameter^2) / 4] x gradient

Question 25

PRACTICAL: How the resistance of a metal wire depends on temperature

Answer 25

Put a waterproofed coil of wire in some water connected to a multimeter on ohms
Start with ice water, then room temperature, then boiling water etc.
Plot a graph