12.3 Resistance

Question 1

What is the resistance of a component in a circuit a measure of

Answer 1

The difficulty of making current pass through the component.

Question 2

What is resistance caused by

Answer 2

The repeated collisions between the charge carriers in the material with each other and with the fixed positive ions of the material

Question 3

What is the resistance of any component defined as

Answer 3

The pd across the component/the current through it

Question 4

For a component which passes current I when the pd across it is V, its resistance R is given by the equation:

Answer 4

R = V/I

Question 5

What is a resistor

Answer 5

A component designed to have a certain resistance, which is the same regardless of the current through it.

Question 6

How can the resistance of a resistor be measured

Answer 6

Ammeter to measure current through, must be in series with resistor
Voltmeter used to measure pd across resistor, must be in parallel, no current should pass through voltmeter otherwise ammeter will not record exact current. Therefore voltmeter must have very high resistance
Variable resistor to adjust current and pd as necessary

Question 7

What does the graph of an ohmic resistor look like

Answer 7

A straight line through the origin

Resistance is same regardless of current

Question 8

Ohm’s law states that…

Answer 8

…the pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change

Question 9

What statement is Ohm’s law equivalent to

Answer 9

The statement that the resistance of a metallic conductor under constant physical conditions (eg. temperature) is constant

Question 10

For a conductor of length L and uniform cross-sectional area A, its resistance R is…

Answer 10

…proportional to L, and inversely proportional to A

Question 11

Equation for resistivity, ρ

Answer 11

ρ = RA/L

Unit of resistivity is the ohm metre Ωm

Question 12

How to work out the area of a conductor with a circular cross section of diameter d,

Answer 12

A = πd^2/4

Question 13

How to determine the resistivity of a wire

Answer 13

Measure diameter of wire using micrometer at several different points along wire to give mean value for d to calculate its cross-sectional area A
Measure resistance R of different lengths L of wire to plot a graph of R against L

Question 14

Finding resistivity of wire from graph

Answer 14

Graph gradient x area

Question 15

What is a superconductor

Answer 15

A wire or device made of material that has zero resistivity at and below a critical temperature that depends on the material
This property of the materia is called superconductivity

Question 16

The wire or device has zero resistance below the ______ temperature of the material. When a current passes through it, there is no ______ across it because its resistance is ______. So the current has no ______ effect.

Answer 16

Critical
pd
Zero
Heating

Question 17

How does a superconductor material lose its superconductivity

Answer 17

If its temperature is raised above its critical temperature.

Question 18

What are superconductors used for

Answer 18

Making high-power electromagnets that generate very strong magnetic fields in devices such as MRI scanners and particle accelerators