Resistance

Question 1

What does resistance measure?

Answer 1

• The resistance of a component in a circuit is a measure of the difficulty of making the current pass through the component.
• Resistance is caused by the repeated collisions between the charge carriers in the material with each other and with the fixed positive ions of the material.

Question 2

The resistance of any component is defined as:

Answer 2

The pd across the component/the current in the component.

Question 3

Cross sectional area Eq

Answer 3

Area = πd²/4

Question 4

Resistivity

Answer 4

• ρ is a constant for the material
• Resistivity is in Ωm

Question 5

Resistance in terms of ρ

Answer 5

• R is proptional to L and inversely proportional to A

Question 6

Ohms law states that:

Answer 6

• The pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change.
• Ohm’s law is equivalent to the statement that the resistance of a metallic conductor under constant physical conditions like temperature is constant.

Question 7

What is a superconductor?

Answer 7

• A superconductor is a wire or a device made of material that has zero resistivity at and below a critical/transitional temperature that depends on the material.
• This property of the material is called superconductivity.
• When a current passes through it, there is no pd across it because its resistance is zero. So the current has no heating effect.

Question 8

Superconducting wires can be used to make:

Answer 8

• Power cables that transmit electricity without any loss of power.
• Really strong electromagnets that have lots of applications, e,g. in MRI scanners particle accelerators, lightweight electric motors, and Maglev trains.
• Electronic circuits that work really fast with minimal energy loss, because there’s no resistance to slow the current down.
• High-power electromagnets that generate very strong magnetic fields in devices such as and .

Question 9

Resistors in series

Answer 9

R = R₁ + R₂ + R₃ …

Question 10

Resistors in parallel

Answer 10

1/R = 1/R₁ + 1/R₂ + 1/R₃ …

Question 11

What causes the heating effect of an electric current?

Answer 11

The heating effect of an electric current in any component is due to the resistance of the component. The charge carriers repeatedly collide with the positive ions of the material. There is a net transfer of energy from the charge carriers to the positive ions as a results these collisions. After a charge carrier loses kinetic energy in such a collision, the force due to the pd across the material accelerates it until it collides with another positive ion.

Question 12

Explaining power supplied

Answer 12

• For a component of resistance R, when current I passes through it, the pd across the component, V = IR.
• Therefore P = IV = I²R = V²/R.
• Hence the energy per second transferred to the component as thermal energy = I²R.
• If the component is at constant temperature, heat transfer to the sourroundings take place at the same rate.
• Therefore the rate of heat transfer = I²R.