Chapter 12 - Electric current

Question 1

What is current?

Answer 1

The rate of flow of charge. The amount of charge carriers flowing through a point per unit time.

Current is due to the movement of charge carriers (flow of charge).

Question 2

In a metallic conductor, the charge carriers are _______ _______. When a voltage is applied across the metal, these ________ ________ are attracted towards the _______ terminal of the metal.

Answer 2

In a metallic conductor, the charge carriers are conduction electrons (delocalised electrons in metal). When a voltage is applied across the metal, these conduction electrons are attracted towards the positive terminal of the metal.

Question 3

In an insulator, each electron is ________ to an atom and ______ move away from the atom. When a voltage is applied across an insulator, _________ passes through it because _______ can _____ through the insulator.

Answer 3

In an insulator, each electron is attracted to an atom and cannot move away from the atom. When a voltage is applied across an insulator, no current passes through it because no electrons can move through the insulator.

Question 4

What is potential difference?

Answer 4

The work done (or energy transferred) per unit charge.

Electrons (charge carriers) receive energy at the battery and carry it around the circuit.

When an electron passes through a component, it does work passing through it and so deposits some or all of its energy.

Question 5

What is the equation for electrical energy produced when charge Q passes through a source?

Answer 5

E = Qε

Question 7

What is the equation for work done by charge carriers?

Answer 7

W = IVt
or
W = Pt

Question 7

What is resistance caused by?

Answer 7

The repeated collisions between charge carriers passing through the component and the positive ions in the component vibrating about fixed positions in the component.

Question 8

Unless specifically stated in questions, ammeters and

voltmeters should be treated as…

Answer 8

…ideal (having zero and
infinite resistance respectively).

Voltmeters (infinite resistance)
Ammeters (zero resistance)

Question 8

What is Ohm’s Law?

Answer 8

The pd across a metallic conductor is directly proportional to the current under constant physical conditions.
I ∝ V

Question 8

How does the resistance of a metal change with increase of temperature? Explain why.

Answer 8

The resistance of a metal increases with increase of temperature.

This is because the positive ions in the conductor vibrate more when its temperature is increased so the charge carriers cannot pass through the metal as easily.

Question 9

What are charge carriers?

Answer 9

Charged particles that move through a substance when a pd is applied across it (e.g. electrons).

Question 10

Work done by charge carrier to pass through component =

Answer 10

Work done by charge carrier to pass through component = Energy loss of charge carrier

Question 11

Ammeters are connected in ______ to a component.

Answer 11

Ammeters are connected in series to a component.

Question 12

Voltmeters are connected in ______ to a component.

Answer 12

Voltmeters are connected in parallel to a component.

Question 13

What is resistance?

Answer 13

The electrical resistance of a material is how well it resists the flow of current.
R = V ÷ I

Question 14

What is an ohmic conductor?

What is the resistance of an ohmic conductor at constant temperature?

Answer 14

A conductor that follows Ohm’s Law.

The resistance of an ohmic conductor at constant temperature is fixed.

Question 15

What is 1kWh in Joules?

Answer 15

1kWh = 3.6x10^6 J

Question 16

Why do semiconductors have an increase of charge carriers with increasing temperature?

Answer 16

As with conductors, when a semiconductor is heated, particles within the lattice gain energy so the ions vibrate faster.

In semiconductors, heating the lattice will provide electrons bound to atoms with the required energy to be freed.
T↑, I↑, R↓

Question 17

What is a superconductor?

Answer 17

A material which has zero resisitivity at or below a certain critical temperature.

There is no pd across a superconductor at or below its critical temperature as no energy is lost.

Question 18

What is an LDR?

Answer 18

A resistor made of semiconducting materials that are sensitive to light.
Light intensity↑, I↑, R↓

Question 19

What is resisitivity?

Answer 19

The constant of proportion in the relationship,
R ∝ L ÷ A

where L = length of wire
where A = cross-sectional area (πr2)

The degree to which the type of material resists the flow of charge carriers.

It is affected by temperature. To have a constant resistivity, the current should remain constant to prevent a temperature increase.

Question 20

Give two uses of superconductors.

Answer 20

The production of strong magnetic fields and the reduction of energy loss in transmission of electric power.

Question 21

A superconductor loses its superconductivity if…

Answer 21

…its temperature is raised at or above its critical temperature.

Question 22

What is superconductivity?

Answer 22

A property of certain materials which have zero resistivity at and below a critical temperature which depends on the material.

Question 23

What is the equation for resistivity?

What is the unit?

Answer 23

ρ = RA/L
Unit: Ωm

Question 25

What is a semiconductor?

Answer 25

A substance in which the number of charge carriers increases when the temperature increases. Therefore, its resistance decreases as temperature increases (opposite of a metallic conductor).

Question 25

Does a thermistor have a negative temperature coefficient?

Explain why.

Answer 25

A thermistor made from a semiconductor has a negative temperature coefficient as the number of charge carriers increases when the temperature is increased.
T↑, I↑, R↓

Question 26

What is meant by the term negative temperature coefficient?

Answer 26

Resistance decreases with increase of temperature.

Question 27

What is meant by the term positive temperature coefficient?

Give an example of something that has a positive temperature coefficient.

Answer 27

Resistance increases with increase of temperature.

A metallic conductor.

Question 28

Give two applications of thermistors.

Answer 28

Temperature sensors and resistance–temperature graphs.

Question 29

What is the forward bias of a diode?

Answer 29

The direction in which the current is allowed to flow.

Most diodes have a threshold voltage of about 0.6V in the forward direction before they will conduct.

In reverse bias, the resistance of the diode is very high and the current that flows is very tiny.