4.1 Charge and Current

Question 1

How is electric current defined?

Answer 1

Electric current is the rate of flow of charge/charged particles.

Question 2

What equation defines electric current?

Answer 2

Current (A) = Change in Charge (C)/Change in Time (s)

Question 3

What is 1 Ampere the same as?

Answer 3

1 Coulomb Second (1Cs)

Question 4

What is electric charge?

Answer 4

Electric charge is a physical property, either positive or negative and is measured in coulombs or relative charge.

Question 5

Define the coulomb.

Answer 5

The electric charge flowing past a point in one second when there is electric current of 1 Ampere.

Question 6

What is the elementary charge?

Answer 6

1.60 x 10⁻¹⁹C

Question 7

What is the relative charge of a proton?

Answer 7

+1

Question 8

How does an object become charged?

Answer 8

Through the loss or gain of electrons.

Question 9

What is meant by quantised charge?

Answer 9

A charge that can only have certain values, such as being an integer multiple of e.

Question 10

What is Millikan’s oil drop experiment?

Answer 10

It was an experiment used to investigate the motion of electrically charged droplets between parallel plates. It was involved in the discovery of quantisation of charge. Depending on the charges, some drops sometimes became stationary as they were balanced by gravity and the upward attractive electrostatic force.

Question 11

What are the charge carriers in a metal?

Answer 11

Charge carriers in metals are the electrons.

Question 12

What is the structure of a metal?

Answer 12

Metal structures are types of giant lattice structures, with positive metal ions surrounded by a number of delocalised electrons. Cations are not free to move but can vibrate in a fixed position.

Question 13

How can you produce an electric current in a wire?

Answer 13

Make one end of the wire positive, and the other negative, all electrons will be attracted to the positive end, moving as an electric current.

Question 14

What is conventional current?

Answer 14

Conventional current is a current from a positive terminal towards a negative one, regardless of electron flow.

Question 15

What is the name given to liquids that conduct electricity?

Answer 15

Electrolytes.

Question 16

What are charge carriers in liquids?

Answer 16

Ions.

Question 17

What are cations and anions?

Answer 17

Cations - positively charged ions.
Anions - negatively charged ions.

Question 18

What instrument is used to measure current and how does it work?

Answer 18

An ammeter is used to measure current at any point in a circuit.
It should be connected in series and places with lowest resistance to reduce its effect on the current.

Question 19

What is meant by conservation of charge?

Answer 19

A conservation law that states that electric charge can neither be created nor destroyed, the total charge in the universe is constant.

Question 20

What is Kirchhoff’s first law?

Answer 20

In an electrical circuit, at any point, the sum of currents into a point is equal to the sum of currents out of that point, charge is conserved.
ΣI in = ΣI out

Question 21

What is number density?

Answer 21

The number density is the number of free electrons per cubic meter of material - the higher number density = better electrical conductor.

Question 22

What are the units of number density?

Answer 22

Cubic metre.

Question 23

What are approximate values of number density for conductors, semi-conductors, and insulators?

Answer 23

Conductor - 10²⁸
Semi-conductor - 10¹⁶
Insulator - close to 0

Question 24

How do electrons move in a wire as they travel towards the positive terminal?

Answer 24

They move slowly and repeatedly collide with positive metal ions as they drift.

Question 25

Describe the motion of electrons in an unconnected wire.

Answer 25

There is a rapid, random motion, like vibrations, due to kinetic energy.

Question 26

What is the equation relating electric current and number density?

Answer 26

I = Anev

I - current (A)
A - cross sectional area (m2)
n - number density (m3)
e - elementary charge
v - drift velocity (ms-1)

Question 27

What is mean drift velocity?

Answer 27

The average velocity of electrons as they move through a wire.

Question 28

What happens to the mean drift velocity of charge carriers when the cross sectional area reduces?

Answer 28

The narrower the wire, the greater the drift velocity so that current remains the same. To maintain current, electrons must travel faster, and so mean drift velocity is inversely proportional to the cross sectional area of the wire.