C8 Charge and Current

Question 1

8.1 What is an electric current?

Answer 1

It is the rate of flow of charge I= /\Q / /\t

Question 2

What are coulombs?

Answer 2

It is the unit of charge

Question 3

What is the elementary charge

Answer 3

It is 1.6 x 10-19 C (coulombs)

Question 4

What is the net charge on a particle?

Answer 4

An object is quantised and a multiple of e

Question 5

What is an electric charge?

Answer 5

A physical property that all bodies which possesses two charges:

Positive charge (+)
Negative charge (-)

Question 6

What are the two possibilities in the way charges act?

Answer 6

• Two of the same charges will repel
• Two opposite charges will attract

Question 7

What is a coulomb (C) and what is it defined as?

Answer 7

The coulomb (C) is the base unit of electric charge.

One coulomb (1C) of charge is defined as the amount of charge that passes a point per second when there is a current of one amp (1A).

Question 8

What is 1e

Answer 8

1e = 1.6x10^-19

Question 9

What does I Q and t mean? In he equation I = Q/t

Answer 9

Q = charge measured in coulombs (C)
I = current measured in amps (A)
t = time measured in seconds (s)

Question 10

What is the net charge and how can it be expressed?

Answer 10

The charge on most objects results from either a gain or a loss of electrons by the object.

The net charge can be expressed as a multiple of e: Q=+-ne

Question 11

8.2 What is electric current and where can you find it?

Answer 11

Electric current is the rate of flow of electric charge.

Electric currents are everywhere not just confined to wires but the Earth’s core, within the layers of the atmosphere and even inside the human cells in your body.

Question 12

Explain how electric current will be produced if one end of a wire is positive and the other end is negative?

Answer 12

(Always mention structure as well and what happens)

• The structure has lattice of positive ions surrounded by a number of free electrons. Positive ions are not free to move, but they vibrate around fixed points as the temperature of the metal increases.
• One way to make electrons move is to make one end of a wire positive and the other negative. The electrons in the metal of the wire will be attracted towards the positive end, and so move through the wire as an electric current.

Question 13

What is the correct way the electrons flow?

Answer 13

The electrons flow from the negative terminal to the positive terminal.

Question 14

What is the conventional current?

Answer 14

Electrons do not flow this way.
The conventional current shows means the electrons flow from the positive terminal

Question 15

What is the difference in the current in a metal vs the current in an electrolyte?

Answer 15

Current in metal is made by a power supply BUT current in an electrolyte is caused by a chemical reaction from the neutralisation of the solution.

Question 16

How does current in metal flow?

Answer 16

It flows due to the sea of delocalised electrons which flow from negative terminal to the positive terminal.

Question 17

How does current flow from electrolytes?

Answer 17

The current flows from the electrodes.
- The electrolytes are liquid that can conduct an electric current.
- The ions carry the charge so they are charge carriers.

Question 18

8.3 What is the conservation of charge?

Answer 18

In any closed system, the total charge in the system is conserved.

Conservation of charge states that electric charge can neither be created nor destroyed. The total amount of charge in the universe is constant.

Question 19

What is Kirchoff’s first law

Answer 19

At any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction.

Question 20

8.4 What is the number density?

Answer 20

Number of electrons per unit volume

Question 21

If he number density increases what happens?

Answer 21

If the number density increases, the greater the number of free electrons per m^3 and so better the electrical conductor.

Question 22

What are the three groups of the number density?

Answer 22

Conductor, semi conductor and insulator

Question 23

What is a conductor?

Answer 23

Large number density of conduction electrons

Question 24

What is an insulator?

Answer 24

Few or no conduction electrons. Number density is number zero.

Question 25

What is a semiconductor?

Answer 25

They have number densities approximately 1 million times smaller than metals such as copper. The conduction electrons travel faster than in conductors and can be changed by adding an impurity to a material such as silicon.

Question 26

What is drift velocity?

Answer 26

The average speed the particle travels at along the conductor. This is called drift velocity.

Question 27

Why do charted particles not travel in a straight line?

Answer 27

They do not travel in a straight line through a conductor, they collide with other particles in the material.

Question 28

What equation for electric current, A (cross-sectional area of a wire), (n) number of charge carriers per m^-3, charge of an electron (e) and mean drift velocity (v)

Answer 28

I = Anev

Question 29

If you narrow a wire, what will happen to the drift velocity?

Answer 29

The narrower the wire, the greater the drift velocity must be in order for the current to be the same.

Question 30

How to derive I=Anev?

Answer 30

m^2 x m^-3 x c x ms^-1 = cs^-1