Chapter 8 Charge and Current

Question 1

What is the unit for current?

Answer 1

Amperes

Question 2

What is the definition for current and what is the equation?

Answer 2

It is the number of coulombs of charge per second passing a point
I = ΔQ / ΔT

Question 3

What is electric charge?

Answer 3

It is a physical property and comes in two forms, positive and negative

Question 4

What is the measure of electric charge? and what is the quantity of 1 coulomb

Answer 4

Coulombs

1 coulomb is the amount of charge that passes in 1 second when there is a current of 1 amp

Question 5

What is the charge of an electron? and a proton?

Answer 5

Electron: -1.6x10^-19 Coulombs
Proton: 1.6x10^-19 coulombs

Question 6

How do you calculate the charge on ions?

Answer 6

Take the relative charges (e.g. +1 or -2 etc) and multiply by 1.6x10^-19 to get the charge in coulombs

Question 7

How is the charge on an object calculated?

Answer 7

Using the equation: Q = ±ne
Q = Charge
n = number of electrons added or removed
e = the elementary charge

Question 8

What is the elementary charge and why is it elementary?

Answer 8

It is the smallest possible charge that something can have. It is the same as the charge of an electron, 1.6x10^-19
This is possible because charge is quantised

Question 9

Explain charge quantisation

Answer 9

It means that the charge has fixed values which are integers of e where e is the elementary charge.
You cannot have any value of charge.

Question 10

What are charge carriers and what are some examples

Answer 10

Charge carriers are anything that carries a charge. Examples include electrons or ions. When current flows through a liquid, the charge carriers are often ions in the liquid

Question 11

What is the structure of a metal?

Answer 11

It is a regular crystalline structure of positive ions surrounded by a sea of delocalised electrons.

Question 12

How does current flow through a metal?

Answer 12

When a metal is connected to different ends of a battery the free electrons move through the metal towards the positive end of the battery. NOT ALL ELECTRONS MOVE ONLY THE FREE ONES

Question 13

What causes a large current?

Answer 13

More electrons moving through the wire at the same time

Electrons moving faster through the wire

Question 14

What is the direction of conventional current vs electron flow

Answer 14

Conventional current flows positive to negative and electron flow is from negative to positive

Question 15

What are electrolytes?

Answer 15

They are solutions that conduct electricity. They have ions that act as charge carriers that move through the solution and transfer charge

Question 16

When conduction through an electrolyte what is the:

a) anode
b) cathode

Answer 16

a) anode is the positively charged end

b) cathode is the negatively charged end

Question 17

What happens to the positive ions in an electrolyte solution

Answer 17

The positive ions move towards the cathode and gain an electron. The negative ions move towards the anode and lose an electron.

Question 18

How is electric current measured?

Answer 18

Using an ammeter, it is placed in series in the circuit. It should have the lowest possible resistance.

Question 19

What is the law of the conservation of charge

Answer 19

Electric charge cannot be created or destroyed and so the total electric charge in the universe is constant

Question 20

What is Kirchhoff’s first law? what is the equation?

Answer 20

The sum of the currents into a point (junction) is equal to the sum of the currents out of that point
ΣI in = ΣI out

Question 21

What is the number density?

Answer 21

It is the number of free charge carriers per unit volume

Question 22

What is the number density of:

a) conductors?
b) semiconductors?
c) insulators?

Answer 22

a) Larger than 10^28 m^-3
b) around 10^17 m^-3
c) Small

Question 23

What is the problem with using semiconductors to transfer electricity?

Answer 23

The electrons have to move faster so the semiconductor heats up more

Question 24

How fast do charge carriers move? and why do lights turn on instantly

Answer 24

They move slowely but they all move at once so the light turns on instantly

Question 25

What is the equation for mean drift velocity?

Answer 25

I = Anev
I = Current
A = Cross sectional area
e = The elementary charge
v = mean drift velocity

Question 26

What is the mean drift velocity?

Answer 26

This is the mean speed of charge carriers

Question 27

What is the affect of changing the cross sectional area on mean drift velocity?

Answer 27

The smaller the cross sectional area the faster the electrons move in order for the current to be the same as a wire with a larger cross sectional area because there is less space for the electrons to pass.