CH8

Question 1

What is meant by quantisation of charge?

Answer 1

It means that electric current comes in discreet, invisible units. The smallest unit of electric charge that exists in nature is the charge of an electron

Question 2

How much is the charge of an electron?

Answer 2

1.60x (10 )-19 C

Question 3

What is the charge of one electron and one proton?

Answer 3

Electron: - 1 elementary charge
Proton: 1 elementary charge

Question 4

What is the Millikan experiment?

Answer 4

Carried out in 1909. It confirmed that electric charge is quantized and the charge of a single electron was found to be -1.59 x (10)-19

Question 5

Describe the Millikan experiment (or the oil-drop experiment)

Answer 5

He analysed the motion of electrically charged oil droplets between two oppositely charged parallel plates. The oil particles experienced gravitational force, air resistance and upthrust. By adjusting the electrical field between the plates he could make the droplets rise, fall or stay suspended. By comparing these measurements with the known properties of air he was able to determine the charge on each droplet. He found that they were all multiples of a fundamental unit of charge, which he identified as the charge of single electron.

Question 6

What is electric charge?

Answer 6

A physical property

Question 7

How is charge measured?

Answer 7

In coulombs (C). C is defined as the electric charge flowing past a point in one second when there is electric current of 1 Ampere

Question 8

How is electric current defined?

Answer 8

It is defined as the rate of flow of charge

Question 9

How is electric current calculated?

Answer 9

I = Delta Q/ Delta t
Where I is the electric current in Ampere, Delta Q is the charge transferred in C and Delta t is the time in seconds

Question 10

What is the net charge of an object?

Answer 10

Q = +- ne
where Q is the net charge in C, n is the number of electrons either added or removed and e is the elementary charge

Question 11

What is meant by conventional current?

Answer 11

It is from the positive terminal to the negative terminal

Question 12

What are liquids that conduct electricity called?

Answer 12

Electrolytes

Question 13

What is the flow of currents in liquids?

Answer 13

A flow of ions

Question 14

What are usually electrolytes?

Answer 14

Ionic solutions

Question 15

Example of ionic solution

Answer 15

Salt (NaCl) dissolved in water. The salt separates into positively charged sodium ions (cations) Na+ and negatively charged chlorine ions (anions) Cl-

Question 16

What does happen to Na+ ions when they reach the cathode?

Answer 16

They accept an electron

Question 17

What does happen to Cl- ions when they reach the anode?

Answer 17

They donate an electron

Question 18

What is an ammeter?

Answer 18

Used to measure current

Question 19

What is a positive charged ion called?

Answer 19

Cation

Question 20

What is a negative charged ion called?

Answer 20

Anion

Question 21

What is a positive electrode called?

Answer 21

Anode

Question 22

What is a negative electrode called

Answer 22

Cathode

Question 23

What is meant by Number Density?

Answer 23

Number Density is the number of free electrons per cubic meter of material

Question 24

How are materials classified based on number density?

Answer 24

Materials are classified into three groups:
- Insulators (eg wax and rubber)
- Semiconductors (eg silicon and germanium)
- Conductors (eg. Lead, copper and zinc)

Question 25

What does the number density of a material determines?

Answer 25

The number density determines how well a material will conduct an electric current. The greater the number the better a material conduct electricity

Question 26

What order of density number have conductors and semiconductors?

Answer 26

(10)28 m-3
(10)17 m-3

Question 27

Can semiconductors carry the same current as conductors?

Answer 27

Yes they can but in order to do so their electrons need to do much faster (as their density number is lower). This results in an increase of temperature which needs to be taken into account in design (eg computer processors are made of silicone so computers need carefully designed cooling systems)

Question 28

What causes the random movement of electrons?

Answer 28

Repeated collisions between positive ions and free electrons

Question 29

How fast do charge carriers move?

Answer 29

They move slowly

Question 30

If charge carriers move slowly, then why do lumps turn on almost immediately when you press the switch button?

Answer 30

Because all the free electrons start moving almost at once

Question 31

What is the equation of electric current which take into account number density and mean drift velocity?

Answer 31

I = Anev
Where: I is the electric current in the conductor in Amp,
A is the cross sectional area of the conductor in m2
e is the elementary charge (1.60 x 10-19 C)
n is the number density
v is the mean drift velocity of the charge carriers in ms-1

Question 32

What is the relationship between mean drift velocity and the cross sectional area of the wire?

Answer 32

The mean drift velocity is inversely proportional to the cross sectional area of the wire.

Question 33

What is mean drift velocity?

Answer 33

Drift velocity is the average velocity of the charge carriers travelling through the conductor