4.1 - Charge and Current

Question 1

What is current?

Answer 1

The rate of flow of charge

Symbol I
Measured in amperes (A)

Question 2

What is the equation for current?

Answer 2

I = ∆Q/∆t

Where:
• I = current in amps
• ∆Q = charge transferred in coulombs
• ∆t = change of time in seconds

Question 3

What is electric charge?

Answer 3

• symbol Q
• a physical property
• either positive or negative
• measured in coulombs, C or as a relative charge

Question 4

What is a coulomb?

Answer 4

Unit of charge
Symbol C

A coulomb passes a point in 1 second when there is an electric current of 1 ampere

1C = 1As

Question 5

What is the elementary charge?

Answer 5

The electric charge equivalent to the charge on a proton, 1.6x10^-19C

Symbol e

An electron has the charge -e

Question 6

Why is the charge on an object quantised?

Answer 6

• the charge of most objects is a result of either a gain or loss of electrons
• therefore the size of the charge on an object must be an integer multiple of e

Q = ±ne

Where:
• Q = charge
• n = number of electrons
• e = elementary charge

Question 7

Why can metals carry current?

Answer 7

• metals have delocalised electrons which are free to move
• electrons are negative, meaning they carry a charge
• if you make one end of a metal positive and the other negative, the electrons in the metal will be attracted to the positive end and will move through the wire as an electric current.

Question 8

What might a larger current in a metal wire be due to?

Answer 8

• a greater number of electrons moving past a given point each second (eg a wire with a larger cross-sectional area)
• the same number of electrons moving faster through the metal

Question 9

What is conventional current?

Answer 9

A model used to describe electric current in a circuit

Conventional current flows from positive to negative

It is the direction in which positive charges would travel

Question 10

What is an electrolyte?

Answer 10

A liquid containing ions (charged particles) that are free to move, and can therefore conduct electricity

They are either molten ionic compounds or ionic solutions

Question 11

How can electrolytes conduct electricity?

Answer 11

If is positive electrode (anode) and a negative electrodes (cathode) are placed in a solution, ions are attracted to the electrodes

• The positive ions (cations) will go to the cathode and the negative ions (anions) will go to the anode
• The movement of ions is a flow of charge, and therefore an electric current
• When cations reach the cathode, they accept an electron. When anions reach the anode, they donate an electron. This means electrons can flow through the metal part of the circuit

Question 12

What is the law of conservation of charge?

Answer 12

States that electric charge can neither be created nor destroyed - the total charge in any interaction must be the same before and after the interaction

Question 13

What is Kirchhoff’s first law?

Answer 13

At any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point

Question 14

What law is Kirchhoff’s first law based on?

Answer 14

Law of conservation of charge

• charge is the product of current and time
• charge cannot be created or destroyed, so the number of charge carriers entering a point in a given time must equal the charge carriers leaving a junction at a given time
• as a result, the sum of currents entering a junction in a given time must equal the sum of currents leaving a junction during that time

Question 15

What is the number density?

Answer 15

The number of free electrons per cubic metre of a material
• Symbol n
• Unit m^-3

The higher the number density, the greater the number of free electrons per m^3, so the better the electrical conductor

Question 16

How can materials be classified in terms of numbers density?

Answer 16

CONDUCTORS:
• have a very high number density, around 10^28 m^-3
• conduct electricity very well

SEMICONDUCTORS
• have an intermediate number density, around 10^-17 m^-3
• electrons must move faster in the semiconductor in order to carry the same current

INSULATORS
• have a very low number density
• does not conduct electricity very well

Question 17

What is mean drift velocity?

Answer 17

The average velocity of electrons as they move through a wire

• symbol v
• unit ms^-1

Question 18

What is the equation for current using mean drift velocity?

Answer 18

I = Anev

Where:
• I = current, A
• A = cross sectional area of conductor, m^2
• n = number density of materials, m^-3
• e = elementary charge (1.6x10^-19C)
• v = mean drift velocity, ms^-1

Question 19

What is an ammeter?

Answer 19

A device used to measure current in a circuit

Always placed in series in the circuit at the point you want to measure current

Should have the lowest possible resistance in order to reduce the effect they have on current (higher resistance would decrease the current)

The ideal ammeter has 0 resistance