# 4.1.1 Electric Current & Charge

### Electric Current

#### Electric Current

• Electric current is defined as the rate of flow of positive charge carriers
• It is measured in units of amperes (A) or amps
• The symbol for current is I
• The charge, current and time are related by the equation:

• Where:
• I = current (A)
• ΔQ = change in charge (Q)
• Δt = time interval (s)
• When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors, causing a current

Charge can flow between two conductors

• There are several examples of electric currents, including in household wiring and electrical appliances
• Current is measured using an ammeter
• Ammeters should always be connected in series with the part of the circuit you wish to measure the current through

An ammeter can be used to measure the current around a circuit and always connected in series

#### Worked Example

When will 8 mA of current pass through an electrical circuit?

A.     When 1 J of energy is used by 1 C of charge
B.     When a charge of 4 C passes in 500 s
C.     When a charge of 8 C passes in 100 s
D.     When a charge of 1 C passes in 8 s

Step 1: Write out the equation relating current, charge and time, rearranging for charge Q

Q = It

Step 2: Rule out any obviously incorrect options

• Option A does not contain charge or time, so can be ruled out

Step 3: Try the rest of the options to determine the correct answer

• Consider option B:

I = 4 / 500 = 8 × 10–3 = 8 mA

• Consider option C:

I = 8 / 100 = 80 × 10–3 = 80 mA

• Consider option D:

I = 1 / 8 = 125 × 10–3 = 125 mA

• Therefore, the correct answer is B

#### Exam Tip

Although electric charge can be positive or negative, since the conventional direction of current is the flow of positive charge the current should always be a positive value for your exam answers.

### Electric Charge

• Charge is a property certain particles have. It can either be:
• A positive charge (+) (eg. proton)
• A negative charge (–) (eg. electron)
• A neutral (no) charge (eg. neutron)
• An atom is neutral. This is because it has an equal number of protons (positive charge) and electrons (negative) charge
• However, just the nucleus which is made up of protons and neutrons is positively charged
• In physics, the charge is represented by the symbol Q or q

#### The Coulomb

• The unit of charge is the Coulomb (C)
• This is defined as the quantity of charge that passes a fixed point per second when a current of 1 A is flowing
• The coulomb (C), in SI base units, is equal to the quantity of electricity conveyed in one second by a current of one ampere i.e. 1 C = 1 A s

Definition of the Coulomb

#### Quantisation of Charge

• The charge on charge carriers is quantised
• This means the charge comes in definite, finite quantities
• In this way, the quantity of charge can be quantised depending on how many protons or electrons are present
• Positive and negative charge has a definite minimum magnitude and comes in multiples of that magnitude
• This magnitude is the elementary charge, e = 1.60 × 10-19 C
• The magnitude of the charge just refers to its value, rather than whether it is positive or negative
• The net charge on a particle can be quantised, meaning it is always a multiple of the charge of an electron by convention
• The charge of an electron, e is -1.60 × 10-19 C
• The charge of a proton, +e is +1.60 × 10-19 C

#### Worked Example

Determine the charge of an ion with charge 3e. State an appropriate unit for your answer.

Step 1: Calculate the charge

1e = –1.60 × 10–19

3e = 3 × (–1.60 × 10–19) = –4.8 × 10–19

Step 2: Include the unit for charge

The units of charge is coulombs (C)

Therefore 3e = –4.8 × 10–19 C

#### Exam Tip

Although the charge of the electron is given on your data sheet, you will be expected to remember that the charge of the proton has the same magnitude

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close