OCR AS Physics

Revision Notes

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:

Current 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

Flow of charge, downloadable AS & A Level Physics revision notes

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

Ammeter in series, downloadable AS & A Level Physics revision notes

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


The Coulomb, downloadable AS & A Level Physics revision notes

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.

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