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Current (Oxford AQA IGCSE Physics)

Revision Note

Ann Howell

Written by: Ann Howell

Reviewed by: Caroline Carroll

Oxford AQA PhysicsOxford AQA Combined Science Double Award: Physics

Current

  • Electric current is defined as the rate of flow of electric charge

    • In other words, the size of an electric current is the amount of charge passing through a component per second

  • Current is measured in amperes, which has the symbol, A

    • A current of 2 A means that 2 coulombs of charge passes through a point in a circuit every second

  • Current flows from the positive terminal to the negative terminal of a cell

    • Note that this is the opposite direction to electron flow

Flow of charge

Flow of charge, GCSE & IGCSE Physics revision notes
Charge flows from the positive terminal to the negative terminal

  • The following equation links charge flow, current and time:

Q space equals space I space cross times space t

  • Where the symbols:

    • Q stands for charge flow (measured in coulombs, C)

    • I stands for current (measured in amperes, A)

    • t stands for time (measured in seconds, s)

  • You can rearrange this equation with the help of the formula triangle shown below

    • Cover up the quantity you need to find

    • The position of the other quantities tells you the operation you need to perform to find them

Charge flow formula triangle

Charge is positioned at the top of the triangle, then current below on the left and then time on the right of current
Use the formula triangle to help you rearrange the equation

Worked Example

Calculate the current through a lamp when a charge of 4 C passes through it in 500 s.

Answer:

Step 1: Write down the known quantities

  • Charge, Q = 4 C

  • Time, t = 500 s

Step 2: Write down the relevant equation

Q space equals space I space cross times space t

Step 3: Rearrange for current, I

I space equals space Q over t

Step 4: Substitute the known quantities into the equation

I space equals space 4 over 500 space equals space 8 space cross times space 10 to the power of negative 3 end exponent space straight A

Examiner Tips and Tricks

Electric currents in everyday circuits tend to be quite small, so it's common for examiners to throw in a unit prefix like 'm' next to quantities of current, e.g. 10 mA (10 milliamperes). Be on the lookout for these prefixes and know you can convert them into standard units, so 10 mA = 10 × 10-3 A.

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    Author
    Reviewer
    Ann Howell

    Author: Ann Howell

    Expertise: Physics Content Creator

    Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students, no matter their schooling or background.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.