National 5PhysicsSQARevision NotesElectricityElectrical Charge CarriersElectric Current

Electric Current (SQA National 5 Physics): Revision Note

Exam code: X857 75

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on

Electric current

  • Electrical current is defined as

Electric charge transferred per unit time

  • Current is measured in units of amperes or amps (A)

    • 1 amp is equivalent to a charge of 1 coulomb flowing in 1 second, or 1 space straight A space equals space 1 space straight C space straight s to the power of negative 1 end exponent

  • This means the size of an electric current is the amount of charge passing through a component each second

  • Current flows

    • when a circuit is formed e.g. when a wire connects the two oppositely charged terminals of a cell

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

current-in-a-circuit

Charge flows from the positive terminal to the negative terminal

Calculating current

  • Current, charge and time are related by the equation:

charge = current × time

Q space equals space I space cross times space t

  • Where:

    • Q = charge, measured in coulombs (C)

    • I = current, measured in amps (A)

    • t = time, measured in seconds (s)

  • The current, charge and time equation can be rearranged with the help of the following relationship triangle:

Relationship triangle for current, charge and time

Current charge time triangle, IGCSE & GCSE Physics revision notes

Relationship triangle for the charge, current and time equation

Worked Example

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

A.     When 8 J of energy is used by 1 C of charge

B.     When a charge of 4 C passes in 0.5 s

C.     When a charge of 8 C passes in 0.1 s

D.     When a charge of 1 C passes in 8 s

E.     When a charge of 0.5 C passes in 4 s

ANSWER:  B

  • The equation relating current, charge and time is:

Q space equals space I space cross times space t

  • Rearrange to make current I the subject of the equation:

I space equals space Q over t

  • Consider option B, where Q = 4 C and t = 0.5 s:

I space equals space fraction numerator 4 over denominator 0.5 end fraction space equals space 8 space straight A

  • Therefore, the correct answer is B

A is incorrect as this is the definition of a voltage of 8 V between two points and does not describe current

C is incorrect as  I space equals space fraction numerator 8 over denominator 0.1 end fraction space equals space 80 space straight A

D is incorrect as  I space equals space 1 over 8 space equals space 0.125 space straight A

E is incorrect as  I space equals space fraction numerator 0.5 over denominator 4 end fraction space equals space 0.125 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). Make sure you can convert these into standard units, e.g. 10 mA = 10 × 10-3 A.

Make sure to only use the triangle to help you rearrange the equation that links charge, current and time. Don't draw it if you are asked to write out the equation in full, such as Q = I × t, as you may lose marks for doing so.

Check out this revision note on speed, distance and time if you need a reminder on how to use relationship triangles.

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    Author
    Reviewer
    Katie M

    Author: Katie M

    Expertise: Physics Content Creator

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

    Caroline Carroll

    Reviewer: Caroline Carroll

    Expertise: Physics & Chemistry 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.