National 5PhysicsSQARevision NotesElectricityElectrical PowerElectrical Power Relationships

Electrical Power Relationships (SQA National 5 Physics): Revision Note

Exam code: X857 75

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on

Electrical power relationships

  • The power of a device depends on:

    • the voltage of the device

    • the current of the device

  • The power of an electrical component (or appliance) is given by the relationship:

P space equals space V I

  • Where:

    • P = power, measured in watts (W)

    • V = voltage, measured in volts (V)

    • I = current, measured in amps (A)

  • This equation can be rearranged with the help of a relationship triangle:

Relationship triangle for power, voltage, and current

pvi-triangle-aqa-gcse-rn

Cover up the variable you need to calculate, and the triangle shows you how the equation is arranged

Worked Example

A 48 W electric motor draws a current of 4 A.

Calculate the voltage across the motor.

Answer:

Step 1: List the known quantities

  • Power, P = 48 W

  • Current, I = 4 A

Step 2: Substitute in the values

V space equals fraction numerator space 48 over denominator 4 end fraction

V space equals space 12 space V

Power & Resistance

  • Ohm's law can be substituted into the power equation to give two other forms

  • The relationship between power, current and resistance is:

P space equals space I V space equals space I open parentheses I R close parentheses

P space equals space I squared R

  • The relationship between power, voltage and resistance is:

P space equals space I V space equals space open parentheses V over R close parentheses V

P space equals space V squared over R

  • Where:

    • R = resistance, measured in ohms (Ω)

  • This means that for a given resistance, if the current is doubled, the power will be four times as great

Worked Example

A current of 6 A passes through a 20 Ω fixed resistor. 

Calculate the power of the resistor.

Answer: 

Step 1: List the known quantities

  • Current, I = 6 A

  • Resistance, R = 20 Ω

Step 2: Write out the relevant equation

P space equals space I squared R

Step 3: Substitute in the known values

P space equals space open parentheses 6 close parentheses squared space cross times space 20

P space equals space 720 space straight W

Examiner Tips and Tricks

There are several different equations for calculating power. Always begin any calculation by writing out the known quantities with their symbols. Then you can choose an equation that contains the quantities that you know. 

For example:

  • If you are given energy transferred, E, and time, t, you would use P space equals space E over t

  • If you are given current, I, and voltage, V, you would use P space equals space I V

  • If you are given resistance, R, and current, I, you would use P space equals space I squared R

  • If you are given resistance, R, and voltage, V, you would use P space equals space V squared over R

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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.