Resistance (Cambridge O Level Physics)

Revision Note

Test yourself
Leander

Author

Leander

Last updated

Did this video help you?

Ohm's Law

Resistance is the opposition to current

    • For a given potential difference, the higher the resistance, the lower the current
    • Therefore resistors are used in circuits to control the current 
    • The unit of resistance is the ohm, represented by the Greek symbol omega Ω 

Ohm's Law

  • The definition of resistance can be given using the equation

R space equals space V over I

  • Where
    • R = resistance (ohms, Ω)
    • V = potential difference (volts, V)
    • I = current (amperes, A)
  • Ohm's Law can be stated in words:

Current is directly proportional to potential difference as long as the temperature remains constant

Equation Triangle for Ohm's Law

VIR triangle (3), IGCSE & GCSE Physics revision notes

Use the formula triangle to help you rearrange the equation until you feel confident to do it unaided

Consequences of Ohm's Law

  • Resistors are used in circuits to control either
    • The current in branches of the circuit (through certain components)
    • The potential difference across certain components
  • This is due to the consequences of Ohm's Law
    • The current in an electrical conductor decreases as its resistance increases (for a constant p.d.)
    • The p.d. across an electrical conductor increases as its resistance increases (for a constant current)

Determining Resistance

Determining Resistance

  • To find the resistance of a component, we can set up a circuit like the one shown below

Circuit Set-up for Determining Resistance

 Resistance circuit, downloadable AS & A Level Physics revision notes

A circuit to determine the resistance of a component includes a power supply, an ammeter connected in series, and a voltmeter connected in parallel to the component being measured

  • The power supply should be set to a low voltage to avoid heating the component, typically 1-2 V
  • Measurements of the potential difference and current should then be taken from the voltmeter and ammeter respectively
  • Finally, these readings should be substituted into the resistance equation

Worked example

A charge of 5.0 C passes through a resistor of resistance R Ω at a constant rate in 3.0 s.

The potential difference across the resistor is 2.0 V. Calculate the value of R.

Answer:

Step 1: List the known quantities

  • Charge, Q = 5.0 C
  • Time, t = 3.0 s
  • Potential difference, V = 2.0 V

Step 2: Rearrange the current & charge equation to make current the subject

Q space equals space I t

I space equals fraction numerator space Q over denominator t end fraction

Step 3: Substitute the known values to calculate

I space equals fraction numerator space 5.0 over denominator 3.0 end fraction space

I space equals space 1.67 space straight A

Step 4: Substitute the known values into the resistance equation to calculate 

R space equals fraction numerator space V over denominator I end fraction

R space equals fraction numerator space 2.0 over denominator 1.67 end fraction

R space equals space 1.2 space straight capital omega

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Leander

Author: Leander

Expertise: Physics

Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.