National 5PhysicsSQARevision NotesElectricityOhm’s LawVoltage-Current (V-I) Graphs

Voltage-Current (V-I) Graphs (SQA National 5 Physics): Revision Note

Exam code: X857 75

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on

Voltage-current (V-I) graphs

Resistance

  • Resistance is defined as:

The opposition to the flow of charge

  • Resistance is measured in units of ohms (Ω)

    • A resistance of 1 Ω is equivalent to a voltage across a component of 1 V which produces a current of 1 A through it

  • The resistance of a component controls the size of the current in a circuit

  • For a given voltage across a component:

    • The higher the resistance, the lower the current that can flow

    • The lower the resistance, the higher the current that can flow

  • All electrical components, including wires, have some value of resistance

  • Wires are often made from copper because it has a low electrical resistance

    • This is why it is known as a good conductor

The effect of resistance on the current in a circuit

Resistance & Current, downloadable IGCSE & GCSE Physics revision notes

       When a circuit has a high resistance, a lower current will flow, and vice versa 

Ohm's law

  • Current, I, potential difference, V, and resistance, R, all affect one another

    • Changing any one of these in a circuit changes all of them

  • The current in a resistor (at a constant temperature) is directly proportional to the potential difference across it

    • This means that the resistance remains constant as the current changes

  • This relationship is described by the following relationship, known as Ohm's law

R space equals space V over I

  • Where

    • R = resistance, measured in ohms (Ω)

    • V = potential difference, measured in volts (V)

    • I = current, measured in amperes or amps (A)

Voltage-current (V-I) graphs

  • Ohm's law can be verified using a voltage-current (V-I) graph

  • If a component obeys Ohm's law:

    • the graph will be a straight line which passes through the origin

    • the resistance is constant

  • If voltage is on the y-axis and current is on the x-axis, then the gradient is equal to:

gradient space equals space fraction numerator increment y over denominator increment x end fraction space equals space fraction numerator increment V over denominator increment I end fraction

  • For a component which obeys Ohm's law, such as a resistor, the ratio of V over I is constant at all values of current

  • Therefore, the resistance is constant and equal to the gradient:

gradient space equals space V over I space equals space R

Voltage-current graph for a fixed resistor

Graph of voltage versus current for a resistor, showing a linear relationship. The gradient is labelled as ΔV/ΔI = R, indicating resistance.
For a fixed resistor at a constant temperature, voltage is directly proportional to the current and the gradient is equal to the resistance

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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.