Resistance and I-V Characteristics

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The quick revision snapshot

Resistance is the ratio V/I for a component at a particular point. A fixed resistor is approximately ohmic at constant temperature, while a filament lamp and diode have non-linear current-potential difference relationships.

By the end, you should be able to…

Explain how resistance changes using precise physics language.
Apply the relevant equations, diagrams or practical method to an exam-style situation.
Use the mini quizzes and practice answers to identify and correct a misconception.

Core learning content

Learn the idea before you practise it

Start with this short teaching sequence for Resistance and I-V Characteristics. Read the model, use the visual, then test your recall before moving into examples and exam practice.

How resistance changes

Resistance is the ratio V/I for a component at a particular point. A fixed resistor is approximately ohmic at constant temperature, while a filament lamp and diode have non-linear current-potential difference relationships.

Build the picture

For an ohmic conductor at constant temperature, doubling potential difference doubles current, so the I-V graph is a straight line through the origin. A filament lamp heats as current rises, increasing resistance; a diode mainly allows current in one direction.

Use the right relationship

Choose a relationship because of the physical change in the question, not because it is familiar. Charge and current: Q = It (C, A, s). Potential difference: V = E ÷ Q (V, J, C). Resistance: V = IR (V, A, Ω). Keep the unit next to each value while you substitute.

What a strong answer does

Read the graph shape, identify the component, describe the physical cause and use V = IR only when the relationship is appropriate. Before writing, underline the command word and identify the information that matters. Keep a calculation as equation, substitution and answer with unit; keep an explanation as a linked chain using because, so or therefore. Finish by checking that the final sentence answers the exact context in the question and that the number, direction or conclusion agrees with the evidence given.

A clean exam-style circuit diagram with an ammeter in series and a voltmeter across a component — **Read the diagram:** identify the quantities, directions or stages before you write an answer.

Charge and currentQ = ItC, A, s

Potential differenceV = E ÷ QV, J, C

ResistanceV = IRV, A, Ω

Electrical powerP = VIW, V, A

Electrical energyE = PtJ, W, s

Retrieval practice

Quick checks

Answer without looking back first. The feedback explains the model, so a wrong answer still helps you learn.

Try it

Resistance and I-V curve

How does changing potential difference reveal a component characteristic?

Potential difference (V)6 Resistance (Ω)3

What to notice: the readout and written explanation remain available if you do not use the controls.

Worked examples

See the method being built

Cover the answer, attempt the method and then compare your physics language as well as the number.

Example 1 · Fixed resistor

A fixed resistor has 6.0 V across it and 0.50 A through it. Calculate resistance.

Use R = V ÷ I.
Substitute 6.0 ÷ 0.50.
Give ohms.

Show the answer

R = 12 Ω.

Example 2 · Read a lamp graph

A lamp’s current rises less for each extra volt. Explain the curve.

Describe the graph shape.
Link current to filament temperature.
Link temperature to resistance.

Show the answer

As current rises the filament heats up; its resistance increases, so current no longer rises proportionally with potential difference.

Example 3 · Diode choice

Choose between a diode and fixed resistor for a circuit that must conduct in one direction only.

Identify the required behaviour.
Match it to the component characteristic.
State the direction condition.

Show the answer

Use a diode because it has very high resistance in the reverse direction and conducts mainly when forward biased.

Misconception clinic

Common wrong turns

Each of these is tempting because it sounds almost right. Replace it with the precise physics before you meet it in a question.

Using one constant resistance for a filament lamp.
Confusing graph gradient with resistance without checking which variable is on each axis.
Forgetting a diode characteristic includes negative p.d.

Independent practice

Exam-style questions

Use the working space first. Reveal the guidance only after you have committed to an answer, then improve the exact part that would gain the next mark.