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Resistance

Resistance is the opposition offered by a conductor to the flow of electric current.

In one line: Resistance means resists current.

Practice This Concept

Understand it

A conductor with higher resistance allows current to flow less easily. Resistance depends on the length, thickness, material, and temperature of the conductor. It is measured in ohm.

Example

A long, thin nichrome wire has more resistance than a short, thick copper wire of the same size.

Formula and rule

R = V/I.

For a wire of the same material, resistance increases with length and decreases with cross-sectional area.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

If a conductor has 12 V across it and carries 3 A, then R = V/I = 12/3 = 4 ohm.

Method you can reuse

  1. 1Use R = V/I when voltage and current are given.
  2. 2Compare length, area, material, or temperature when a wire is changed.
  3. 3Decide whether resistance increases or decreases.
  4. 4Write the final answer clearly.

Sketch it in your answer

Show two wires of the same material, one long and thin and one short and thick, with current arrows to compare resistance.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does a long, thin wire usually offer more resistance than a short, thick wire?

Think first, then show answer
A long, thin wire offers more resistance because current has a longer path to travel and less cross-sectional area is available for charge flow.

Common wrong answer

A common wrong answer is to say a longer wire has less resistance because it has more material. That is incorrect; a longer path increases opposition to current.

Don't mix this up

Students often think resistance is the amount of current, not the opposition to current.

Exam tip

Remember: longer wire means more resistance, and thicker wire means less resistance.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as comparison questions, numerical questions using V and I, or reasoning questions about wire size and material.
  1. 1

    Enough for 1 mark

    Resistance is the opposition offered by a conductor to the flow of electric current.

  2. 2

    Add for 2 marks

    Example: A long, thin nichrome wire has more resistance than a short, thick copper wire of the same size.

  3. 3

    Add for 3 marks

    Resistance depends on the length, thickness, material, and temperature of the conductor.

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