What is meant by describing a component as ohmic?

An ohmic component is one for which the current is directly proportional to the potential difference at constant temperature, so its resistance stays the same however the potential difference is varied.

Describe the shape of the current–voltage graph for an ohmic conductor.

It is a straight line passing through the origin, with the same gradient for positive and negative values of the potential difference.

Describe the shape of the current–voltage graph for a non-ohmic component.

It is a curve rather than a straight line, so the ratio of potential difference to current is different at different points on the graph.

Give two examples of components that behave ohmically.

A metal wire held at constant temperature and a carbon composition resistor both behave ohmically over their normal working range.

CCEA A-Level Physics 1210

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Resistance and Resistivity

Ohmic and non-ohmic behaviour

3 things to know
Unit AS 1

Overview

Describing a component as ohmic, describing a component as non-ohmic and shape of the current–voltage graph for an ohmic conductor.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why must the line on a current–voltage graph pass through the origin before a component can be called ohmic, and not merely be straight?

Key term

What is meant by describing a component as non-ohmic?

Key term

What test would you apply to a set of current and voltage readings to decide whether a component is ohmic?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by describing a component as ohmic?
Model answer
An ohmic component is one for which the current is directly proportional to the potential difference at constant temperature, so its resistance stays the same however the potential difference is varied.
2
What is meant by describing a component as non-ohmic?
Model answer
A non-ohmic component is one whose resistance changes as the potential difference across it changes, so the current is not proportional to the potential difference.
3
Describe the shape of the current–voltage graph for an ohmic conductor.
Model answer
It is a straight line passing through the origin, with the same gradient for positive and negative values of the potential difference.
4
Describe the shape of the current–voltage graph for a non-ohmic component.
Model answer
It is a curve rather than a straight line, so the ratio of potential difference to current is different at different points on the graph.
5
What test would you apply to a set of current and voltage readings to decide whether a component is ohmic?
Model answer
Calculate V/I for every pair of readings across the whole range; if the value is constant within experimental uncertainty the component is ohmic, and if it drifts systematically it is not.
6
Give two examples of components that behave ohmically.
Model answer
A metal wire held at constant temperature and a carbon composition resistor both behave ohmically over their normal working range.

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