7 min readConcept sheet1-mark MCQ, and 2 or 3-mark questions using the right-hand thumb rule, often with a diagram

Magnetic field of a straight current-carrying wire

A current in a straight wire makes concentric circles of magnetic field lines around it. The right-hand thumb rule gives their direction.

Discovered byHans Christian Oersted, 1820: a current made a compass needle move

Shape of the fieldConcentric circles centred on the wire, in a plane at right angles to it

Rule for directionRight-hand thumb rule (Maxwell's corkscrew rule)

StrengthIncreases with current; decreases with distance from the wire

Oersted's discovery

In 1820, Hans Christian Oersted noticed that a compass needle moved when a current flowed in a nearby wire. This showed that electricity and magnetism are linked: a current produces a magnetic field around it. When the current is reversed, the needle turns the other way, so the direction of the field also reverses. The oersted, a unit of magnetic field strength, is named after him.

Activity: the field around a straight wire

1
Pass a thick copper wire vertically through the centre of a horizontal piece of cardboard.
2
Connect the wire in series with a battery, a plug key and a rheostat.
3
Sprinkle iron filings on the cardboard, close the key and tap the card gently. Observation: the filings form concentric circles around the wire.
4
Place a compass on one of the circles and note where its north pole points. Then reverse the current. Observation: the needle turns to face the opposite way.
5
Increase the current with the rheostat, then move the compass further from the wire. Observation: more current gives more deflection; more distance gives less.

Right-hand thumb rule

Hold the wire in your right hand with the thumb pointing in the direction of the current. Your curled fingers then show the direction of the magnetic field lines around the wire. This rule is also called Maxwell's corkscrew rule. For example, if the current flows upwards in a vertical wire, the field lines go anticlockwise when you look down from above.

How to draw the field of a straight wire

Draw a vertical line for the wire with an arrow showing the current, for example upwards. Draw the cardboard as a flat oval that the wire passes through.
On the card, draw 3 or 4 concentric circles centred on the wire, spaced wider as they go outwards.
Put arrows on the circles: anticlockwise seen from above for an upward current.
Draw a small compass on one circle with its needle along the arrow.
Label the wire, the direction of current, the magnetic field lines and the cardboard.

In real life

Every wire that carries a current, from house wiring to overhead power cables, has a magnetic field around it. A compass kept close to a cable carrying a large direct current, such as a car or tractor battery cable, can point the wrong way.

Exam angle

2 marks: “State the right-hand thumb rule.” Hold the conductor in the right hand with the thumb pointing along the current. The curled fingers give the direction of the magnetic field lines.

3 marks: “A current flows from north to south in a horizontal power line. What is the direction of the magnetic field at a point directly below the wire, and at a point directly above it?” Using the right-hand thumb rule with the thumb pointing south: directly below the wire the field points east; directly above it, the field points west.

2 marks: “How does the magnetic field of a straight current-carrying wire change when (a) the current is increased and (b) the point moves further from the wire?” (a) The field becomes stronger. (b) The field becomes weaker.

Remember it

Thumb = current, fingers = field. Grip the wire in your right hand like the handle of a cricket bat.