K326 / K327 / 2027
Magnetism and electromagnetism overview

Topic 5 of 7

Magnetic fields from currents

An electric current produces a magnetic field around the conductor. The field's pattern depends on the conductor's shape, while the current affects its strength and direction.

Conventional current follows the direction in which positive charge would flow. In a metal it is opposite to electron motion. A settled compass N end gives the local magnetic field direction.

A straight wire: circular field lines

Look along a straight wire, so that the wire runs perpendicular to the page. Its magnetic field lines form circles centred on the wire in this end view. The field direction at any point is tangent to a circle, rather than pointing radially towards or away from the wire.

Dot: towards you
A dot represents an arrow tip coming out of the page towards the reader.
Cross: away from you
A cross represents an arrow tail going into the page, away from the reader.

In the wire diagrams these marks label current direction. They are not positive and negative charge signs. Always read the nearby label: a dot or cross can also be used for another quantity perpendicular to the page.

Right-hand grip rulePoint your right thumb along the conventional current. Your curled fingers show the magnetic field direction around the wire.

Look along the wire

The central dot means current towards you; the central cross means current away from you. Teal arrows show the field. Each compass's labelled N end follows the local tangent.

Current towards you: anticlockwise field

Current towards you: anticlockwise fieldAn end view looks along a straight wire. The central dot means conventional current comes out of the page towards the viewer. Teal arrows on three concentric circles run anticlockwise. Four small compasses have their north ends tangent to the outer circle: at the right the N end points up, at the bottom it points right, at the left it points down, and at the top it points left. None points radially at the wire. The two panels keep the same viewpoint.NNNN

Current away from you: clockwise field

Current away from you: clockwise fieldFrom the same end-view position, a central cross means conventional current goes into the page away from the viewer. Teal arrows on the concentric circles now run clockwise. The compass north ends reverse: at the right the N end points down, at the bottom it points left, at the left it points up, and at the top it points right. The drawn line count and current magnitude are not numerical scales.NNNN

Keep the viewpoint fixed when comparing directions. The circles represent field lines, not paths travelled by the compasses or by charges around the wire.

In this fixed end view, current out of the page gives an anticlockwise field. Reversing it into the page gives a clockwise field. A compass N end follows the local tangent.

If you look from the opposite end of the wire, what appears clockwise changes. State the viewpoint before applying a direction rule.

Predict a compass direction

A compass is just to the right of the wire

For current out of the page, the circular field is anticlockwise, so the local field on the wire's right points up the page.

Reverse the current while keeping the compass and viewpoint fixed. The field becomes clockwise and the compass N end points down the page, assuming the wire's field dominates other fields there.

Increasing current magnitude makes the field stronger at a fixed position for the same wire geometry. Reversing current reverses the field direction. These are separate changes: a stronger current in the original direction does not reverse the field.

Within a consistently drawn pattern, closer field-line spacing can show greater strength qualitatively. The drawn lines do not move faster when current increases, and an arbitrary number of lines is not a numerical field measurement.

A solenoid: the field of a coil

A solenoid is a coil with many turns. The fields due to its turns combine to give a pattern like that of a bar magnet. In the central region of a sufficiently long solenoid, the field is approximately uniform: its lines are nearly parallel and similarly spaced.

Outside, lines leave the solenoid's N end and curve round to its S end. Inside, they return from S towards N, forming continuous loops.

Grip the coil with your right hand, curling your fingers in the conventional-current direction around its turns. Your thumb points towards its north end, along the field inside. Looking directly at one end, anticlockwise current makes that end N; clockwise current makes it S.

Match the winding current to the poles

Brown arrows show conventional current; teal arrows show magnetic field. This projected coil has N on the left and S on the right.

Side view: the near-side current rises

A powered solenoid with north at its left endA continuous seven-turn winding connects to a cell by two distinct leads. The positive terminal is on the right, so conventional current travels up the right lead, through the winding from right to left, then down the left lead to the negative terminal. On the near or front half of each turn the current rises; the paler back halves are behind the coil axis. The left end is north and the right end south. Two closed teal field loops point left inside the coil and right along their outside return portions. Their central straight sections represent an approximately parallel field. Field lines and the winding are different quantities shown in projection.-+d.c. supplyNS

The field returns from S to N inside the coil. Outside, it runs from N to S. The paler wire sections are on the far side of the coil.

Look from the left end

Anticlockwise current viewed at the north endLooking into the same coil from its left end, a circular brown outline represents the end-view winding, with anticlockwise conventional-current arrows. At the right side of this end view, corresponding to the near side in the side view, current points up. The centre is labelled north. A separate teal dot in a small circle explicitly means magnetic field coming towards the viewer through the coil's north end. In this panel the dot labels field, not a wire current. The end-view projection does not draw the distinct supply connections again.NField towards youCurrent anticlockwise

Here the teal dot represents field out of the page. The brown circular arrows represent current around the turns.

This is a qualitative pattern. The turn spacing and the number of drawn field lines do not give a numerical field strength.

The supplied winding gives N on the left and S on the right. Inside, the field points left. Viewed from the left end, current is anticlockwise and the field points towards the reader; the dot in that end view is labelled as field.

Reversing the current swaps the N and S ends and reverses the field throughout the pattern. Increasing the current magnitude strengthens the field when the coil geometry and core remain the same.

A suitable soft-iron core becomes magnetised in the coil's field and strengthens the magnetic effect. It loses much of this induced magnetism when the current is switched off. Its contribution depends on the magnetising field and material.

Investigate the field with a compass

  1. Use suitable low-voltage equipment. Pass a straight wire through a horizontal card and keep the wire vertical. Looking down at the card gives an end view of the wire.
  2. With current off, record the compass's background direction. With current on, allow a small compass to settle at several marked positions and draw arrows along its N end.
  3. Keep the wire, positions and viewpoint fixed. Reverse only the current and compare the new directions. Record the current direction as seen from your chosen view.
  4. Map a solenoid in the same way, recording its current direction and the compass directions near both ends and along the outside. Compare with the bar-magnet-like pattern.

Keep nearby magnets and magnetic objects away, and use a freely moving compass. The needle responds to the resultant field, including Earth's, so the measured pattern need not match the ideal wire or coil pattern where its field is weak.

To investigate current magnitude, keep the geometry, compass position and background field fixed and record the actual current. A reproducible change in compass deflection can show that the wire's contribution has changed. It does not by itself give a calibrated numerical field strength, and a compass already aligned almost fully with the wire's field may turn very little further.

Stay within the equipment's current ratings and switch off between readings where appropriate to limit heating. Heating or moving the apparatus would change more than the intended variable.

Optional check Viewed end-on, a straight wire initially carries current out of the page. The current is then reversed and increased in magnitude, with the wire unchanged. What happens to its magnetic field?
Viewed end-on, a straight wire initially carries current out of the page. The current is then reversed and increased in magnitude, with the wire unchanged. What happens to its magnetic field?