The magnetic effect of a current

Definition

Magnetic effect of a current

The magnetic effect of a current is the production of a magnetic field in the space around a conductor whenever there is a current in that conductor.

A wire with no current in it has no magnetic field around it and has no effect at all on a nearby compass.
Switch the current on and a magnetic field appears in the space around the wire. Switch it off and the field vanishes at once.
The field is produced by the moving charges in the wire. Any moving charge is surrounded by a magnetic field, and a current is simply a great many charges moving together.
The conductor itself is not made of a magnetic material. Copper is not attracted by a magnet, yet a copper wire carrying a current still produces a field.
The current is not used up in making the field. The same current arrives back at the supply, and the field lasts exactly as long as the charge keeps flowing.
This single fact is the root of the whole of electromagnetism. Every electromagnet, relay, loudspeaker and electric motor works because a current makes a magnetic field.

Practical

Showing the field around a straight conductor

Aim: to show that a current in a long straight conductor produces a magnetic field, to find the shape and direction of that field, and to test how its strength depends on the current and on the distance from the wire.
Apparatus: a thick insulated copper wire, a stiff sheet of white card with a hole punched at its centre, a retort stand and clamps to hold the card level, a low-voltage direct current power supply or battery pack, a rheostat, an ammeter, a switch, four or more plotting compasses, iron filings in a shaker, a ruler and connecting leads.
Variables: the current and the distance from the wire are the independent variables, the deflection of the compass needle is the dependent variable, and the wire, its vertical alignment, the compasses used and the surroundings are all kept the same.
Method, setting up:
- Thread the wire vertically through the hole in the card and clamp the card horizontally, so the wire passes through it at a right angle.
- Connect the wire in series with the switch, the ammeter, the rheostat and the supply, keeping the leads well away from the card so their own fields do not confuse the result.
- With the switch open, place four plotting compasses on the card at equal distances round the wire and note that every needle points the same way, along the Earth's field.
- Set the rheostat to its highest resistance before closing the switch, so the first current through the wire is small.
Method, finding shape and direction:
- Close the switch briefly and watch the compasses. Each needle swings round until it lies at a tangent to a circle drawn round the wire.
- Record the direction of every needle by sketching it on the card, then read off whether the pattern runs clockwise or anticlockwise when viewed from above.
- Open the switch, swap the two leads at the supply so the current now flows the other way, close the switch again and record the new directions.
- Clear the compasses away, sprinkle iron filings thinly over the card, close the switch and tap the card gently. The filings settle into circles centred on the wire.
Method, testing strength:
- Put one compass a fixed $cm2\ \text{cm}$ from the wire and record the angle its needle turns through for currents of $A1\ \text{A}$ , $A2\ \text{A}$ , $A3\ \text{A}$ and $A4\ \text{A}$ , set with the rheostat and read on the ammeter.
- Then hold the current steady and move the same compass to $cm1\ \text{cm}$ , $cm2\ \text{cm}$ , $cm4\ \text{cm}$ and $cm8\ \text{cm}$ from the wire, recording the deflection each time.
- Switch off between readings so that the wire does not heat up, and take three readings at each setting to find a mean.
Expected pattern: the needles form circles round the wire, the whole pattern turns the other way when the current is reversed, larger currents give larger deflections, and the deflection falls off as the compass is moved away from the wire.
Comparing the two detectors: iron filings give a sharper picture of the circular shape, but only the compasses show which way round the field points.
Watch out: a sloping or sagging wire, leaving the current on so long that the wire gets hot and the reading drifts, iron or steel under the bench, connecting leads lying close to the card, and forgetting that the Earth's field is still acting on every compass.
Safety: keep the current low and switch on only in short bursts, since a thick wire carrying several amperes becomes hot enough to burn. Never connect the wire straight across the supply without the rheostat, and keep iron filings away from eyes.

Shape and direction of the field

Definition

Conventional current

Conventional current is the direction of flow of charge taken to be from the positive terminal of the supply to the negative terminal around the external circuit.

Definition

Right-hand grip rule

The right-hand grip rule states that if the right thumb points along the conventional current in a straight conductor, the curled fingers give the direction of the circular magnetic field lines around it.

The magnetic field around a long straight conductor is a set of concentric circles lying in planes at right angles to the wire, with the wire itself at the centre of every circle.
Concentric means the circles share the same centre. They do not overlap and they do not cross.
The field goes around the wire, not along it. The current runs along the wire while the field wraps round it, and the two directions are at right angles.
This field has no north pole and no south pole. The lines are closed loops that never begin or end anywhere, which is why a straight wire cannot be labelled N and S.
The way round the circles run is decided by the direction of the conventional current, and it is found with the right-hand grip rule.
To use the rule, grip the wire in your right hand as though holding a pen:
1. point the thumb along the wire in the direction of the conventional current
2. the curled fingers then sweep round the wire the same way as the magnetic field lines
The rule uses the right hand and it uses conventional current, which flows from the positive terminal of the supply round to the negative terminal. Electrons in the metal drift the opposite way.
Reversing the current reverses the field. Every circle keeps its shape and its size, but every arrow on it turns the other way round.
In a diagram of a vertical wire seen from above, a current flowing upwards out of the page gives anticlockwise circles, and a current flowing downwards into the page gives clockwise circles.

Example

Finding which way the field circles

A vertical wire passes through a horizontal card and the conventional current in it flows upwards. The card is viewed from above.
Grip the wire with the right hand and point the thumb straight up, matching the direction of the conventional current.
Looking down on your own hand from above, the curled fingers sweep round anticlockwise.
The field lines on the card are therefore circles centred on the wire with their arrows pointing anticlockwise.
A compass placed to the north of the wire on the card would settle pointing due west, because west is the tangent direction there.
If the supply leads are swapped so the current flows downwards, the same reasoning gives clockwise circles and that compass swings right round to point due east.

How strong the field is

Two things decide the strength of the field around a long straight conductor: the size of the current and the distance from the conductor.
A larger current gives a stronger field, because more charge is flowing past each point every second.
A greater distance from the wire gives a weaker field, so the field is strongest right at the surface of the conductor and fades away as you move outwards.
On a field-line diagram this is drawn by spacing the circles unevenly: they are packed tightly close to the wire and drawn further and further apart towards the outside.
There is no equation to learn or use for this field. What is assessed is the qualitative link: more current means a stronger field, more distance means a weaker one.
The field around a single straight wire is weak. A wire carrying a few amperes barely disturbs a compass held a hand's width away, which is why bigger effects need the current concentrated into a coil.

Exam technique

Describing the field around a wire

A describe question here is normally worth three marks, one each for the circular shape, the link between field direction and current direction, and the effect of current or distance on strength.
Use the word concentric and say the circles are centred on the wire, because circles alone leaves the position of the centre unstated.
In a method question, the observation that scores is the change when the switch is closed, so describe what the compasses do before as well as after.
Name plotting compasses whenever the question asks about direction, and iron filings only when it asks about shape.
The mark most often dropped is the reversal one. Add a sentence saying that swapping the supply connections reverses the field direction, since that shows the link rather than just stating it.

Common Mistake

Do not draw the field lines running along the wire. The current runs along it and the field circles around it.
Do not label a north pole and a south pole on a straight current-carrying wire. Its field lines are closed loops with no poles.
Do not use your left hand for the grip rule, and do not use the direction electrons drift. The rule needs the right hand and conventional current.
Do not write that the current is used up producing the field, or that only a wire made of iron would work.

Self review

Describe an experiment that shows a current in a straight wire produces a magnetic field.
State the shape of the magnetic field around a long straight current-carrying conductor.
Explain how the right-hand grip rule is used to find the direction of that field.
State what happens to the field when the direction of the current is reversed.
Give the two factors that determine how strong the field is, and say how each one affects it.
Explain why plotting compasses are better than iron filings for showing the direction of this field.

The magnetic effect of a current

Definition

Magnetic effect of a current

The magnetic effect of a current is the production of a magnetic field in the space around a conductor whenever there is a current in that conductor.

A wire with no current in it has no magnetic field around it and has no effect at all on a nearby compass.
Switch the current on and a magnetic field appears in the space around the wire. Switch it off and the field vanishes at once.
The field is produced by the moving charges in the wire. Any moving charge is surrounded by a magnetic field, and a current is simply a great many charges moving together.
The conductor itself is not made of a magnetic material. Copper is not attracted by a magnet, yet a copper wire carrying a current still produces a field.
The current is not used up in making the field. The same current arrives back at the supply, and the field lasts exactly as long as the charge keeps flowing.
This single fact is the root of the whole of electromagnetism. Every electromagnet, relay, loudspeaker and electric motor works because a current makes a magnetic field.

Practical

Showing the field around a straight conductor

Aim: to show that a current in a long straight conductor produces a magnetic field, to find the shape and direction of that field, and to test how its strength depends on the current and on the distance from the wire.
Apparatus: a thick insulated copper wire, a stiff sheet of white card with a hole punched at its centre, a retort stand and clamps to hold the card level, a low-voltage direct current power supply or battery pack, a rheostat, an ammeter, a switch, four or more plotting compasses, iron filings in a shaker, a ruler and connecting leads.
Variables: the current and the distance from the wire are the independent variables, the deflection of the compass needle is the dependent variable, and the wire, its vertical alignment, the compasses used and the surroundings are all kept the same.
Method, setting up:
- Thread the wire vertically through the hole in the card and clamp the card horizontally, so the wire passes through it at a right angle.
- Connect the wire in series with the switch, the ammeter, the rheostat and the supply, keeping the leads well away from the card so their own fields do not confuse the result.
- With the switch open, place four plotting compasses on the card at equal distances round the wire and note that every needle points the same way, along the Earth's field.
- Set the rheostat to its highest resistance before closing the switch, so the first current through the wire is small.
Method, finding shape and direction:
- Close the switch briefly and watch the compasses. Each needle swings round until it lies at a tangent to a circle drawn round the wire.
- Record the direction of every needle by sketching it on the card, then read off whether the pattern runs clockwise or anticlockwise when viewed from above.
- Open the switch, swap the two leads at the supply so the current now flows the other way, close the switch again and record the new directions.
- Clear the compasses away, sprinkle iron filings thinly over the card, close the switch and tap the card gently. The filings settle into circles centred on the wire.
Method, testing strength:
- Put one compass a fixed $cm2\ \text{cm}$ from the wire and record the angle its needle turns through for currents of $A1\ \text{A}$ , $A2\ \text{A}$ , $A3\ \text{A}$ and $A4\ \text{A}$ , set with the rheostat and read on the ammeter.
- Then hold the current steady and move the same compass to $cm1\ \text{cm}$ , $cm2\ \text{cm}$ , $cm4\ \text{cm}$ and $cm8\ \text{cm}$ from the wire, recording the deflection each time.
- Switch off between readings so that the wire does not heat up, and take three readings at each setting to find a mean.
Expected pattern: the needles form circles round the wire, the whole pattern turns the other way when the current is reversed, larger currents give larger deflections, and the deflection falls off as the compass is moved away from the wire.
Comparing the two detectors: iron filings give a sharper picture of the circular shape, but only the compasses show which way round the field points.
Watch out: a sloping or sagging wire, leaving the current on so long that the wire gets hot and the reading drifts, iron or steel under the bench, connecting leads lying close to the card, and forgetting that the Earth's field is still acting on every compass.
Safety: keep the current low and switch on only in short bursts, since a thick wire carrying several amperes becomes hot enough to burn. Never connect the wire straight across the supply without the rheostat, and keep iron filings away from eyes.

Shape and direction of the field

Definition

Conventional current

Conventional current is the direction of flow of charge taken to be from the positive terminal of the supply to the negative terminal around the external circuit.

Definition

Right-hand grip rule

The magnetic field around a long straight conductor is a set of concentric circles lying in planes at right angles to the wire, with the wire itself at the centre of every circle.
Concentric means the circles share the same centre. They do not overlap and they do not cross.
The field goes around the wire, not along it. The current runs along the wire while the field wraps round it, and the two directions are at right angles.
This field has no north pole and no south pole. The lines are closed loops that never begin or end anywhere, which is why a straight wire cannot be labelled N and S.
The way round the circles run is decided by the direction of the conventional current, and it is found with the right-hand grip rule.
To use the rule, grip the wire in your right hand as though holding a pen:
1. point the thumb along the wire in the direction of the conventional current
2. the curled fingers then sweep round the wire the same way as the magnetic field lines
The rule uses the right hand and it uses conventional current, which flows from the positive terminal of the supply round to the negative terminal. Electrons in the metal drift the opposite way.
Reversing the current reverses the field. Every circle keeps its shape and its size, but every arrow on it turns the other way round.
In a diagram of a vertical wire seen from above, a current flowing upwards out of the page gives anticlockwise circles, and a current flowing downwards into the page gives clockwise circles.

Example

Finding which way the field circles

A vertical wire passes through a horizontal card and the conventional current in it flows upwards. The card is viewed from above.
Grip the wire with the right hand and point the thumb straight up, matching the direction of the conventional current.
Looking down on your own hand from above, the curled fingers sweep round anticlockwise.
The field lines on the card are therefore circles centred on the wire with their arrows pointing anticlockwise.
A compass placed to the north of the wire on the card would settle pointing due west, because west is the tangent direction there.
If the supply leads are swapped so the current flows downwards, the same reasoning gives clockwise circles and that compass swings right round to point due east.

How strong the field is

Two things decide the strength of the field around a long straight conductor: the size of the current and the distance from the conductor.
A larger current gives a stronger field, because more charge is flowing past each point every second.
A greater distance from the wire gives a weaker field, so the field is strongest right at the surface of the conductor and fades away as you move outwards.
On a field-line diagram this is drawn by spacing the circles unevenly: they are packed tightly close to the wire and drawn further and further apart towards the outside.
There is no equation to learn or use for this field. What is assessed is the qualitative link: more current means a stronger field, more distance means a weaker one.
The field around a single straight wire is weak. A wire carrying a few amperes barely disturbs a compass held a hand's width away, which is why bigger effects need the current concentrated into a coil.

Exam technique

Describing the field around a wire

A describe question here is normally worth three marks, one each for the circular shape, the link between field direction and current direction, and the effect of current or distance on strength.
Use the word concentric and say the circles are centred on the wire, because circles alone leaves the position of the centre unstated.
In a method question, the observation that scores is the change when the switch is closed, so describe what the compasses do before as well as after.
Name plotting compasses whenever the question asks about direction, and iron filings only when it asks about shape.
The mark most often dropped is the reversal one. Add a sentence saying that swapping the supply connections reverses the field direction, since that shows the link rather than just stating it.

Common Mistake

Do not draw the field lines running along the wire. The current runs along it and the field circles around it.
Do not label a north pole and a south pole on a straight current-carrying wire. Its field lines are closed loops with no poles.
Do not use your left hand for the grip rule, and do not use the direction electrons drift. The rule needs the right hand and conventional current.
Do not write that the current is used up producing the field, or that only a wire made of iron would work.

Self review

Describe an experiment that shows a current in a straight wire produces a magnetic field.
State the shape of the magnetic field around a long straight current-carrying conductor.
Explain how the right-hand grip rule is used to find the direction of that field.
State what happens to the field when the direction of the current is reversed.
Give the two factors that determine how strong the field is, and say how each one affects it.
Explain why plotting compasses are better than iron filings for showing the direction of this field.

12.2.1 Magnetic field around a current-carrying conductor

The magnetic effect of a current

Showing the field around a straight conductor

Shape and direction of the field

Finding which way the field circles

How strong the field is

Describing the field around a wire

Review 12.2.1 Magnetic field around a current-carrying conductor by teaching Genie

12.2.1 Magnetic field around a current-carrying conductor

The magnetic effect of a current

Showing the field around a straight conductor

Shape and direction of the field

Finding which way the field circles

How strong the field is

Describing the field around a wire

Review 12.2.1 Magnetic field around a current-carrying conductor by teaching Genie

12.2.1 Magnetic field around a current-carrying conductor

The magnetic effect of a current

Showing the field around a straight conductor

Shape and direction of the field

Finding which way the field circles

How strong the field is

Describing the field around a wire

Review 12.2.1 Magnetic field around a current-carrying conductor by teaching Genie

12.2.1 Magnetic field around a current-carrying conductor Revision Guide

12.2.1 Magnetic field around a current-carrying conductor

The magnetic effect of a current

Showing the field around a straight conductor

Shape and direction of the field

Finding which way the field circles

How strong the field is

Describing the field around a wire

Review 12.2.1 Magnetic field around a current-carrying conductor by teaching Genie

12.2.1 Magnetic field around a current-carrying conductor Revision Guide