The magnetic effect of a current
Definition
Magnetic effect of a current
The production of a magnetic field around a conductor whenever an electric current flows through it.
- Whenever an electric current flows through a wire, a magnetic field is produced around the wire.
- This is called the magnetic effect of a current, and it happens in any conductor, including copper wires that are not made of a magnetic material.
- The field exists only while the current flows, so switching the current off makes the field disappear.
- A plotting compass placed near the wire responds to this field, because its needle is a small magnet that lines up with any magnetic field around it.
- Iron filings sprinkled near the wire also line up with the field and show its shape.
Practical
Showing the magnetic effect of a current
- Aim: to show that a current in a straight wire produces a magnetic field, and to find the shape and direction of that field.
- Apparatus: a low-voltage d.c. power supply, a switch, a variable resistor, an ammeter, connecting leads, a long straight piece of thick copper wire, a square of stiff card with a small hole in its centre, a clamp stand, four small plotting compasses and iron filings in a shaker.
- Method:
- Clamp the card horizontally and push the copper wire vertically through the hole, so that the wire stands at right angles to the card.
- Connect the wire in series with the switch, variable resistor, ammeter and power supply, leaving the switch open.
- Place the four plotting compasses on the card in a square around the wire, each about 2 cm2\ \text{cm}2 cm from it, and check that every needle points the same way, along the Earth's field.
- Close the switch, adjust the variable resistor until the ammeter reads about 5 A5\ \text{A}5 A, and watch the needles.
- Record the direction of each needle with an arrow on a sketch of the card seen from above, and note whether the current flows up or down the wire.
- Open the switch and check that all the needles swing back to their original direction.
- Swap the connections at the power supply to reverse the current, close the switch again and record the new needle directions.
- Remove the compasses, sprinkle a thin layer of iron filings on the card, close the switch and tap the card gently to reveal the shape of the field.
- Results: with no current every needle points north, but with the current on each needle swings round towards a direction lying along a circle around the wire, never pointing at the wire, and reversing the current makes every needle swing the opposite way.
- Conclusion: a current in a wire produces a magnetic field made of circles centred on the wire, and the direction of the field depends on the direction of the current.
- Watch out: a small current gives a field too weak to overcome the Earth's field, so the needles barely move, and the iron filings show the circular shape but not the direction.
- Safety: keep the switch closed only for short bursts, because a large current heats the wire enough to burn your fingers.
Field direction around a wire
Definition
Right-hand grip rule
A rule in which the thumb of the right hand points along the conventional current in a wire and the curled fingers give the direction of the magnetic field around it.
Definition
Conventional current
The direction of current shown on circuit diagrams, from the positive terminal of the supply, around the circuit, to the negative terminal.
- The magnetic field lines around a long straight wire are concentric circles centred on the wire, lying in planes at right angles to the wire.
- The field never points straight towards or away from the wire, so a compass needle beside the wire always lies along one of these circles.
- The direction of the circles depends on the direction of the conventional current, which flows from the positive terminal of the supply to the negative terminal.
- The direction of the field is found with the right-hand grip rule.
- Imagine gripping the wire with your right hand, with your thumb pointing in the direction of the conventional current.
- Your fingers then curl around the wire in the direction of the magnetic field lines.
- On diagrams, a wire running into or out of the page is drawn as a small circle, with a cross ⊗\otimes⊗ for current flowing into the page and a dot ⊙\odot⊙ for current flowing out of the page.
- For a current flowing into the page, the field lines circle clockwise.
- For a current flowing out of the page, the field lines circle anticlockwise.
- Reversing the current reverses the direction of the field, so every compass needle around the wire turns to point the opposite way.
- The circular shape of the field stays the same when the current is reversed, and only the arrows on the circles change direction.
Example
Finding the field direction with the grip rule
- A vertical wire carries a conventional current upwards, and the arrangement is viewed from above.
- Seen from above, a current flowing upwards is coming out of the page towards you, so it is drawn as a dot.
- Pointing the right thumb up out of the page makes the fingers curl anticlockwise.
- The field lines around the wire therefore circle anticlockwise when viewed from above.
Example
Predicting a compass needle beside a wire
- A wire carrying a current into the page is drawn as a cross, and a plotting compass sits directly to the right of it.
- A current into the page gives field lines that circle clockwise.
- Going clockwise, the circle passes the point to the right of the wire travelling downwards, towards the bottom of the page.
- The north pole of the compass needle therefore points towards the bottom of the page.
- If the current is reversed, the field circles anticlockwise and the needle swings round to point towards the top of the page.
Exam technique
Describing the field around a wire
- When asked how to show the magnetic effect, name the plotting compass, say what its needle does when the current is switched on, and explain that it moves because the current has produced a magnetic field.
- Describe the field as concentric circles centred on the wire, because a vague phrase about the field going round the wire may not earn the mark.
- State the right-hand grip rule by naming both the thumb and the fingers and saying what each one shows.
- Say what happens when the current is reversed, because the field reversing is a common extra mark.
Common Mistake
- Do not say that the current flows in circles around the wire, because the current flows along the wire and it is the field that forms circles.
- Do not use your left hand for the grip rule, because it gives the opposite direction.
- Do not draw the field lines pointing outwards from the wire like the spokes of a wheel, because a compass beside the wire never points straight at it.
Self review
- What is produced around a wire when a current flows through it?
- Describe the shape of the magnetic field lines around a long straight wire.
- State the right-hand grip rule.
- In which direction do the field lines circle for a current flowing into the page?
- What happens to the field around a wire when the current is reversed?
