Magnetic Field
Electricity and Magnetism

Magnetic fields due to currents in wires

for 14-16

This collection of experiments is designed for students to do themselves and so gain experience of the phenomenon of electromagnetism. They will need plenty of time to experiment and develop their understanding of the ideas.

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Magnetic field due to an electric current in a wire

Magnetic Field
Electricity and Magnetism

Magnetic field due to an electric current in a wire

Practical Activity for 14-16

Class practical

Iron filings reveal the pattern of magnetism around a current-carrying wire, and factors that affect it.

Apparatus and Materials

For each student group

Health & Safety and Technical Notes

Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.

The power supplies used in this experiment must be able to allow a current of up to 10 amps to flow when a short piece of wire is connected across its 1 or 2 volt terminals. Many 0-12-volt power supplies will not pass such high currents and so the trip switch will cut out or, worse still, the power supply will be damaged. Students should be encouraged to switch off the power supply when they are not using it because the wires and coils will become hot.

Read our standard health & safety guidance


Procedure

  1. Make a hole in the centre of the board. Pass a length of insulated copper wire through the hole, and connect it to the black and red DC terminals on the low-voltage power supply.
  2. Fix the white board in a horizontal position, perhaps by clipping it under a terminal on the power supply (use one of the yellow a.c. terminals not otherwise used in this experiment).
  3. Switch on the power supply and sprinkle iron filings onto the white board.
  4. Tap the board gently with a pencil and watch the pattern develop.
  5. Pour away the iron filings and explore the field with the small plotting compass. Reverse the supply connections to see what effect this has on the compass needle.

Teaching Notes

  • Fast students could also try the effect of a larger number of turns.
  • Students will see that:
    • a current-carrying wire produces a magnetic field around itself
    • the field reverses when the current is reversed
    • the field is stronger with more turns of the wire
  • A simple rule to use to show the direction of the current in a wire and the direction of its associated field is the right hand grip rule. With the thumb of a clenched right hand pointing in the direction of the conventional current (positive flow), then the fingers point around the current in the direction of the conventional magnetic field.
  • The field direction is defined as the way in which the north-seeking pole of a compass needle points.

This experiment was safety-tested in July 2007

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Oersted's experiment

Electromagnetism
Electricity and Magnetism

Oersted's experiment

Practical Activity for 14-16

Class practical

This is one of the greatest demonstrations in the history of electromagnetism.

Apparatus and Materials

Health & Safety and Technical Notes

The power supplies used in this experiment must be able to allow a current of up to 10 amps to flow when a short piece of wire is connected across its 1 or 2 volt terminals. Many 0-12 volt power supplies will not pass such high currents and so the trip switch will cut out or, worse still, the power supply will be damaged. Students should be encouraged to switch off the power supply when they are not using it because the wires and coils will become hot.

Read our standard health & safety guidance


Procedure

  1. Using wire strippers, bare approximately 3 cm of each end of the wire.
  2. Connect the ends of the wire to the DC terminals (red and black) of the power supply.
  3. Switch the supply on so a large current flows through the wire.
  4. Hold the compass above and then below the wire. Observe how it orientates itself in each position.

Teaching Notes

  • When an electric current is flowing through the wire, compass needles placed above and below the wire will point in opposite directions.
  • How Science Works Extension: Oersted discovered this effect while giving a public demonstration in 1820. For more about this, see

    Oersted, electric current and magnetism.


This experiment was safety-tested in July 2007

Up next

Magnetic field due to a coil carrying a current

Magnetic Field
Electricity and Magnetism

Magnetic field due to a coil carrying a current

Practical Activity for 14-16

Class practical

A plotting compass and iron filings show that the shape of this magnetic field resembles that of a permanent magnet.

Apparatus and Materials

Health & Safety and Technical Notes

Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.

Read our standard health & safety guidance


The power supplies used in this experiment must be able to allow a current of up to 10 amps to flow when a short piece of wire is connected across its 1 or 2 volt terminals. Many 0-12-volt power supplies will not pass such high currents and so the trip switch will cut out or, worse still, the power supply will be damaged. Students should be encouraged to switch off the power supply when they are not using it because the wires and coils will become hot.

Procedure

  1. Wind five or six turns, closely spaced, onto the wooden cylinder. Twist the ends to prevent unwinding.
  2. Slide it off the cylinder and into the slots cut in the board.
  3. Connect the ends of the wire to the DC terminals of the power supply.
  4. Investigate whether there is a field when there is no current.
  5. Sprinkle the board lightly with iron filings, switch on the current, tap the board gently with a pencil, and note the pattern.
  6. Remove the iron filings and repeat step 5 using the plotting compass inside and outside the coil.
  7. Reverse the current and investigate the effect on the plotting compass.

Teaching Notes

  • The best results with the compass needles will be found when the axis of the coil is aligned in a north-south direction.
  • The pattern near to each of the vertical parts of the coil is the same as when a single vertical wire is connected to a power supply.
  • The direction of the field can be found using a plotting compass. When the current is reversed, the direction of the magnetic field through the centre of the coil and around the wires is reversed.
  • The magnetic field is strongest inside the coil. This is because the effects of both sides of the vertical part of the wires add up at the centre.
  • The standard right hand grip rule can be modified to predict the direction of the magnetic field. If the right hand fingers curl round the coil in the direction of the current, then the thumb points in the direction of the magnetic field down the centre of the coil.

This experiment was safety-tested in July 2007

Up next

A model galvanometer

Magnetic Field
Electricity and Magnetism

A model galvanometer

Practical Activity for 14-16

Class practical

The deflection of a compass needle increases with the current carried by a wire.

Apparatus and Materials

  • Copper wire, PVC-covered, 150 cm with bare ends
  • Plotting compass
  • Carbon resistor, 10 ohm
  • Hardboard piece to take plotting compass
  • Carbon resistor, 100 ohm
  • Rheostat (10 - 15 ohms) e.g. from circuit teaching kit
  • Power supply, low voltage


Health & Safety and Technical Notes

Read our standard health & safety guidance


The piece of hardboard should have a central hole to take the plotting compass, and four notches to hold the wire.

The power supplies used in this experiment must be able to allow a current of up to 10 amps to flow when a short piece of wire is connected across its 1 or 2-volt terminals. Many 0-12-volt power supplies will not pass such high currents and so the trip switch will cut out or, worse still, the power supply will be damaged. Students should be encouraged to switch off the power supply when they are not using it because the wires and coils may become hot.

Procedure

  1. Leaving a length of about 30 cm (for making connections to the power supply), wind five turns of wire on one side of the board. Leave a gap, and wind a further five turns on the other side.
  2. Insert the compass into the hole in the board.
  3. Place this device on the bench so that the compass needle is parallel to the winding.
  4. Connect up to the power supply through a 100 Ω carbon resistor. A small current (about 10 mA.) will flow; observe the deflection of the compass needle. Reverse the current and repeat.
  5. Repeat with a 10 Ω carbon resistor, and then with a rheostat (so that you can vary the current).

Teaching Notes

  • Direct connection to the power supply with no resistor should be avoided. The large current sets the needle spinning.
  • For those interested in historic apparatus, this is a model of the tangent galvanometer in which the size of the current passing through the coil is proportional to the tangent of the angle through which the needle moves when there is a current in the coil. The Earth's magnetic field pulls the needle one way, and the coil's field pulls it at right angles.

This experiment was safety-tested in April 2006

Up next

Magnetic field inside an open coil

Magnetic Field
Electricity and Magnetism

Magnetic field inside an open coil

Practical Activity for 14-16

Class practical

Using a plotting compass and iron filings to explore the field pattern.

Apparatus and Materials

For each student group

Health & Safety and Technical Notes

Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.

Read our standard health & safety guidance


Procedure

  1. You are going to explore the magnetic field inside a current-carrying coil of wire. Wind five spaced turns on the wooden cylinder.
  2. Slide this coil off the cylinder and into slots on the board.
  3. Connect the ends up to the DC terminals of the power supply.
  4. Sprinkle iron filings onto the board, paying particular attention to the region inside the coil.
  5. Switch on the current, tap the board, and observe the pattern.
  6. Use a plotting compass to investigate the direction of the field inside and outside the coil. Investigate what happens if the current is reversed.

Teaching Notes

This is a chance to explore the field along the centre of the coil and around some of the coils as they enter the card. Students should see that the field inside the coil is in the opposite direction to the field outside the coil.

This experiment was safety-tested in July 2007

Up next

Magnetic field due to a long close-wound coil

Magnetic Field
Electricity and Magnetism

Magnetic field due to a long close-wound coil

Practical Activity for 14-16

Class practical

Iron filings show that a long, closely wound current-carrying coil behaves just like a bar magnet.

Apparatus and Materials

  • Copper wire, PVC-covered, 100 cm with bare ends
  • Iron filings


  • Cardboard, rectangle
  • Pencil
  • Plotting compass (optional)
  • Power supply, low-voltage ('Westminster pattern' very-low-voltage supplies are best)
  • Power supplies


Health & Safety and Technical Notes

Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.

Read our standard health & safety guidance


Procedure

  1. Make a coil of twenty or thirty turns by winding the wire around a pencil. (Leave enough wire free at either end to make connections to the power supply.) The coils should be wound firmly and closely on the pencil.
  2. Lay the coil on the cardboard and sprinkle iron filings onto the board.
  3. Switch on the current, tap the board, and observe the pattern.
  4. Try using a plotting compass after you have tried the iron filings. Investigate what happens if the connections are reversed.

Teaching Notes

  • The long, closely wound coil behaves just like a bar magnet.
  • The direction of the magnetic field reverses when the current is reversed.

This experiment was safety-tested in April 2006

Up next

Exhibiting magnetic field patterns

Magnet
Electricity and Magnetism

Exhibiting magnetic field patterns

Teaching Guidance for 14-16

Here are five ways of exhibiting magnetic field patterns:

  • Large posters of magnetic fields can be plotted out with a compass to make a classroom display.
  • Paper can be waxed with paraffin wax and placed on top of a magnet. After the iron filing pattern is produced, melt the wax slightly with a candle placed underneath the waxed paper. When the candle is removed and the paper cooled, the iron filings stick to the paper. This method was used by Faraday.
  • An iron filing pattern can be made permanent by spraying gently with hair spray (lacquer).
  • Student or teacher can make a quick demonstration using an overhead projector. Instead of covering the magnet with card, before sprinkling iron filings, place a piece of transparent plastic on top of the magnet. Sprinkle iron filings on it.
  • Photographs of magnetic fields can be produced by replacing the card with photographic paper. In the dark, or with a red safety light on, sprinkle iron filings onto the paper more densely than normal. Switch on the laboratory lights for 20 seconds. Then switch off the lights and shake the iron filings off the photographic paper. Develop the photographic paper in the normal way, taking the usual safety precautions when handling photographic developer.

Safety note: Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.

Resources

Download the following images of magnetic field patterns.

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