Magnetic Force
Electricity and Magnetism

Forces on moving charges

for 14-16

Forces on current-carrying wires are at the heart of many practical devices. They are also used to define the unit of current.

Up next

Force on a wire carrying a current in a magnetic field

Magnetic Force
Electricity and Magnetism

Force on a wire carrying a current in a magnetic field

Practical Activity for 14-16

Class practical

This effect is the basis of all electric motors.

Apparatus and Materials

For each student group

  • Iron yoke
  • Magnadur (ceramic) slab magnets, 2
  • Copper wire, stiff, bare, SWG 32 and SWG 26
  • Clamp, or wooden support blocks
  • Crocodile clips, 2
  • Leads, 4 mm, 2
  • Power supply, low-voltage


Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Make a long rectangular loop of thin copper wire.
  2. Clamp it in a wooden support block with wing nut, or between two pieces of wood in the jaws of a clamp. The closed end of the loop will project out horizontally, sagging a little.
  3. Connect the ends of the copper wire to the low-voltage DC supply, using cleaned crocodile clips and 4 mm leads.
  4. Place the slab magnets on the yoke, ensuring that opposite poles are facing each other. Bring it near the free end of the loop when a current is flowing.
  5. Find the position in which the magnets have the greatest effect on the current-carrying wire.
  6. Now, using two 5-cm lengths of the thicker copper wire, make a pair of parallel horizontal rails. Clamp them as shown, and connect up to the power supply, or clamp them directly to the DC terminals of a Westminster pattern power supply.
  7. Place a third piece of copper wire across the rails.
  8. Bring up the magnets; how should they be held to produce a force on the third wire?
  9. Investigate what happens if you reverse the current, or if you reverse the magnets.

Teaching Notes

  • In this experiment, students may use the knowledge that a current-carrying wire has an associated magnetic field. When the wire is placed in a magnetic field it is likely that these two fields will interact.
  • In practice, students will see the motion and know that there must be forces at play, but the three-dimensional geometry will remain obscure.
  • Students will find that there is a force on the wire at right angles to both the current and the magnetic field. (If the current-carrying wire is not at right angles to the field, then only a component of the current will create a force.) If the wire lies along the magnetic field, there will be no force. If the wire is perpendicular to the magnetic field then the force will be maximum. A reversal of the current or of the field will reverse the direction of the force.
  • You could introduce the left hand rule here in order to summarize what students have discovered.

This experiment was safety-tested in July 2007

Up next

Catapult magnetic field

Magnetic Force
Electricity and Magnetism

Catapult magnetic field

Practical Activity for 14-16

Class practical

A spectacular demonstration showing how magnetic fields interact to produce forces on wires.

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 card must be sturdy. Strips of thin plywood would be better, with a central hole already drilled.

The current through the wire should be 100 amps obtained by winding enough turns (Total current = current through wire x number of turns in wire).

Procedure

Diagram 1

  1. Make a hole, approx 5 mm diameter, in the centre of the card.
  2. Place the support blocks about 30 cm apart and use them to support the long card.
  3. Wind a hoop coil of several turns, of diameter about 10 cm. (Form the coil by passing the wire again and again through the hole in the card.)
  4. Support the coil with a lump of modelling clay.
  5. Place two slab magnets upright on the card near the ends, about 25 cm apart.
  6. Sprinkle iron filings on the card and look for the magnetic field pattern. See diagram 1.
  7. Sweep the filings away, remove the slab magnets, and connect the coil to the power supply.
  8. Sprinkle iron filings on the card and look for the magnetic field pattern. See diagram 1.
  9. Sweep the filings away. Connect the coil to the power supply and replace the slab magnets.
  10. Sprinkle iron filings on the card and look for the magnetic field pattern due to the coil and the slab magnets. This is called a 'catapult' field. See diagram 1.
  11. Diagram 2 (below) shows the catapult analogy.

Teaching Notes

  • You will need to explain the catapult field pattern to students. The Magnadur magnets produce a uniform, parallel magnetic field. The current-carrying vertical wire produces a circular magnetic field around itself. When the two fields are combined, the pattern produced by the iron filings indicates a complex field pattern showing how the wire, if free to move, will be catapulted from the stronger field towards the weaker field; in this case towards a neutral point.
  • Diagram 2
  • The 'catapult force' is a sideways force. It does not act along the wire carrying a current, nor does it act along the magnetic field. It acts perpendicular to both the current and the magnetic field. If the wire carrying the current is horizontal and runs North-South, and the magnetic field is horizontal and runs East-West, the force on the wire is vertical, up or down. The left-hand rule neatly sums up this observation. Spread the thumb, first and second fingers of the left hand at right angles to each other. Then:
    • the second finger represents the current direction
    • the first finger the field direction
    • the thumb the force (thrust) direction
  • Diagram 3 (below) shows how to set up a large-scale version of this experiment for demonstration purposes. This uses 50 turns of PVC-covered copper wire to form the coil (about 20 cm side) and a current of 2 to 3 A.
  • Diagram 3
  • In the case of the magnetic force on a beam of electrons the expression for the force of a magnetic field on a current carrying wire (F=BIL) must be changed into the force of a magnetic field on a moving charge (F=Bev).
  • Helmhotz pairIn a fine beam tube the catapult force of the magnetic field is perpendicular to the stream of negatively charged electrons and so a uniform magnetic field will hold the stream in a circular orbit, provided the electrons move at a constant speed. The magnetic field pulls the electrons into an orbit rather like a tether that holds a whirling ball. If the tube is twisted slightly in its holder then the circular motion of the beam combines with a linear component of the beam to make a spiral.

This experiment was safety-checked in July 2007

Up next

Moving coil meter

Magnetic Force
Electricity and Magnetism

Moving coil meter

Practical Activity for 14-16

Class practical

A moving coil meter makes use of the catapult field.

Apparatus and Materials

For each student group

  • Steel yoke
  • Copper wire, PVC-covered, 150 cm with bare ends
  • Base
  • Armature
  • Magnadur magnets
  • Split pins
  • Knitting needle
  • Rivets
  • Wire strippers
  • Drinking straw
  • Rheostat (10-15 ohms, rated at 5A or more)
  • Power supply, low-voltage


Health & Safety and Technical Notes

InsertLink{Read our standard health & safety guidance}/{health-and-safety-statement}

Kits are commercially available for making model electric motors; this experiment is a variation on them.

The armature is made from a wooden or plastic block with an aluminium tube through the clearance hole drilled through the wood.

The base is also of wood or plastic, with holes positioned to take the rivets which hold the wires.

A good length of wire must be used for each spiral. If they are too tight (because the overall length is too short), the meter will be insensitive.

Care should be taken to ensure that the turns of the spiral do not foul each other, or the supports, or the magnets. The leads must also be fixed rigidly to the rivets, so that moving the connecting leads does not move the spirals.

Procedure

  1. Wind a coil of ten turns of PVC-covered copper wire on the armature block, with a couple of tight turns round the tube at the end in order to fix the ends. Plenty of wire should be left at each end.
  2. Coil each end into a loose spiral of four to five turns.
  3. Pass the knitting needle through the aluminium tube in the armature. Support it above the wooden base using the two split pins.
  4. Check that the knitting needle is firmly held, and that the armature can turn freely. (The split pins can be rotated so that the knitting needle jams in the eye of the split pin; this encourages rigidity.)
  5. Put rivets in the holes in the wooden base. Fix the leads of the spirals, passing them twice under each rivet head, as shown above. Check that, when you rotate the armature, it springs back approximately to the horizontal position.
  6. Insert a drinking straw in the hole in the armature block so that it sticks up to act as an indicating needle.
  7. Place the slab magnets on the yoke, ensuring that opposite poles are facing each other. This provides the magnetic field.
  8. Slip the yoke into place under the wooden base.
  9. Connect the leads to the DC terminals of the power supply.
  10. Switch on; the needle will go hard over.
  11. Add a rheostat (10-15 ohms, rated at 4 to 5 A) in series with the moving coil meter, so that the deflection can be varied by varying the current. Check that reversing the current reverses the deflection.

Teaching Notes

  • This goes much better if you have already built a model and then challenge students to produce their own. It can be a bit fiddly, especially for large fingers, but once one student has got it working then others soon follow.
  • Students will be amazed that, with care, they can build an ammeter which responds to a varying current in a circuit consisting of a cell, lamp, potentiometer and the model ammeter.
  • Once students have their own working model, they can be asked to describe the electromagnetic forces which act on the armature, producing a force couple which causes the armature to turn. This couple is counterbalanced by the couple produced by the spring, so that the armature settles into a position with the two couples balanced.
  • The meter could be tried on AC. It will be found to buzz furiously, but will give no deflection.
  • After students have made their own moving coil instruments, they should look at and examine any available commercially made galvanometers or moving coil ammeters, observing the moving coils within them.
  • This experiment can act as an introduction to electric motors. Ask students, Can the ammeters motion be made to continue?' The motion of the armature is certainly rotary, but it ends when the two forces, the restoring forces of the springs, and the deflecting forces of the magnetic fields, balance. 'If the springs were removed, what then?' The armature would move until it could no longer continue, because the couple produced by the magnetic forces no longer rotated it. If it did rotate, because of its momentum, then the leads would become tangled. (To solve this problem a commutator would need to be constructed.)

This experiment was safety-tested in July 2007

Up next

The electric motor

Magnetic Force
Electricity and Magnetism

The electric motor

Practical Activity for 14-16

Class practical

A motor makes use of the catapult field.

Apparatus and Materials

For each student group

  • Copper wire, PVC-covered, 150 cm with bare ends
  • Base
  • Armature
  • Magnadur magnets, 2
  • Steel yoke
  • Split pins, 2
  • Knitting needle
  • Rivets, 4
  • Wire strippers
  • Bicycle valve rubber tubing
  • Adhesive tape
  • Power supply, low-voltage


Health & Safety and Technical Notes

Read our standard health & safety guidance


Kits are commercially available for making model electric motors like this.

The armature is made from a wooden or plastic block with an aluminium tube through the clearance hole drilled through the wood. The base is also of wood or plastic, with holes positioned to take the rivets which hold the wires.

This experiment is linked to the experiment

Using ammeters and Moving coil meter


Procedure

  1. To make the commutator, first insulate one end of the aluminium tube with Sellotape. Then cut two slices off the length of valve-rubber tubing to make two rubber bands which are slid over the end of the tube.
  2. Bare one end of the PVC-covered copper wire, loop it as shown in the detail of the illustration, and fix it in place with the rubber bands. Note that the bared wires are in the same plane as the coil, not at right angles to it. This is essential.
  3. Wind about ten turns on the armature and cut off the wire, leaving enough to finish the other side of the commutator. Bare this end and loop it.
  4. Slide this end against the Sellotape on the opposite side of the aluminium tube from the original loop, under the rubber bands, to retain both ends of the coil in position.
  5. Pass the knitting needle through the aluminium tube in the armature. Support it above the wooden base using the two split pins. Check that the knitting needle is firmly held, and that the armature can turn freely. (The split pins can be rotated so that the knitting needle jams in the eye of the split pin; this encourages rigidity.)
  6. Take two further lengths of insulated wire, with bare ends. These will form the brushes and leads to the supply. Bend the ends so that they will press against the commutator.
  7. Put the rivets in the holes in the wooden base. Wind the wires from the brushes around the rivets and connect the free ends to the power supply. (Don't switch on yet.) Make sure that the brush wires press gently against the commutator wires.
  8. Place the slab magnets on the yoke, ensuring that opposite poles are facing each other. This provides the magnetic field.
  9. Slip the yoke into place under the wooden base.
  10. Switch on the power supply. Give the armature a gentle push to start it turning.

Teaching Notes

  • This goes much better if you have already built a model and then challenge students to produce their own. It can be a bit fiddly, especially for large fingers, but once one student has got it working then others soon follow.
  • The motor will run very easily on DC, but if it is spun to begin with at the synchronous speed then it will also run on AC. The commutator enables the current in the armature to reverse, so that the current in the right-hand side of the armature, for example, is always in the same direction, forcing that side either up or down and so continuing the rotary motion.
  • The looped ends of the wire forming the commutator are not strictly necessary - a straight end would suffice - but the loop enables contact with the brushes to be maintained over a greater part of a revolution, with a consequent increase in power.
  • Contact with the brushes when the coil is in the horizontal plane is particularly important, as this is when the force on the coil is maximum.
  • When the motor has been running for a long time, the brushes will become dirty and have a high resistance from the sparking. The commutator should be stripped down, and the wires emery-papered and cleaned before being reassembled.
  • For those with poor manipulative ability, there is an easier way to make the commutator. It is not as good as the first version, as the frictional torque is greater with a commutator of larger diameter. In place of the Sellotape, rubber tubing is slid over the aluminium tube to provide insulation. Larger rubber bands are needed to secure the bared wire loops, and the assembly proceeds as before.
  • Success with this motor is really impressive and students may want to construct one at home with odds and ends. The only problem may be acquiring the magnets. Many science centres now sell the magnets on their own, or even complete motor kits.

This experiment was safety-tested in July 2007

  • A video showing a much simpler electric motor:

Up next

Fractional horse-power motor

Magnetic Force
Electricity and Magnetism

Fractional horse-power motor

Practical Activity for 14-16

Demonstration

If students have built their own model electric motor, it is useful if they can see a commercial motor doing a useful job and find out how it is constructed.

Apparatus and Materials

Health & Safety and Technical Notes

Read our standard health & safety guidance


The fractional horse-power motor should operate from approximately 12 volts DC, which is conveniently obtainable from the variable low-voltage supply. The field and armature connections should both be connected, in parallel, to the voltage supply.

It is helpful to use a motor with a removable plate which can be taken off to reveal the commutator and brushes.

Procedure

  1. Show the motor in operation.
  2. Remove the plate on the end to show the brushes and internal movement. Allow students to look at this so that they can identify the parts. Replace the plate.
  3. Attach the string to the spindle of the motor. Do this by tying it to a spoke of the pulley wheel and winding it several times round the spindle.
  4. Attach the other end to the demonstration force meter which is suspended from a retort stand.
  5. Increase the voltage of the supply gradually. Observe the force with which the motor pulls on the force meter. (Note that the motor should not be in this condition for long, as it is being heated with many watts of electrical power. Raise the voltage carefully to avoid overloading the power supply and the motor.)

Teaching Notes

  • If students have built their own model electric motor, it is useful if they can see a commercial motor doing a useful job and find out how it is constructed. In most cases the magnets are actually electromagnets. (Beware: most commercial motors are induction motors rather than moving coil motors!) The armature is likely to be wound in slots in a soft iron block so that it acts as several armatures placed at an angle to each other. This is so that the motor runs more smoothly.
  • An ammeter placed in the armature circuit will show how the current changes when the motor is doing a job such as hauling up a load.

This experiment was safety-tested in July 2007

Up next

Model loudspeaker

Magnetic Force
Electricity and Magnetism

Model loudspeaker

Practical Activity for 14-16

Class practical

Students will be surprised how simple it is to make a loudspeaker, and are likely to remember their need for permanent magnets.

Apparatus and Materials

For each student group

  • Magnadur (ceramic) slab magnets, 2
  • Iron yokes, 2
  • Adhesive tape
  • Copper wire, thin, insulated, SWG 36, 2.5 m
  • Scissors
  • Paper, stiff
  • Thread
  • Retort stand, boss, and clamp
  • Signal generator, with high current output
  • Power supply, low-voltage


Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Make a simple loudspeaker with paper and adhesive tape:
    • Cut a circle of fairly stiff paper. Cut a 45º wedge out of the circle. Bring the cut edges together to make a shallow cone. Tape those edges together so that the cone will keep its shape.
    • Cut a strip of the same paper about 4 cm by 20 cm. Roll the strip up to make a tube about 3 cm diameter, and tape it so that it keeps its shape.
    • Place the tube on the point of the cone and fix it there with several strips of tape.
    • Wind two dozen or more turns of thin insulated wire (SWG 36) round the tube.
  2. Prepare the magnets as follows:
    • Use two C-cores side by side so that they form a W. (You may tape the central pair of legs together if you like.) Make the paper tube wide enough to fit over that pair of legs.
    • Place slab magnets on the inside faces of the two outer legs. Make sure the magnets have the same poles pointing inward.
  3. Hang the loudspeaker using thread from a horizontal rod. Position the magnets with their poles outside the lower end of the coil.
  4. Connect the coil to a low-voltage AC supply. Can you make the coil broadcast a buzz? Try a signal generator instead of the power supply. Vary the frequency; can you hear different notes?

Teaching Notes

  • Students could be asked to explain how their loudspeaker works. They could also look at commercial loudspeakers, and identify how the force on a current in a magnetic field produces the motion of the loudspeaker cone.
  • Most loudspeakers have a permanent magnet with a special shape, a sort of all round horse-shoe magnet.
  • The radio drives a rapidly changing current through the coil. The current follows the vibrations of speech and the electromagnetic force follows the current changes, pushing the paper cone. Finally the air in front of the loudspeaker is set into vibration following the cone's motion, and sound waves are transmitted to the listener.

This experiment was safety-tested in April 2006

Up next

The current balance

Magnetic Force
Electricity and Magnetism

The current balance

Practical Activity for 14-16

Demonstration

The current balance is a useful meter. Sophisticated versions of it are used in very sensitive current-measuring experiments. However, it is not very portable or easy to use.

Apparatus and Materials

  • Sellotape
  • Lamps with holders, 3
  • Copper wire, bare, 26 SWG, 1 reel
  • Leads
  • Cells, 1.5 V, with holders, 2
  • Current balance

Health & Safety and Technical Notes

Read our standard health & safety guidance


Circuit boards with 2 cells and 3 lamps may be used.

If the straw gives trouble by slewing round, cut a shallow groove accurately in the vertical rails with a file to localize the straw.

Beware of draughts. Warn students not to wait for the straw to come to rest.

The magnet/straw assemblies are quite fragile, and care should be taken to preserve them between lessons.

Procedure

  1. Connect the balance into a circuit of one lamp and one cell in order to weigh one lamps worth' of current.
  2. Reverse the connections to the balance and see what happens.
  3. With one lamp in the circuit, move the rider along the drinking straw until it is again horizontal with the end against the reference mark. This position of the rider will represent the current and can be marked.
  4. Put an extra lamp in series so that the lamps are under-run and only glow faintly. Move the rider to balance the straw again.
  5. Increase the voltage so that one lamp glows more brightly and the current is greater. Again move the rider to balance the straw.

Teaching Notes

  • The magnetic effect of an electric current can be used to indicate the size of an electric current. When a current passes through a coil of wire, the coil of wire behaves like a magnet. The magnet attached to the straw is then attracted towards the coil, and the straw becomes tilted. The force between the two magnets can then be balanced by putting a counterweight on the other end of the straw.
  • The greater the current, the greater the force between the magnets. The counterweight must be positioned further away from the fulcrum to return the straw to the horizontal position.
  • The current balance is useful in showing what happens when its leads are connected the opposite way round. The straw moves down and cannot be brought into balance with the rider. A useful lesson is that instruments may have to be connected in a particular way and that is why they frequently have red and black terminals. Of course the current balance does not indicate the current direction.

This experiment was safety-tested in March 2005

Up next

Faraday's motor

Magnetic Force
Electricity and Magnetism

Faraday's motor

Practical Activity for 14-16

Demonstration

To demonstrate the motor effect in a wire carrying an electric current.

Apparatus and Materials

  • Shallow dish, 80 mm diameter and 20 mm high (Petri dish)
  • Sheet of conducting metal, i.e. lead or copper approx 30 mm by approx 100 mm placed in base of dish with one end folded at right angles to clear top of dish and form electrode terminal
  • Neodymium magnet (10 mm diameter) glued to top face of metal sheet and placed centrally
  • Copper sulfate solution, saturated
  • Fuse wire, 5 amp. Use rheostat to limit current
  • Copper rod, 16 SWG or thicker, approx 100 mm long
  • Stand and clamp
  • Crocodile clips, 2
  • Power supply (0-12 V DC), 5 amps
  • Reversing switch (optional)
  • Ammeter (optional) to keep current to around 5 amps

Health & Safety and Technical Notes

The apparatus can produce some gas if current is too high.

Fuse wire may rupture if the current is too high, so the demonstration should be done behind a screen. Why not use a rheostat and ammeter to adjust the current? (Copper sulfate is toxic.)

The copper sulphate should not be left in the dish after the experiment is complete since it tends to attack the iron of the magnet.

Read our standard health & safety guidance


Neodymium magnets are available from...

Rapid Electronics


...or...

Middlesex University teaching resources


Insulate side of magnet with glue or ring of adhesive tape.

Recent developments in magnet technology have made this possible using brine or copper sulfate solution as the liquid conductor, instead of mercury (used by Faraday).

If you use copper sulfate solution, the device will deposit copper from solution so can only be operated for short periods. Brine does not prevent the problem.

Copper wire can be salvaged from offcuts of electrical earthing cable or use a nail / toy car axle etc.

Procedure

  1. Connect the lead strip electrode, using a crocodile clip, to the reversing switch.
  2. Wrap or solder fuse wire to one end of the copper rod. Suspend the copper rod from clamp with 5 amp fuse wire so that the rod just clears the lead strip and can rotate freely around the magnet without fouling the magnet or strip.
  3. Connect the free end of fuse wire, using a crocodile clip, to the reversing switch.
  4. Pour sufficient copper sulfate into dish to just cover the magnet.
  5. Starting at zero, increase the voltage until the copper wire starts to rotate around the magnet (it may need a helping hand to start). Reduce the voltage once rotation is established in order to keep the current to around 5 amps.
  6. Reversing the polarity will reverse the direction of rotation.

Teaching Notes

  • The copper sulfate solution forms an electrical conductor since it has free ions and completes the electrical circuit.
  • An interesting but unwanted side effect is the reduction of copper sulfate by electrolysis to copper metal. This can be seen forming like iron filings at the end of copper rod as it rotates.
  • This was the first motor ever produced repeatedly by Michael Faraday in his laboratory at the Royal Institution, London.

This experiment was submitted by Richard Walder from Eastbourne College.

Up next

Fleming's left hand rule (using the Earth's magnetic field)

Magnetic Force
Electricity and Magnetism

Fleming's left hand rule

Practical Activity for 14-16

Demonstration

The catapult field seen when two magnetic fields interact can reveal the presence of the Earth's magnetic field, making Fleming's rule more memorable to students.

Apparatus and Materials

  • Strip of aluminium foil approx 2 metres long cut to the same width as adhesive tape (25 mm)
  • Metre strip of adhesive tape, 2
  • Crocodile clips, 2
  • Connecting wires of suitable length, 2
  • Stands and clamps, 2
  • Power supply (5 to 8 amp DC), or car battery charger, set to 6 volts
  • Magnetic compass (optional)
  • Reversing switch (optional)

Health & Safety and Technical Notes

The resistance of the aluminium strip is about an ohm, so it is almost a short-circuit. Make sure the power supply is capable of withstanding such a low-resistance load. Do not use a battery.

Unroll the aluminium foil and apply adhesive tape to one surface; this makes the foil much more robust and easily cut without tearing.

Ensure power is briefly applied otherwise tape will rupture.

Read our standard health & safety guidance


Photograph courtesy of Richard Walder

Procedure

  1. Connect crocodile clips to each end of the tape and suspend with an East-West orientation between the two stands allowing the tape to almost touch the bench.
  2. Connect to the power supply and allow tape to settle.
  3. Briefly switch on and observe a kick in the tape. Reverse polarity and repeat; the kick is in the opposite direction. Rotate the set-up to align North-South and repeat. There will be no movement.

Teaching Notes

  • There is a force on the wire only when the current is perpendicular to the magnet's field. The interaction with a current carrying conductor in this experiment demonstrates that the Earth's magnetic field is strong and aligned North to South.
  • The Earth's magnetic field is used by earth scientists to explain aurorae and by biologists to explain how life on Earth is protected from cell damage.

This experiment was submitted by Richard Walder from Eastbourne College.

Resources

For some more detailed images of the apparatus used in this experiment.

IOP DOMAINS Physics CPD programme

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