4.Main Content
experiments
Catapult magnetic field
Class experiment
A spectacular demonstration showing how magnetic fields interact to produce forces on wires.
Apparatus and materials
For each student group:
- Copper wire, PVC-covered, 150 cm with bare ends
- Iron filings
- Magnadur magnets, 2
- Support blocks, 2
- Stiff card or hardboard, 35 cm x 10 cm
- Plasticine
- Power supply, low-voltage ('Westminster pattern' very-low-voltage supplies are best)
Technical notes
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).
Safety
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
Diagram 1
a Make a hole, approx 5 mm diameter, in the centre of the card.
b Place the support blocks about 30 cm apart and use them to support the long card.
c 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.)
d Support the coil with a lump of modelling clay.
e Place two slab magnets upright on the card near the ends, about 25 cm apart.
f Sprinkle iron filings on the card and look for the magnetic field pattern. See diagram 1.
g Sweep the filings away, remove the slab magnets, and connect the coil to the power supply.
h Sprinkle iron filings on the card and look for the magnetic field pattern. See diagram 1.
i Sweep the filings away. Connect the coil to the power supply and replace the slab magnets.
j 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.
k Diagram 2 (below) shows the catapult analogy
Teaching notes
1 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
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.
3 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
4 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).
This diagram shows the magnetic field produced by a Helmholtz pair, used with a fine beam tube.
In 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