Catapult magnetic field
A spectacular demonstration showing how magnetic fields interact to produce forces on wires.
Apparatus and materials
Copper wire, PVC-covered, 150 cm with bare ends
Magnadur magnets, 2
Support blocks, 2
Stiff card or hardboard, 35 cm x 10 cm
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 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).
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.
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.
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.
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).
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