RP10-Magnetic force on a wire

• Equipment: Wire, two stands and clamps, two identical magnets and a cradle, mass balance, ammeter, variable power supply, ruler.
• Set up: Place the magnets in the cradle on top of a mass balance and tare it. Use the stands to hold the wire between (but not touching) the magnets, using Fleming's left hand rule to ensure the force on the wire is vertically upwards (and therefore vertically downwards for the cradle). Measure the length of the wire in the magnetic field and then connect to an ammeter and variable power supply in series.
• Method: Increase the voltage in intervals and record the ammeter reading and the mass displayed on the mass balance. As it is difficult to get exact repeats, do as many different readings as possible with small intervals between them.
• Graph: Plot a graph of F against I. The gradient is B multiplied by L.
• Analysis: Force is mass in kilograms multiplied by gravity. F = B I L.
• Safety: Wire will get hot - do not touch wire, keep currents high for as little time as possible.