Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Force on a wire carrying a current in a magnetic field

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 ('Westminster pattern' very-low-voltage supplies are best)

Health & Safety and Technical notes


Read our standard health & safety guidance

 

Procedure


Apparatus set-up

a Make a long rectangular loop of thin copper wire. 

b 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. 
 
c Connect the ends of the copper wire to the low-voltage DC supply, using cleaned crocodile clips and 4 mm leads. 
 
d 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. 
 
e Find the position in which the magnets have the greatest effect on the current-carrying wire. 

clamp
 
f 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. 
 
g Place a third piece of copper wire across the rails. 
 
h Bring up the magnets; how should they be held to produce a force on the third wire? 
 
i Investigate what happens if you reverse the current, or if you reverse the magnets. 


Teachingn notes


1 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. 
 
2 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. 
 
3 You could introduce the left hand rule here in order to summarize what students have discovered. 

 
This experiment was safety-checked in July 2007

 

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Catapult magnetic field