Forces Between Parallel Wires
Forces Between Parallel Wires
- Each current produces a magnetic field. Each wire is therefore inside the field produced by the other wire.
Why the wires exert forces
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- Wire P produces a magnetic field at wire Q.
- The current in Q is perpendicular to that field, so a magnetic force acts on Q.
- The same reasoning gives a force on P due to the field from Q.
Currents in the same direction attract. Currents in opposite directions repel.
The two forces form a Newton's third-law pair
- The forces have equal magnitudes and opposite directions, even when the two currents are different.
- Reversing both currents does not change the force direction. Each current reverses, and the field produced by the other wire also reverses.
Worked example
Force per unit length
The field from wire P at wire Q is 2.6 mT. Wire Q carries 5.0 A. Find the force per unit length on Q. The field from Q at P is 1.5 mT; find the current in P (9702/41/M/J/23 Q6(c)).
- For one metre of wire:
- Newton's third law gives the same force per unit length on P.
Answer
Common mistake
Do not say that the wire with the larger current experiences the larger force. The forces are always equal and opposite.
Your turnquick check
Two parallel wires carry currents in opposite directions. Do they attract or repel? Compare the two force magnitudes.
Show worked answer
They repel. The two force magnitudes are equal.