Magnetic fields and the force between parallel currents
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Question
What is a magnetic field?
Answer
The region around a magnet **or a current** where a magnetic force is felt. We picture it with **field lines** — closer lines mean a stronger field.
Question
What shape is the magnetic field around a straight current-carrying wire?
Answer
**Concentric circles** centred on the wire. Use the **right-hand grip rule**: thumb along the current I, fingers curl the way the circles point.
Question
How do magnetic field lines run between two bar magnets?
Answer
From the **N pole to the S pole** (outside the magnet). Unlike poles (N–S) attract; like poles (N–N) repel.
Question
Two parallel wires carry current in the SAME direction — attract or repel?
Answer
They **attract** (parallel currents come together).
Question
Two parallel wires carry current in OPPOSITE directions — attract or repel?
Answer
They **repel** (anti-parallel currents push apart).
Question
Formula for the force per unit length between parallel wires?
Answer
$\dfrac{F}{L} = \mu_{0}\dfrac{I_{1}I_{2}}{2\pi r}$ — given in the data booklet.
Question
In F/L = μ_{0} I_{1} I_{2} / (2π r), what is μ_{0}?
Answer
The **permeability of free space**, a constant equal to 4π × 10⁻⁷ T m A⁻¹.
Question
How does the force per unit length depend on the separation r?
Answer
It is **inversely proportional** to r: F/L ∝ 1/r. Doubling r halves F/L.
Question
How does F/L change if one current is doubled?
Answer
It **doubles** — F/L is proportional to each current (F/L ∝ I_{1} I_{2}).
Question
Why do two current-carrying wires exert a force on each other?
Answer
Each wire sits in the **magnetic field** created by the other, so each feels a force. By Newton's third law the forces are equal and opposite.
Question
Reverse the current in ONE of two parallel wires — what happens to the force?
Answer
It flips between attraction and repulsion (the currents become anti-parallel, or parallel, instead).
Question
Two wires 0.10 m apart carry 2.0 A and 5.0 A the same way. Direction of the force?
Answer
Attraction — same-direction (parallel) currents attract.
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Topic 4.2 hub
Electric and magnetic fields
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