D.3 · IB Physics
Force between parallel wires
Look end-on at two long parallel wires. Change the size and direction of each current and the separation (drag a wire or use the slider). Then see the field each wire makes at the other and the resulting force per unit length.
B = μ₀I / (2πr)
F/L = μ₀I₁I₂ / (2πd)
μ₀ = 4π × 10⁻⁷ T m A⁻¹
+ current = out of page (•)
− current = into page (×)
F/L = μ₀I₁I₂ / (2πd)
μ₀ = 4π × 10⁻⁷ T m A⁻¹
+ current = out of page (•)
− current = into page (×)
Try this
- Set I₁ = I₂ = +5.0 A and d = 10.0 cm. Note F/L, then halve d to 5.0 cm. By what factor does the force per unit length change?
- Set I₁ = 10.0 A and I₂ = 2.0 A. B₁ at wire 2 and B₂ at wire 1 are different. Compare the force arrows on the two wires.
- Reverse just one current and see attraction turn into repulsion. Then reverse both currents: does the interaction change?
Exam tip: work in two steps. First use the right-hand grip rule to find the direction of B from one wire at the other wire. Then apply F = BIL with the left-hand (or right-hand palm) rule to find the force direction. Remember the two forces are always equal and opposite.