The solenoid, the force on a wire and parallel wires
Answers are for parents and teachers.
1
A solenoid of 1200 turns and length 0.6 m carries 0.75 A in air. Find B inside it (μ0 = 1.256 × 10⁻⁶ T·m/A).
B = 1.256 × 10⁻⁶ × 1200 × 0.75 ÷ 0.6 = 1.256 × 10⁻⁶ × 1500 = 1.88 × 10⁻³ T.
2
What current gives B = 4.0 × 10⁻³ T inside an air-filled solenoid with 2500 turns per metre (μ0 = 1.256 × 10⁻⁶ T·m/A)?
I = B ÷ (μ0n) = 4.0 × 10⁻³ ÷ (1.256 × 10⁻⁶ × 2500) = 4.0 × 10⁻³ ÷ 3.14 × 10⁻³ = 1.27 A.
3
A wire 0.35 m long carries 6 A in a field of 0.2 T. Find the force when the wire is at 90° to the field and when it is at 30°.
At 90°: F = 0.2 × 6 × 0.35 = 0.42 N. At 30°: F = 0.42 × 0.5 = 0.21 N.
4
Two long parallel wires 0.1 m apart carry 20 A upward and 5 A downward. Find the force per metre and say whether they attract or repel.
F ÷ ℓ = 2 × 10⁻⁷ × 20 × 5 ÷ 0.1 = 2 × 10⁻⁴ N/m. The currents are opposite, so they repel.
5
The field points down and the current in a wire flows east. Which way does the force point?
Forefinger down, middle finger east: the thumb of the left hand points north.