PHY049 · Magnetism · Classes 10–12
Force on conductor
Objective: To show that a current-carrying conductor in a magnetic field feels a force (the motor effect).
🧰 Apparatus
- Horseshoe magnet
- Two metal rails
- Copper rod
- Low-voltage supply
- Wires
📐 Formulas and SI units
F = BIL sin θ (N)
Fleming's left-hand rule
💡 Theory
A wire carrying current across a magnetic field is pushed sideways with F = BIL sin θ. Fleming's left-hand rule: first finger = field, second finger = current, thumb = motion. This force turns electric motors.
🪜 Procedure
- Place the rails between the poles of the magnet and lay the rod across them.
- Connect the rails to the supply and switch on briefly.
- Watch the rod roll and note its direction.
- Reverse the current, then reverse the magnet: observe.
- Increase the current and compare.
📋 Observations Simulated values
✅ Result and conclusion
In 0.5 T, a 0.1 m rod carrying 2 A is pushed with 0.1 N, perpendicular to both the field and the current.
🔬 In a real lab
Friction between the rod and rails must be overcome first; the magnet's field is not uniform at the edges.
⚠️ Safety
Short circuits through the rails heat them: switch on only briefly with a current-limited supply.
❓ Quick check
1. The force is zero when the wire is…
2. Fleming's left-hand rule: the thumb shows…
3. Which device uses this effect?
🏋️ Practice
EasyB = 1 T, I = 3 A, L = 0.2 m. F?
Answer: 0.6 N
MediumSame at 30° to the field?
Answer: 0.3 N
ChallengeTwo ways to reverse the force?
Answer: Reverse the current or the field