4.5.4 Force on a Current‑Carrying Conductor
Experiment Overview
⚡️ Goal: Show that a magnetic field exerts a force on a conductor carrying an electric current, and see how the force changes when we reverse the current or the field direction.
Materials
- Thin copper wire (≈ 1 m long)
- Power supply (5 V battery or DC adapter)
- Switch (toggle or push‑button)
- Strong permanent magnet (neodymium, 2 cm × 2 cm)
- Support stand for the magnet (e.g., a small tripod)
- Lightweight support for the wire (e.g., a plastic rod or a small wooden block)
- Protractor or ruler (to measure angle of deflection)
- Notebook for observations
Setup Diagram (text‑only)
[Power] ──► [Switch] ──► [Wire] ──► [Ground]
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▼
[Magnet] (north pole up)
Procedure
- Place the magnet on the support stand so that its north pole faces the wire.
- Thread the copper wire through the centre of the magnet’s field, leaving a small loop hanging freely.
- Connect the wire to the power supply via the switch.
- Close the switch to allow current to flow. Observe the motion of the wire loop.
- Measure the angle of deflection from the vertical using the protractor.
- Record the direction of the force (upward, downward, left, right).
Physics Behind It
The force on a segment of wire in a magnetic field is given by the Lorentz force law:
\$\mathbf{F} = I\,\mathbf{L} \times \mathbf{B}\$
where:
- \$I\$ – current (A)
- \$\mathbf{L}\$ – length vector of the wire (m, direction of current)
- \$\mathbf{B}\$ – magnetic field (T, direction from north to south)
The cross product means the force is perpendicular to both the current direction and the magnetic field.
Reversing the Current (a)
- Switch the polarity of the power supply (swap the + and – terminals).
- Repeat the experiment.
- Observe that the direction of the force on the wire reverses (e.g., from left to right).
Reversing the Magnetic Field (b)
- Flip the magnet so that the south pole now faces the wire.
- Keep the current direction the same.
- Observe that the force direction again reverses.
Summary Table
| Condition | Force Direction |
|---|
| Normal current, North pole up | Right (→) |
| Reversed current, North pole up | Left (←) |
| Normal current, South pole up | Left (←) |
| Reversed current, South pole up | Right (→) |
Analogy: The Magnetic Field as a Crowd
Imagine the magnetic field as a crowd of people (magnetic lines) moving in a straight line. If you walk (current) through the crowd, you feel a push from the side. If you walk in the opposite direction, the push comes from the other side. If the crowd changes direction, the push also changes. This helps visualise why reversing either the current or the field reverses the force.
Exam Tip
• Remember the formula \$ \mathbf{F} = I\,\mathbf{L} \times \mathbf{B} \$ – the force is perpendicular to both \$I\$ and \$B\$.
• Use the right‑hand rule: point your thumb in the direction of current, fingers in the direction of \$B\$, and your palm shows the force direction.
• In questions, check if reversing the current or the field will change the sign of the force.