4.5.3 Magnetic Effect of a Current
Objective
🔍 Identify the pattern of the magnetic field, including its direction, produced by currents in straight wires and in solenoids.
Experiment 1: Straight Wire
⚡️ A simple experiment using a straight wire, a compass, and a battery can reveal the magnetic field pattern around the wire.
- 🔌 Connect a straight copper wire to a 9 V battery using alligator clips.
- 📏 Place the wire horizontally on a table and secure it so it stays straight.
- 🧲 Hold a small compass close to the wire, keeping the compass needle perpendicular to the wire.
- 🔄 Observe the deflection of the compass needle as the current flows.
- 🗺️ Repeat the experiment with the wire rotated 90° to confirm the direction changes accordingly.
🧠 How to read the results: The compass needle points in the direction of the magnetic field. Using the right‑hand rule (thumb in the direction of current, fingers curl in the direction of the magnetic field) you can predict the field lines around the wire.
Experiment 2: Solenoid
🔩 A solenoid (a coil of wire) creates a stronger, more uniform magnetic field inside. This experiment demonstrates the field inside and outside the solenoid.
- 🔌 Wrap a coil of insulated copper wire (e.g., 200 turns) around a cardboard tube.
- 📏 Measure the length of the solenoid, \(l\), and the number of turns, \(N\).
- 🔋 Connect the solenoid to a 9 V battery.
- 🧲 Place a compass at the centre of the solenoid and note the needle’s direction.
- 📐 Move the compass along the axis of the solenoid and record the field direction at different positions.
- 🧪 Place a small iron nail inside the solenoid to see the magnetic field lines forming a clear north–south axis.
🧠 Key observations: Inside the solenoid the field is uniform and points along the axis; outside the field is weak and loops back, forming closed loops.
Data Table
| Experiment | Parameter | Value |
|---|
| Straight Wire | Current \(I\) | ≈ 0.5 A |
| Solenoid | Turns \(N\) | 200 |
| Solenoid | Length \(l\) | 10 cm |
| Solenoid | Current \(I\) | ≈ 0.5 A |
Analysis & Theory
🔬 The magnetic field around a straight wire is given by Ampère’s law:
\$B = \frac{\mu_0 I}{2\pi r}\$
where \(r\) is the distance from the wire. The direction follows the right‑hand rule.
🔬 For a solenoid, the field inside is:
\$B = \mu_0 n I\$
with \(n = \frac{N}{l}\) the turn density. Outside the solenoid, \(B\) is negligible.
Exam Tips
- 📝 Remember to state the right‑hand rule when describing field direction.
- 📐 Use the formula \(B = \mu_0 n I\) for solenoids and explain why the field is uniform inside.
- 🧲 Illustrate the field lines with a diagram (use arrows and emojis if allowed).
- 🔁 Practice describing how the field changes if you reverse the current direction.
- 💡 In exam answers, include a brief explanation of why the field outside a solenoid is weak.