Describe an experiment to identify the pattern of the magnetic field (including direction) due to currents in straight wires and in solenoids

Published by Patrick Mutisya · 14 days ago

IGCSE Physics 0625 – Magnetic Effect of a Current

4.5.3 Magnetic Effect of a Current

Objective

Describe an experiment that identifies the pattern of the magnetic field (including its direction) produced by:

  • a straight current‑carrying wire, and

  • a solenoid.

Key Theory

The magnetic field around a long straight conductor is given by

\$B = \frac{\mu_0 I}{2\pi r}\$

where μ₀ is the permeability of free space, I the current and r the radial distance from the wire.

Inside an ideal solenoid the field is uniform and parallel to the axis:

\$B = \mu_0 n I\$

with n the number of turns per unit length.

Apparatus

ItemPurpose
Power supply or batteryProvides a steady current
AmmeterMeasures the current in the circuit
Straight copper wire (≈ 1 m)Source of magnetic field
Solenoid (e.g., 200 turns, length 10 cm)Creates a uniform magnetic field inside
Connecting leadsComplete the circuit
Compass (or a set of small magnetic needles)Detects direction of magnetic field lines
Paper sheet and pencilRecord needle positions

Experimental Procedure

  1. Set up the circuit as shown in the diagram below: connect the straight wire to the power supply through the ammeter.

  2. Place the compass on a flat surface and position it at a known distance (e.g., 5 cm) from the wire. Record the initial needle direction (pointing to magnetic north).

  3. Switch on the current (choose a moderate value, e.g., 2 A). Observe and record the new needle direction. Repeat for several distances (2 cm, 5 cm, 10 cm) on both sides of the wire.

  4. Reverse the direction of the current and repeat the observations. Note the change in needle orientation.

  5. Remove the straight wire and replace it with the solenoid. Connect the solenoid to the same power source.

  6. Place the compass at the centre of the solenoid and record the needle direction with the current off.

  7. Switch on the current (same current as before). Record the needle direction. Then move the compass to positions at the ends of the solenoid and repeat.

  8. Reverse the current in the solenoid and repeat the observations.

  9. Tabulate all observations and sketch the inferred field lines.

Suggested diagram: (a) straight wire with compass at various positions; (b) solenoid with compass inserted along its axis.

Observations (Typical)

SetupCurrent DirectionCompass DeflectionInterpretation
Straight wire – left sideInto pageNeedle turns clockwiseField circles wire (right‑hand rule)
Straight wire – right sideInto pageNeedle turns anticlockwiseOpposite sense on opposite side
Solenoid – centreCurrent clockwise when viewed from leftNeedle aligns with axis, pointing from left to rightUniform field inside, direction given by right‑hand grip rule
Solenoid – outsideSame as aboveVery little deflectionField outside is weak and loops back

Analysis

From the compass deflections around the straight wire we infer that the magnetic field forms concentric circles centred on the wire. The direction of the field follows the right‑hand rule: thumb in the direction of conventional current, fingers curl in the direction of the magnetic field lines.

Inside the solenoid the compass aligns with the axis, showing a uniform field parallel to the coil’s length. Reversing the current reverses the field direction, confirming the right‑hand grip rule for solenoids (fingers follow the coil winding, thumb points in the field direction).

Conclusion

The experiment successfully demonstrates the characteristic magnetic field patterns:

  • For a straight conductor, the field is circular and its direction is given by the right‑hand rule.

  • For a solenoid, the field inside is uniform and parallel to the axis; outside the field is weak and forms closed loops.

These observations underpin the use of the right‑hand rule in predicting magnetic field directions for various current configurations.