Describe the effect on the magnetic field around straight wires and solenoids of changing the magnitude and direction of the current

Published by Patrick Mutisya · 14 days ago

IGCSE Physics 0625 – Magnetic Effect of a Current

4.5.3 Magnetic Effect of a Current

Learning Objective

Describe how the magnetic field around straight wires and solenoids changes when the magnitude or direction of the current is altered.

1. Magnetic Field Around a Straight Conductor

A current‑carrying straight wire produces concentric circular magnetic field lines centred on the wire. The direction of the field is given by the right‑hand rule: point the thumb in the direction of conventional current (positive to negative) and the curled fingers show the direction of the magnetic field.

1.1 Effect of Changing the Current Magnitude

The magnetic field strength \$B\$ at a distance \$r\$ from a long straight wire is given by

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

where \$\mu_0\$ is the permeability of free space and \$I\$ is the current. From this relation:

  • If \$I\$ increases, \$B\$ increases proportionally.

  • If \$I\$ decreases, \$B\$ decreases proportionally.

  • If \$I = 0\$, the magnetic field disappears.

1.2 Effect of Reversing the Current Direction

Reversing the direction of the current reverses the direction of the magnetic field lines (they change from clockwise to anticlockwise or vice‑versa). The magnitude of \$B\$ remains the same for the same absolute current value.

Suggested diagram: Straight wire with current to the right, magnetic field circles shown using right‑hand rule; another diagram with current reversed.

2. Magnetic Field Inside a Solenoid

A solenoid is a coil of many turns of wire. When a current flows, the field inside is nearly uniform and parallel to the axis of the solenoid, while the external field is weak.

The magnetic field inside an ideal solenoid is given by

\$B = \mu_0 n I\$

where \$n\$ is the number of turns per unit length and \$I\$ is the current.

2.1 Effect of Changing the Current Magnitude

  • Increasing \$I\$ increases \$B\$ linearly.

  • Decreasing \$I\$ reduces \$B\$ linearly.

  • When \$I = 0\$, the interior field vanishes.

2.2 Effect of Reversing the Current Direction

Reversing the current reverses the direction of the magnetic field inside the solenoid (the north and south poles swap). The field strength remains unchanged for the same absolute current.

Suggested diagram: Solenoid with current flowing clockwise when viewed from the left, field lines inside pointing to the right; a second diagram with current reversed showing opposite field direction.

3. Summary Table

ObjectChange MadeResult on Magnetic Field
Straight wireIncrease \$I\$ (same direction)\$B\$ increases proportionally; direction unchanged.
Straight wireDecrease \$I\$ (same direction)\$B\$ decreases proportionally; direction unchanged.
Straight wireReverse current direction\$B\$ magnitude unchanged; direction reverses (clockwise ↔ anticlockwise).
SolenoidIncrease \$I\$ (same winding sense)\$B\$ inside increases linearly; external field still weak.
SolenoidDecrease \$I\$ (same winding sense)\$B\$ inside decreases linearly.
SolenoidReverse current direction\$B\$ magnitude unchanged; direction inside reverses (north ↔ south).

4. Key Points to Remember

  1. The magnetic field around a straight conductor is circular and follows the right‑hand rule.

  2. Field strength varies directly with current magnitude and inversely with distance from the wire.

  3. Reversing current reverses field direction but not its magnitude.

  4. Inside a solenoid the field is uniform, parallel to the axis, and proportional to the product \$nI\$.

  5. Changing the current in a solenoid changes the field strength linearly; reversing current swaps the poles.