Cambridge IGCSE Physics 0625 – 4.5.3 Magnetic Effect of a Current
4.5.3 Magnetic Effect of a Current
Objective
State the qualitative variation of the strength of the magnetic field around straight wires and solenoids.
Key Concepts
A current‑carrying conductor produces a magnetic field that forms concentric circles around the wire.
The magnetic field inside a solenoid is approximately uniform and parallel to the axis of the coil.
Field strength depends on several factors, which can be expressed qualitatively (and sometimes quantitatively).
Magnetic Field Around a Straight Wire
The magnetic field (\$B\$) produced by a long, straight conductor varies with:
Current (\$I\$): stronger current → stronger magnetic field.
\$B \propto I\$
Distance from the wire (\$r\$): the farther from the wire, the weaker the field.
\$B \propto \frac{1}{r}\$
Direction of current: reversing the current reverses the direction of the magnetic field (right‑hand rule).
Suggested diagram: Concentric magnetic field lines around a straight wire with arrows indicating direction according to the right‑hand rule.
Magnetic Field Inside a Solenoid
A solenoid is a coil of many turns of wire. Inside a long solenoid the field is nearly uniform and parallel to the axis. Its strength depends on:
Current (\$I\$): increasing current increases the field.
\$B \propto I\$
Number of turns per unit length (\$n\$): more turns per metre give a stronger field.
\$B \propto n\$
Core material: inserting a ferromagnetic core multiplies the field by the relative permeability (\$\mu_r\$).
\$B = \mu0 \mur n I\$
Length of the solenoid (for a given total number of turns): a longer solenoid spreads the same number of turns over a greater length, reducing \$n\$ and thus the field.
Suggested diagram: Magnetic field lines inside and outside a solenoid, showing uniform field inside and weaker, divergent field outside.
Summary Table – Qualitative \cdot ariation of Magnetic Field Strength
Configuration
Parameter that Increases \$B\$
Parameter that Decreases \$B\$
Straight wire
Increase current \$I\$ Move closer to the wire (decrease \$r\$)
Decrease current \$I\$ Move farther from the wire (increase \$r\$)
Solenoid (long, tightly wound)
Increase current \$I\$ Increase turns per unit length \$n\$ (more turns or shorter length for same total turns) Insert a ferromagnetic core (higher \$\mu_r\$)
Decrease current \$I\$ Decrease \$n\$ (fewer turns or longer solenoid) Use a non‑magnetic core (lower \$\mu_r\$)
Practical Implications
When designing electromagnets, use a high current and many turns of wire to obtain a strong field.
Placing a soft iron core inside a solenoid can increase the field by a factor of several hundred.
In laboratory setups, the magnetic field strength can be measured with a tangent‑galvanometer; the readings will follow the qualitative trends listed above.
Key Take‑away Statements
For a straight conductor, the magnetic field strength is directly proportional to the current and inversely proportional to the distance from the wire.
Inside a solenoid, the magnetic field strength is directly proportional to the product of current and the number of turns per unit length, and is enhanced by magnetic core material.
These relationships are qualitative guides; the exact quantitative formulas involve constants such as \$\mu_0\$ (the permeability of free space).