Magnetic Force on a Current-Carrying Conductor (AQA A Level Physics): Revision Note

Magnetic force on a current-carrying conductor

• A current-carrying conductor produces its own magnetic field

  • When interacting with an external magnetic field, it will experience a force

• The force $F$ on a conductor carrying current $I$ at an angle $\theta$ to a magnetic field with flux density $B$ is defined by the equation

$F=BIL\sin \theta$

• Where:

  • $F$ = force on a current-carrying conductor in a $B$ field (N)

  • $B$ = magnetic flux density of applied $B$ field (T)

  • $I$ = current in the conductor (A)

  • $L$ = length of the conductor (m)

  • $\theta$ = angle between the conductor and applied $B$ field (degrees)

• This equation shows that the force on the conductor can be increased by:

  • increasing the strength of the magnetic field

  • increasing the current flowing through the conductor

  • increasing the length of the conductor in the field

• Note: The length $L$ represents the length of the conductor that is within the field

• A current-carrying conductor (e.g. a wire) will experience the maximum magnetic force if the current through it is perpendicular to the direction of the magnetic field lines

  • It experiences no force if it is parallel to magnetic field lines

• The maximum force occurs when $\sin \theta =1$

  • This means $\theta =90°$ and the conductor is perpendicular to the $B$ field

• The equation for the magnetic force becomes:

$F=BIL$

• The minimum force, i.e. $F=0\mathrm{N}$, is when $\sin \theta =0$

  • This means $\theta =0°$ and the conductor is parallel to the $B$ field

• It is important to note that a current-carrying conductor will experience no force if the current in the conductor is parallel to the field

  • This is because the $F$, $B$ and $I$ must be perpendicular to each other

Observing the force on a current-carrying conductor

• The force due to a magnetic field can be observed by:

  • placing a copper rod in a uniform magnetic field

  • connecting the copper rod to a circuit

• When current is passed through the copper rod, it experiences a force

  • This causes it to accelerate in the direction of the force