Magnetic Effects of Electric Current
What is tested
This domain tests magnetic fields around currents, the right-hand thumb rule, solenoids, electromagnets, force on a current-carrying conductor, electric motors, electromagnetic induction, generators and AC versus DC.
Core rules
- A current-carrying conductor produces a magnetic field. Field lines are closed curves and never cross.
- Right-hand thumb rule: point the thumb in the direction of current; curled fingers give the field direction.
- In a solenoid, the field inside is uniform and similar to a bar magnet. A soft iron core makes an electromagnet.
- Fleming's left-hand rule gives the force on a current-carrying conductor in a magnetic field: forefinger field, middle finger current, thumb force.
- Electromagnetic induction: a changing magnetic field induces a current. Fleming's right-hand rule gives the induced current direction.
- A motor converts electrical energy to mechanical energy; a generator converts mechanical energy to electrical energy.
Worked example
A straight wire carries current to the right in a magnetic field directed into the page. Using Fleming's left-hand rule, the forefinger points into the page and the middle finger points right; the thumb points upward. So the magnetic force on the wire is upward.
Traps
Field lines do not cross. The left-hand rule is for force on a conductor; the right-hand rule is for induced current. AC reverses direction periodically; DC does not. A solenoid's field is strongest inside.