electromagnetic effects

4.5.1 electromagnetic induction

an electromotive force is induced when a conductor cuts magnetic field lines.

• the conductor must move relative to the magnetic field
• or the magnetic field must change

the magnitude of the induced e.m.f. depends on:

• speed of motion
• strength of the magnetic field
• length of the conductor in the field

the direction of the induced e.m.f. can be determined using fleming’s right-hand rule.

4.5.2 the a.c. generator

an a.c. generator converts kinetic energy into electrical energy.

• a coil rotates in a magnetic field
• an alternating e.m.f. is induced by the right hand rule

factors affecting the magnitude of the output:

• speed of rotation of the coil
• strength of the magnetic field
• number of turns in the coil

the induced current changes direction every half turn.

4.5.3 magnetic effect of a current

a current-carrying conductor produces a magnetic field.

• the field pattern around a straight wire consists of concentric circles
• the direction of the field depends on the direction of current

the direction of the magnetic field can be found using the right-hand grip rule.

the strength of the magnetic field increases when:

• the current increases
• the conductor is closer to the point

4.5.4 force on a current-carrying conductor

a current-carrying conductor placed in a magnetic field experiences a force.

• the force is perpendicular to both the current and the field
• the direction is given by fleming’s left-hand rule

the size of the force depends on:

• current in the conductor
• strength of the magnetic field
• length of the conductor in the field

4.5.5 the d.c. motor

a d.c. motor converts electrical energy into kinetic energy.

• a current flows through a coil in a magnetic field
• forces act on opposite sides of the coil due to left hand rule
• the coil rotates

a split-ring commutator:

• reverses the current every half turn
• keeps the coil rotating in the same direction

4.5.6 the transformer

a transformer changes the size of an alternating voltage.

• works only with alternating current
• uses electromagnetic induction

step-up transformer:

• increases voltage
• has more turns on the secondary coil

step-down transformer:

• decreases voltage
• has fewer turns on the secondary coil

transformer equation:

vp / vs = np / ns

power losses occur due to:

• heating of coils
• eddy currents in the core (out of syllabus, just saying)