Magnetism

4.5 Electromagnetic effects

4.5.1 Electromagnetic induction

 1 Know that a conductor moving across a magnetic field or a changing magnetic field linking with a conductor can induce an e.m.f. in the conductor

 2 Describe an experiment to demonstrate electromagnetic induction

 3 State the factors affecting the magnitude of an induced e.m.f.

 4 Know that the direction of an induced e.m.f. opposes the change causing it

 5 State and use the relative directions of force, field and induced current

4.5.2 The a.c. generator

 1 Describe a simple form of a.c. generator (rotating coil or rotating magnet) and the use of slip rings and brushes where needed

 2 Sketch and interpret graphs of e.m.f. against time for simple a.c. generators and relate the position of the generator coil to the peaks, troughs and zeros of the e.m.f.

4.5.3 Magnetic effect of a current

 1 Describe the pattern and direction of the magnetic field due to currents in straight wires and in solenoids

 2 Describe an experiment to identify the pattern of the magnetic field (including direction) due to currents in straight wires and in solenoids

 3 Describe how the magnetic effect of a current is used in relays and loudspeakers and give examples of their application

 4 State the qualitative variation of the strength of the magnetic field around straight wires and solenoids

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

4.5 Electromagnetic effects

4.5.4 Force on a current-carrying conductor

 1 Describe an experiment to show that a force acts on a current-carrying conductor in a magnetic field, including the effect of reversing:

 (a) the current

 (b) the direction of the field

 2 Recall and use the relative directions of force, magnetic field and current

 3 Determine the direction of the force on beams of charged particles in a magnetic field

4.5.5 The d.c. motor

 1 Know that a current-carrying coil in a magnetic field may experience a turning effect and that the turning effect is increased by increasing:

 (a) the number of turns on the coil

 (b) the current

 (c) the strength of the magnetic field

 2 Describe the operation of an electric motor, including the action of a split-ring commutator and brushes

4.5.6 The transformer

 1 Describe the construction of a simple transformer with a soft iron , as used for voltage transformations

 2 Use the terms primary, secondary, step-up and step-down

 3 Recall and use the equation Vp/Vs =  Np/Ns where p and s refer to primary and secondary

 4 Describe the use of transformers in high-voltage transmission of electricity

 5 State the advantages of high-voltage transmission

 6 Explain the principle of operation of a simple iron-d transformer

 7 Recall and use the equation for 100% efficiency in a transformer IpVp = IsVs where p and s refer to primary and secondary

 8 Recall and use the equation P = I^2R to explain why power losses in cables are smaller when the voltage is greater