## 4- Elec & Mag

Electricity and Magnetism

Main Points

Simple phenomena of magnetism

• State the properties of magnets

• Give an account of induced magnetism

• Distinguish between ferrous and non-ferrous materials

• Describe methods of magnetisation and of demagnetisation

• Describe an experiment to identify the pattern of field lines round a bar magnet

• Distinguish between the magnetic properties of iron and steel

• Distinguish between the design and use of permanent magnets and electromagnets

Electrical quantities

Electric charge

• Describe simple experiments to show the production and detection of electrostatic charges

• State that there are positive and negative charges

• State that unlike charges attract and that like charges repel

• Describe an electric field as a region in which an electric charge experiences a force

• Distinguish between electrical conductors and insulators and give typical examples

Current

• State that current is related to the flow of charge

• Use and describe the use of an ammeter

Electro-motive force

• State that the e.m.f. of a source of electrical energy is measured in volts

Potential difference

• State that the potential difference across a circuit component is measured in volts

• Use and describe the use of a voltmeter

Resistance

• State that resistance = p.d./current and understand qualitatively how changes in p.d. or resistance affect current

• Recall and use the equation V/I

• Describe an experiment to determine resistance using a voltmeter and an ammeter

• Relate (without calculation) the resistance of a wire to its length and to its diameter

Electric circuits

Circuit diagrams

• Draw and interpret circuit diagrams containing sources, switches, resistors (fixed and variable), lamps, ammeters voltmeters, magnetising coils, transformers, bells, fuses and relays

Series and parallel circuits

• Understand that the current at every point in a series circuit is the same

• Give the combined resistance of two or more resistors in series

• State that, for a parallel circuit, the current from the source is larger than the current in each branch

• State that the combined resistance of two resistors in parallel is less than that of either resistor by itself

• State the advantages of connecting lamps in parallel in a lighting circuit

Action and use of circuit components

• Describe the action of a variable potential divider (potentiometer)

• Describe the action of thermistors and light-dependent resistors and show understanding of their use as input transducers

• Describe the action of a capacitor as an energy store and show understanding of its use in time delay circuits

• Describe the action of a relay and show understanding of its use in switching circuits

Dangers of electricity

• state the hazards of damaged insulation, overheating of cables, damp conditions
• Show an understanding of the use of fuses and circuit-breakers

Electromagnetic effects

Electromagnetic induction

• Describe an experiment that shows that a changing magnetic field can induce an e.m.f. in a circuit

a.c. generator

• Describe a rotating-coil generator and the use of slip rings

• Sketch a graph of voltage output against time for a simple a.c. generator

Transformer

• Describe the construction of a basic iron-cored transformer as used for voltage transformations

• Recall and use the equation (Vp /Vs) = (Np /Ns)

• Describe the use of the transformer in high voltage transmission of electricity & advantages

The magnetic effect of a current

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

• Describe applications of the magnetic effect of current, including the action of a relay

Force on a current-carrying conductor

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

(i) the current

(ii) the direction of the field

d.c. motor

• State that a current-carrying coil in a magnetic field experiences a turning effect and that the effect is increased by increasing the number of turns on the coil

• Relate this turning effect to the action of an electric motor

Cathode-ray oscilloscopes

Cathode rays

• Describe the production and detection of cathode rays

• Describe their deflection in electric fields

• State that the particles emitted in thermionic emission are electrons

Simple treatment of cathode-ray oscilloscope