Electricity and magnetism

• magnetic forces are due to interactions between magnetic fields
• temporary magnets (made of soft iron)
• permanent magnets (made of steel)
• a magnetic field is a region in which a magnetic pole experiences a force
• the relative strength of a magnetic field is represented by the spacing of the magnetic field lines
• the direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point
• there are positive and negative charges
• positive charges repel other positive charges
• negative charges repel other negative charges
• positive charges attract negative charges
• charge is measured in coulombs
• an electric field is a region in which an electric charge experiences a force
• the direction of an electric field at a point is the direction of the force on a positive charge at that point
• charging of solids by friction involves only a transfer of negative charge (electrons)
• electric current is related to the flow of charge
• electric current is the charge passing a point per unit time
• conventional current is from positive to negative
• the flow of free electrons is from negative to positive
• electromotive force (e.m.f.) is the electrical work done by a source in moving a unit charge around a complete circuit
• e.m.f. is measured in volts (V)
• potential difference (p.d.) is the work done by a unit charge passing through a component
• the p.d. between two points is measured in volts (V)
• resistance is directly proportional to length
• resistance is inversely proportional to cross-sectional area
• the energy unit is the kWh
• electric circuits transfer energy from a source of electrical energy, such as an electrical cell or mains supply, to the circuit components and then into the surroundings
• the current at every point in a series circuit is the same
• the sum of the currents entering a junction in a parallel circuit is equal to the sum of the currents that leave the junction
• the total p.d. across the components in a series circuit is equal to the sum of the individual p.d.s across each component
• the p.d. across an arrangement of parallel resistances is the same as the p.d. across one branch in the arrangement of the parallel resistances
• for a parallel circuit, the current from the source is larger than the current in each branch
• the combined resistance of two resistors in parallel is less than that of either resistor by itself
• connecting lamps in parallel in a lighting circuit
• the sum of the currents into a junction is the same as the sum of the currents out of the junction
• the p.d. across an electrical conductor increases as its resistance increases for a constant current
• a mains circuit consists of a live wire (line wire), a neutral wire and an earth wire
• a switch must be connected to the live wire for the circuit to be switched off safely
• the outer casing of an electrical appliance must be either non-conducting (double-insulated) or earthed
• a fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance
• 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