P2 - Physics Mains Electricity

P2 – Physics - Aims  to recognise errors in the wiring of a three-pin plug  to recognise dangerous practice in the use of mains electricity  to compare potential differences of d.c. supplies and the peak potential difference of a.c. supplies from diagrams of oscilloscope traces  (HIGHER ONLY) to determine the period and hence the frequency of a supply from diagrams of oscilloscope traces Mr. P. R. Collins

P2 – Physics – Mains Electricity  Mains electricity is useful but can be very dangerous. It is important to know how to use it safely Mr. P. R. Collins

P2 – Physics – Currents  Cells and batteries supply current which always passes in the same direction. This is called direct current (d.c.)  An alternating current (a.c.) is one which is constantly changing direction. Mains electricity is an a.c. supply. In the UK it has a frequency of 50 cycles per second (50 hertz)  UK mains supply is about 230 volts  Most electrical appliances are connected to the mains using cable and a three-pin plug Mr. P. R. Collins

P2 – Physics - Currents Voltage Time Direct Current – flows in one direction only 1/50 th second 230V Voltage Time Alternating Current – changes direction 50 times every second (frequency = 50 hertz) Mr. P. R. Collins

P2 – Physics – The Three-Pin Plug Earth Wire Fuse Live Wire Neutral Wire Insulation Cable Grip Mr. P. R. Collins

P2 – Physics – The Three-Pin Plug The neutral wire of a plug stays at a potential close to zero with respect to the earth The live wire of a plug alternates between positive and negative potential with respect to the neutral wire Mr. P. R. Collins

P2 – Physics - Safety  If an electrical fault causes too great a current the circuit should be switched off by a fuse or a circuit breaker  When the current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit  Appliances with metal cases are usually earthed  The earth wire and fuse together protect the appliance and the user Mr. P. R. Collins

P2 – Physics - Earth wires Earth wires are always used if an appliance has a _____ case. If there is a _____ in the appliance, causing the live wire to ______ the case, the current “_______” down the earth wire and the ______ blows. Words – fuse, fault, metal, surges, touch Mr. P. R. Collins

P2 – Physics - Fuses Fuses are _______ devices. If there is a fault in an appliance which causes the ____ and neutral (or earth) wire to cross then a ______ current will flow through the _____ and cause it to _____. This will break the _______ and protect the appliance and user from further _____. Words – large, harm, safety, melt, live, circuit, fuse Mr. P. R. Collins