Like charges repel, unlike attract. resistance. = voltage / current The diode has a very high R in the reverseDischarging is when a charged object is connected Ohmic conductors obey Ohm’s law where R is direction.to earth. constant. Eg metal wire. Graph:Friction removes electrons from insulators. Eg a The resistance of a filament lamp increases as the temperature ofpolythene rod becomes negative while the cloth the filament increases so itbecomes positive. is non-ohmic. The R of a thermistor decreases as the temperature increases. The R of a light-dependent resistor (LDR)Electrostatic charges are used in photocopiers to decreases as light intensity increases.attract the toner ink,in dust precipitators a positive grid gives the dust resistanceparticles a positive charge so they are attracted tothe negative grid. In computers we use digital signals consistingIn a defibrillator large paddles are charged up andplaced over your chest. static of 1 (on) or 0 (off). Analogue signals haveIn electrostatic paint sprayers the paint picks up electronic continuously varying values.charge as it comes out and the particles repel each More information or data can be transmittedother but get attracted to the oppositely charged communications using multiplexing – interleaving extra signalsobject. Electricity on the same line. Also less interference from noise.Aircraft need to be earthed when refuelling to Analogue signals from a microphone areprevent the inflammable vapour from exploding. sampled 6400 times a second and coded into a string of 8 ones or zeros. Eg 10110110. mains supply A laser reads a DVD or CD which has a spiral Logic gates of pits (0) or no pits (1).The mains supply is alternating in direction at 50Hz.The p.d. is about 230 V. for all the sockets in parallel The output Vout of a potential divider (a resistor and a sensor inBrown wire = live neutral = blue To change AC to DC we use a diode, which series) can be fed to a decision making logic gate.Earth = green and yellow. only conducts in one direction, to give half- The output of an OR gate is high (1) when either input A or inputIf the current exceeds the fuse rating, it will melt and wave rectification. B is high.break, protecting the cable from overheating. A circuitbreaker is used in a fuse box. The AND gate is high when input AMetal casings are connected to the earth wire. Full-wave rectification can be got with a and input B are high.Finding the cost: bridge circuit with 4 diodes.Energy = power x time We can smooth the output by using a The NOT gate inverts the input. 1 becomes 0. Kilowatt-hours = kW x hours capacitor, which will discharge its charge If an alarm system is activated it needs to stay on – we can use aYou will be told the cost of one kWh, usually about 8p when the voltage starts to drop. bistable latch circuit, which uses two NOR gates.