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Electricity (Electricity in home (2 types of electricity supply-…
Electricity
Electricity in home
2 types of electricity supply- alternating and direct current.
In ac supplies, current is constantly changing direction. Alternating currents produced by alternating voltages in which positive and negative ends keep alternating.
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Cells and batteries supply direct current. Direct current- current that is always flowing in same direction.- created by direct voltage.
Electrical appliances connected to mains supply by 3-core cables--3 wires with core of copper and coloured plastic coating.
Neutral wire (blue)- completes circuit and carries away current- electricity flows in through live wire and out through neutral- 0V.
Live wire (brown)- provides alternating p.d from mains supply.
Earth wire (green + yellow)- protecting wiring, and safety- stops appliance casing from becoming live. Doesn't carry current; only when fault- 0V.
Body is 0V. If touch live wire ; large p.d produced across body and current flows through you; large electric shock- injure or kill you. Even if plug socket or light switch turned off, still danger or electric shock. Current isn't flowing but still p.d in live wire. If made contact with live wire, body would provide a link between supply and earth so current flow through you.
Connection between live and earth can be dangerous- if link creates low resistance path to earth, huge current flow- fire.
Circuit devices
LDR- light dependent resistor. Dependent on intensity of light. In bright light, resistance falls. In darkness, resistance is highest. e.g. automatic night lights, outdoor lighting and burglar detectors.
Thermistor- temperature dependent resistor. In hot conditions, resistance drops. In cool conditions resistance rises. Make useful temperature detectors e.g. car engine temperature sensors and electronic thermostats.
Sensing circuits used to turn on or increase power to components depending on conditions. In sensing circuits, fixed resistor and fan will have same p.d across them as connected in parallel. p.d of power supply is shared out between thermistor and loop made up of fixed resistor and fan according to resistances- bigger components resistance, more of p.d it takes. As room gets hotter, resistance of thermistor decreases and takes smaller share of p.d from power supply- p.d across fixed resistor and fan rises, making fan go faster.
Also connect component across variable resistor. The greater the p.d across component, more energy it gets.
Series circuit
Different components connected in line, end to end between +ve and -ve of power supply. If remove or disconnect 1 component, circuit broken and all stop.
Total p.d of supply shared between various components- p.d around series circuit always add up to equal source p.d.
Current flows through all components. Size of current determined by total p.d of cells and total resistance of current.
Total resistance of 2 components is sum of resistances- as adding resistors make p.d shared.
P.d across each resistor is lower, so current through each resistor is also lower. Total current also reduced when resistor added- total resistance of circuit increases. The bigger a component's resistance, bigger its share of total p.d. Bigger p.d when more cells in series if all connected in same way.
Parallel circuits
Each component separately connected to +ve and -ve of supply. If remove or disconnect 1, hardly affect others. All components get full source p.d so same across all components.
Total current flowing around circuit is equal to total of all currents through separate components. In parallel circuit, there are junctions where current splits or rejoins. Total current going to junctions has to equal current leaving. If 2 identical components connected in parallel, same current flow through each component.
If 2 resistors in parallel, total resistance is less than resistance of smallest of 2 resistors: in parallel, both resistors have same p.d across them as source. So, pushing force making current flow = source p.d for each resistor you add.
Adding loop- current has more than 1 direction to go in; increase total current that can flow around circuit. Increase in current is decrease in total resistance of circuit.
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IV characteristics
For some components, as current is changed, resistance changes. Resistance of ohmic conductors doesn't change with current. At constant temperature, current flowing through ohmic conductor is directly proportional to p.d across it.
When electrical charge flows through filament lamp, transfers same energy to thermal energy store of filament. Resistance increases with with temperature so as current increases, filament lamp heats up more and resistance increases.
Diodes- resistance depends on direction of current. Lets current flow in 1 direction, but have very high resistance if reversed.
National grid
Throughout day, electricity usage changes. Power stations have to produce enough electricity for everyone to have it when needed. Demand increases when people get up in morning, come home from school or work, when gets dark or cold.
Power stations often run at well below maximum power output, so space capacity to cope with high demand when unexpected shut down of another station- smaller power stations kept on standby.
to transmit huge amount f power needed, high p.d or current. High current loses loads of energy as carries heat up and energy transferred to thermal energy store of surroundings. Cheaper to boos t p.d (400000V) and keep current low as possible. Increasing p.d decreases current. Transformers and big pylons with huge insulators allow p.d to reach 400000V. Transformers have to step p.d up at 1 end- efficient transmission and bring back down to safe, usable levels. P.d increased by step-up transformer. Reduced by step-down transformer- domestic use.
Resistance
Resistance depends on whether component is in series or parallel circuit or length of wire in circuit.
Ammeter- measures current (amps) flowing through test wire. Must be placed in series with whatever is tested.
Voltmeter- measures p.d (volts) across test wire. Must be placed in parallel of what investigating.
Power
When electrical charge goes through change in p.d, energy is transferred. Energy supplied to charge at power source to raise it through potential- gives up energy when charge falls through any potential drop in components elsewhere in circuit.
Battery with bigger p.d will supply more energy to circuit for every coulomb of charge which flows round it because charge raised up higher at start.