Electricity

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Static Electricity

  • build up of static elecrticity is caused by friction
  • when certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and transferred on to the other 
  • this will leave a positive static charge on one and a negative static charge on the other 
  • the way of which the electrons are transferred depends on the materials involved 
  • electrically charged objects attract smaller objects put near them 
  • examples are polythene and acetate rods being rubbed with a cloth. With polythene rods, electrons move from the duster to the rod. With the acetate rod, electrons move from the rod to the duster. 
  • only electrons move - the positive charges dont
  • both +ve and -ve electrostatic charges are only produced by movement of electrons.
  • A positive static charge is caused by electrons moving away. the material that loses the electrons loses some negative charge, and is left with an equal charge.
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Static Electricity

  • Like charges repel and Opposite charges attract
  • two things with opposite electric charges are attracted to each other
  • two things with the same electric charge repel each other 
  • the forces get weaker the further apart the two things are
  • charges can move easily through conductors
  • electrical charges move easily through some materials = conductors
  • metals are known as good conductors = electrons are free to move
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Current and Potential Difference

  • current, A = the flow of elecrtric charge round the circuit. Current only flows through a component if there is a potential difference across the component. 
  • potential difference, V = the driving force that pushes the current round
  • resistance, Ω = anything in the circuit that slows the flow down
  • The greater the resistance across a component, the smaller the current that flows. 
  • Total charge through a circuit depends on current and time
  • current = the rate of flow of charge. when current, I, flows past a point in a circuit for a length of time, t, the charge, Q, that has passed is given by the formula.. I = Q/t, current, A = charge, C / time, s
  • more charge passes around the circuit when a bigger current flows 
  • Potential Difference (P.D) is the work done per unit charge
  • the p.d of voltage is the work fone per coulomb of charge that passes between two points in an electrical circuit.
  • the p.d across an electrical component is the amount of energy that is transferred by that electrical component per unit of charge
  • voltage and p.d mean the same thing... P.D, V = work done, J /charge, C...  V= W/Q
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Circuits

  • need to know: cell, battery, switch open, switch closed, filament lamp, fuse, LED, resistor, variable resistor, ammeter, voltmeter, diode, LDR, thermistor
  • Testing a circuit:
  • to know the resistance of a component, you find the resistance by measuring the current through the potential difference across the component
  • the ammeter: measures the current, A, flowing through the component...must be placed in series...can be put anywhere in series in the main circuit but never in parallel.
  • the voltmeter: measures P.D, V, across the component...must be placed in parallel around the component under test 
  • the basic circuit is used for testing components and getting V-I graphs 
  • the component, the ammeter and variable resistor are all in series - can be put in any order in the main circuit. The voltmenet only in parallel around the component under test.
  • as you vary the variable resistor, the current lowing through the circuit is altered = allows you to take several pairs of readings from ammeter and voltmeter
  • then able to plot the values for current and voltage on a V-I graph and find resistance. 
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Resistance

  • Resistance increases with temperature 
  • when an electrical charge flows through a resistor, some electrical energy is transferred to heat energy and the resistor gets hot. 
  • heat energy causes the ions in the conductor to vibrate more = more difficult for charge carrying electrons through to the resistor - the current can't flow as easily as the resistance increases
  • for most resistors there is a limit to the amount of current that can flow. more current means an increase in temperature, meaning an increase in resistance = current decreases again 
  • Potential Difference, V = Current, A x Resistance, Ω       V = I x R
  • For straight line graphs, the resistance of the component is steady and equal to the inverse of the gradient of the line 1/gradient 
  • If the graph curves, it means the resistance is changing. 
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Resistance Graphs

1 = resistor..current through a resistor id directly proportional to P.D, different resistors have different resistances. different slopes but straight lines 

2 = filament lamp...as the temp of the filament increases, the resistance increases. curve

3 = diode...current only flows through in one direction - has high resistance in opposite direction. increase

1.      2. (http://www.docbrown.info/page20/page20images/IVgraph2.jpg)   3. (http://www.docbrown.info/page20/page20images/IVgraph3.jpg)

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Circuit Devices

  • current only flows in one direction through a Diode
  • diode = special device made from semiconductor material, used to regulate the potential difference in circuits, lets current flow freely through it in one direction, not in the other, useful in electronic circuits.
  • LED = light-emitting diodes, very useful, emits light when a current flows through in a forward direction, being used more as lighting, use a smaller current than other forms of lighting, indicate the presence of current in a circuit, often used in appliances, used for traffic lights.
  • LDR = light dependent resistor, dependent on the intensity of light, in bright light the resistance falls, in darkness, the resistance is highest, lots of applications inc. automatic night lights, outdoor lighting and burglar detectors 
  • Resistance of a Thermistor decreases as temp increases = a thermistor is a temp dependent resistor, in hot conditions, the resistance drops, in cold condidtions, the resistance goes up, thermistors make useful temp detectors eg..car engine 
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Series Circuits

  • series circuits - all or nothing
  • in series circuits, the different components are connected in a line end to end between +ve and -ve of the power supply, if you remove or disconnect one component, the circuit is broken, this isnt very handy, very few things are connected in series
  • Potential Difference is shared: in series, the total P.D of the supply is shared between other components, the voltages roung a series circuit always add up to equal the source voltage - V = V1+V2+...
  • Current is the same everywhere: in series the same current flows through all parts of the circuit - A1 = A2, the size of the current is determined by the total P.D of the cells and the total resistance of the circuit. 
  • Resistance adds up: in series, the total resistance is just the sum of all resistances - R = R1+R2+R3, the bigger the resistance of a component, the bigger its share of total P.D.
  • Cell voltages add up: bigger P.D when more cells are in series, provided the cells are all connected in the same way 
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Parallel Circuits

  • parallel circuits - independence and isolation
  • in parallel circuits, each component is seperately connected to the +ve and -ve of the supply.
  • if you remove or disconnect one of them, it will hardly affect the others.
  • this is how most things are connected. 
  • P.D is the same across all components: in parallel circuits, all components get the full sourcE P.D so the voltage is the same across all components - V1 = V2 = V3, identical bulbs connected in parallel will be at the same brightness
  • Current is shared between branches: in parallel, the total current flowing round the circuit = the total of all currents through seperate components -  A = A1+A2+..., there are junctions where the current splits or rejoins, the total current going into a junction has to equal the total current leaving, if 2 identical components are connected in parallel, the same current will flow through each component
  • Voltmeters and Ammeters are exceptions: ammeters are always connected in series, voltmeters are always connected in paralles with a component. 
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Mains Electricity

  • mains supply is AC (alternating current) and battery supply is DC (direct current - same direction)
  • UK mains supply is 230V, it is an AC supply - constantly changing direction, frequency of an AC supply is 50 cycles per second, or 50Hz. 
  • Electricity supplies can be shown on an Oscilloscope screen - cathode ray oscilloscope(CRO) is a better voltmeter. by plugging an AC supply to an oscilloscope, a "trace" appears on screen showing how the voltage of the supply changes with time, the trace goes up and down in a regular pattern.....with a DC supply, the trace is a straight line
  • the vertical height of the AC trace shows the input of voltage at that point, by measuring the height of the trace, you can find potential difference of the AC supply
  • for DC - the voltage is the distance from the straight line trace to the centre line 
  • Frequency, Hz, = 1/time period, s
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Electricity in the Home

  • hazards in the home: -long cables, -frayed cables -cables in contact with something hot or wet, -water near sockets, -shoving things into sockets, -damaged plugs, -too many plugs in one socket, -lighting sockets without bulbs, -appliances without covers on
  • Cables have 3 separate wires (three-core cables):...1. brown LIVE WIRE alternating between high +ve and -ve voltage.....2. NEUTRAL WIRE always at OV - electricity normally flows through live and neutral wire only.....3. EARTH WIRE protects wiring and works together with a fuse preventing fire and shocks - attatched to metal casing of the plug and carries electricity to the earth if something went wrong. 
  • 3-pin plugs - no bare wires showing inside the plug, different appliances need different amounts of electrical energy, thicker cables have less resistance so carry more current. 
  • plug features: -metal parts made of copper or brass, very good conductors  -the case, cable grip and cable insulation are made of rubber / plastic, good insulators and flexible
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Energy and Power in Circuits

  • energy is transferred from cells and other sources, anything that supplies electricity also supplies energy 
  • cells, batteries, generators are all transfering energy to components in a circuit
  • Motion (kinetic energy) - motors, Light (light energy) - light bulbs, Heat (heat energy) - hair dryers/kettles, Sound (sound energy) - speakers...the cell provides the energy
  • all resistors produce heat when a current flows through them
  • when a current flows through anything with electrical resistance, electrical energy is converted into heat energy, the more surrent that flows, the more heat energy is produced, a bigger voltage means more heating as it pushes more current through. 
  • Filament bulbs work by passing a current through a thin wire, heating it up so it glows - waste a lot of energy as heat. 
  • if an appliance is efficient, it wastes less energy 
  • when you buy electrical appliances there is a choice to buy ones that are more energy efficient
  • these appliances transfer more of their total electrical energy output to useful energy
  • Energy Transferred, kJ  = Power rating x time, s
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Power and Energy Change

  • Power = current x potential difference....P = I x V 
  • electrical goods show power rating and voltage rating, to work out the size of the fuse needed, work out the current that the item will normally use. 
  • the potential difference is the energy transferred per charge passed 
  • energy is supplied to the power source to "raise" it through a potential
  • the charge gives up the energy when it falls through a potential drop in components in the circuit
  • Energy transformed = Charge x Potential Difference,   E = Q x V
  • the bigger the change in P.D the more energy is transferred for a given amount of charge passing through the circuit = the battery with bigger voltage supplies more energy to the circuit for every coulomb of charge that flows around it, as the charge is raised up higher at the start and more energy will be dissipated into the circuit
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