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Electricity

Electricity. Lesson 4. Revision of basics Mains Electricity AC/DC Safety Power Cost of electricity Electric Charge. Basic ideas…. Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.

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Electricity

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  1. Electricity

  2. Lesson 4 • Revision of basics • Mains Electricity • AC/DC • Safety • Power • Cost of electricity • Electric Charge

  3. Basic ideas… Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____. Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta. Resistance is anything that resists an electric current. It is measured in _____.” (Words: volts, amps, ohms, voltage, ammeter, voltmeter)

  4. More basic ideas… If a battery is added the current will ________ because there is a greater _____ on the electrons If a bulb is added the current will _______ because there is greater ________ in the circuit

  5. Summary In a SERIES circuit: Current is THE SAME at any point Voltage SPLITS UP over each component In a PARALLEL circuit: Current SPLITS UP down each “strand” Voltage is THE SAME across each”strand”

  6. Resistance Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. Georg Simon Ohm 1789-1854 V Resistance = Voltage (in V) (in ) Current (in A) I R The resistance of a component can be calculated using Ohm’s Law:

  7. Mains electricity • Mains electricity (230 volts in the UK) is dangerous and can kill if not used safely • Electrical appliances can be connected to the mains using; • a cable (containing an inner core of copper, and an outer layer of plastic) • a plug (containing a plastic case, brass pins, fuse, earth pin and cable grip) • Mains electricity is an a.c. supply

  8. AC/DC • An alternating current (a.c.) constantly changes direction. 50Hz means the current changes direction 50 times in one second • Mains electricity is an a.c. supply • A direct current (d.c.) is found in battery/cell circuits. The current always flows in the same direction

  9. Direct current (d.c.) Direct current (d.c.)

  10. Alternating current (a.c.) Alternating current

  11. What is the difference between a.c. and d.c.? a.c. peak forward voltage d.c. voltage voltage time time peak reverse voltage The voltage of an a.c. supply follows a repeated pattern: it rises to a peak, returns to zero changes direction and so on. The difference between alternating current (a.c.) and direct current (d.c.) can be seen using an oscilloscope. For each current, the oscilloscope trace is a graph showing how the voltage of an electricity supply varies with time. The voltage of a d.c. supply is steady and always in the same direction.

  12. Frequency of alternating current 1 complete cycle a.c. peak forward voltage voltage time peak reverse voltage The frequency of a.c. electricity is the number of complete cycles per second, which is measured in hertz (Hz). The a.c. frequency can be determined from an oscilloscope by counting the number of complete waves per unit time. If the frequency is increased, the number of complete waves shown on the screen increases. For example, if the frequency is doubled, the number of waves doubles.

  13. Comparing a.c and d.c.

  14. Mains Circuits • Every mains circuits has a live wire and a neutral wire • Neutral wire is Earthed at a substation • Live wire alternates its voltage from + to – every cycle • It goes alternates between 325V (+to-)

  15. 230V • Because mains electricity alternates from +325V to -325 V • This is equivalent to a DC voltage 230V • We say that the mains voltage is 230volts • Changes 50 times per second (50Hz)

  16. Plug Plug

  17. Plug Plug

  18. What is a fuse? A fuse is a safety device that protects an electric cable from overheating so that the insulation does not catch fire. A fuse also makes appliances with a metal case safer. If the live wire becomes loose and touches the metal case, a very large current flows along the earth wire and blows the fuse. This makes it safe to touch the appliance.

  19. How does a fuse work? When a large current flows the wire becomes hot. If too much current flows, the wire overheats and melts, which breaks the circuit. A fuse is a built-in weak point in a circuit. It contains a thin wire with a higher resistance than normal wire. terminals case made of insulating material thin wire with high resistance

  20. How do you choose the correct fuse? A fuse is labelled with the maximum current that it will allow to flow through it. To choose the correct fuse for an electrical device, always choose the one with the closest rating that is greater than the operating current of the device. Example: If a kettle operates with an electrical current of 4.3A, what fuse should it be fitted with? You can choose from fuses of 3A, 5A and 13A. 5A fuse

  21. Selecting a fuse • Fuses may be 3, 5 or 13A • Calculate current of device, then select a Fuse • 500w heater at 230V • 500/230 = 2.2A • Therefore use 3A fuse

  22. Fuses • A fault in an appliance can cause the current to be too great and can lead to the wire overheating and possibly causing a fire. • A fuse is designed to melt if the current through a circuit is too high, thereby breaking the circuit • The fuse should be just higher than the normal working current • A circuit breaker can be reset rather than replaced

  23. You choose which fuse to use!

  24. Fuses Fuses

  25. Fuse Fuse

  26. Earth • If an appliance has a metal case it needs to be earthed. • A fault could cause the live wire to touch the metal case. If the appliance is earthed a large current flows down the earth wire and melts the fuse. If the appliance was not earthed there would be a risk of electrocution.

  27. Earth wire Earth wire

  28. An electrical fault (1) An electrical fault (1)

  29. An electrical fault (2) An electrical fault (2)

  30. What is a circuit breaker? The electrical wiring in a building must be protected from being overloaded so that it does not overheat. This is the job of a ‘fuse box’, which used to contain fuse wire but now contains circuit breakers to protect the wiring. Circuit breakers do the same job as fuses but they are electromagnetic switches which are easy to reset. A house has several circuits and each one is protected by a separate circuit breaker. Circuit breakers have different ratings as shown by the coloured dots.

  31. How does a circuit breaker work? switch electromagnet The circuit breakers in a ‘fuse box’ are some of the most important safety mechanisms in your home. Each circuit breaker is an electromagnetic switch which is designed to break the circuit when the current gets too high. Too much current makes the magnetic field produced by the electromagnet strong enough to open the switch. The circuit breaker is said to ‘trip’ and switches off the current.

  32. residual current device (RCD) A residual current device (RCD), also called a residual current circuit breaker (RCCB), is another type of circuit breaker. It prevents electric shocks when using extension cables to appliances like lawnmowers. This safety device compares the current in the live and neutral wires, which are the same when the appliance is working properly. If the current is leaking, the RCD detects a difference between the live and neutral wires and instantly shuts down the power.

  33. electrical power Electrical power is the rate at which an electrical appliance uses electrical energy. All appliances have a power rating. Power is measured in watts (W). 1000 watts = 1 kilowatt (kW).1 watt of power means that 1 joule of energy is used every second. Appliances that need to create heat, such as washing machines, cookers, hair dryers and kettles, usually use the most power. TVs, radios and computers usually use the least amount of power.

  34. Power • Power = Current x Voltage • Power is measure in Watts • 1 Watt means that 1 Joule of energy was changed in 1 second • Energy transferred = Voltage x Charge • Charge = Current x Time

  35. Energy and Power E P T The POWER RATING of an appliance is simply how much energy it uses every second. In other words, 1 Watt = 1 Joule per second E = Energy (in joules) P = Power (in watts) T = Time (in seconds)

  36. How is power calculated? A filament bulb has a potential difference of 200V across it and a current of 0.2A running through it. At what power is the filament bulb operating? P = IV = 0.2A x 200V = 40W

  37. Power Power

  38. Calculating the units of electricity electrical energy = power x time 1 unit of electricity = 1 unit of electrical energy The amount of electrical energy (i.e. the amount of electricity) used by an appliance depends on its power and how long the electricity is used for. Power is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in? = 1 kilowatt hour (kWh) Example: How many units of electricity is 17.6 kWh? 17.6 units

  39. How is electricity paid for? Electricity costs money, which is why every home has an electricity meter. The meter records how much electricity is used in a house in units of electrical energy. The units of electrical energy are called kilowatt hours (kWh). The cost of an electricity bill is calculated from the number of units used.

  40. How much does electricity cost? cost = number of units x cost per unit The cost of electricity is the number of units of electrical energy multiplied by the cost per unit. Example: How much would 10 units of electricity cost at a price of 9p per unit? cost = 10units x 9p/unit = 90p

  41. Buying electricity – example A kettle uses 45.2 kWh of energy.If electricity costs 10p per unit, how much does it cost to use the kettle? Number of units: number of units of electricity = number of kilowatt hours =45.2 units Cost of electricity: cost = number of units x cost per unit = 45.2units x 10p/unit = 452 p or £4.52

  42. Buying electricity – example An iron that operates at a power of 3 kW for 4 hours uses electricity that costs 8p per unit. How much does it cost for the electricity used by the iron in that time? Number of units: number of units of electricity = number of kilowatt hours = 3kW x 4h = 12kWh = 12units Cost of electricity: cost = number of unitsx cost per unit = 12 units x 8p/unit = 96p

  43. Electric charge • When a device is on, electrons are forced through the device by the P.D • The P.D causes a flow of charge through the device carried by the electrons • Unit of charge is a Coulomb (C).

  44. Charge Q • Unit is Coulomb (C) • Equal to the charge flow when current is 1 ampere for 1 second • Charge flow = I x t • Example – 5A in 200s is 1000 C • 1 A is 1 coulomb per second

  45. Calculations • Q=I x t • Calculate Q, (0.4A, 10s) • Calculate Q, (0.4A, 10 minutes) • Calculate I, (5s, 10C) • Calculate I, (15s, 15C) • Calculate t, (15C, 3A) • Calculate t, (10.5C, 9A)

  46. Answers • 4C • 240C • 2A • 1A • 5s • 1.17s

  47. Electrical power – calculations

  48. Buying electricity – calculations

  49. Quiz Quiz

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