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19/04/2017 Electricity.

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Presentation on theme: "19/04/2017 Electricity."— Presentation transcript:

1 19/04/2017 Electricity

2 Lesson 4 Revision of basics Mains Electricity AC/DC Safety Power
19/04/2017 Revision of basics Mains Electricity AC/DC Safety Power Cost of electricity Electric Charge

3 (Words: volts, amps, ohms, voltage, ammeter, voltmeter)
Basic ideas… 19/04/2017 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… 19/04/2017 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 Current is THE SAME at any point
19/04/2017 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 V R I Resistance = Voltage (in V) (in ) Current (in A)
Georg Simon Ohm 19/04/2017 Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. The resistance of a component can be calculated using Ohm’s Law: V R I Resistance = Voltage (in V) (in ) Current (in A)

7 Mains electricity 19/04/2017 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 19/04/2017 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.)

11 What is the difference between a.c. and d.c.?
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. d.c. time voltage a.c. time voltage peak forward peak reverse The voltage of a d.c. supply is steady and always in the same direction. The voltage of an a.c. supply follows a repeated pattern: it rises to a peak, returns to zero changes direction and so on.

12 Frequency of alternating current
The frequency of a.c. electricity is the number of complete cycles per second, which is measured in hertz (Hz). 1 complete cycle a.c. time voltage peak forward peak reverse 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. Teacher notes
This virtual experiment uses a oscilloscope connected to a power supply to enable students’ to compare a.c. and d.c. It could be used as a precursor to running the practical in the lab, or as a revision exercise. The current can be switched between alternating current and direct current. The dials can be used to alter the voltage and frequency. It should be highlighted that altering the frequency has no effect on the d.c. current. The static/dynamic button allows the wave to appear stationary (static) or move across the oscilloscope screen (dynamic).

14 Mains Circuits Every mains circuits has a live wire and a neutral wire
19/04/2017 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
19/04/2017 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 thin wire with high resistance case made of insulating material
How does a fuse work? A fuse is a built-in weak point in a circuit. It contains a thin wire with a higher resistance than normal wire. terminals Teacher notes The image shows a glass fuse, which is commonly used in cars but not with mains electricity. The advantage of this type of fuse is that the fuse wire is visible. thin wire with high resistance case made of insulating material When a large current flows the wire becomes hot. If too much current flows, the wire overheats and melts, which breaks the circuit.

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 4.3 A, what fuse should it be fitted with? You can choose from fuses of 3 A, 5A and 13A. 5 A fuse

21 Selecting a fuse Fuses may be 3, 5 or 13A
19/04/2017 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 19/04/2017 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!
Teacher notes This multiple-choice activity could be used to assess students’ understanding of the use of fuses. Coloured traffic light voting cards (green=3A, yellow=5A, red=13A) could be used to increase class participation.

24 Fuses Fuses

25 Fuse Fuse

26 Earth If an appliance has a metal case it needs to be earthed.
19/04/2017 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?
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. switch electromagnet 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 Boardworks GCSE Science: Physics Electricity to the Home
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) 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
19/04/2017 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 In other words, 1 Watt = 1 Joule per second
Energy and Power 19/04/2017 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 T P E = Energy (in joules) P = Power (in watts) T = Time (in seconds)

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

37 Power Power

38 Calculating the units of electricity
Boardworks GCSE Science: Physics Electricity to the Home Calculating the units of electricity 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. electrical energy = power x time Power is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in? 1 unit of electricity = 1 unit of electrical energy = 1 kilowatt hour (kWh) Example: How many units of electricity is 17.6 kWh? 17.6 units

39 How is electricity paid for?
Boardworks GCSE Science: Physics Electricity to the Home 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?
Boardworks GCSE Science: Physics Electricity to the Home The cost of electricity is the number of units of electrical energy multiplied by the cost per unit. cost = number of units x cost per unit Example: How much would 10 units of electricity cost at a price of 9p per unit? cost = 10 units x 9 p/unit = 90 p

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

42 Buying electricity – example
Boardworks GCSE Science: Physics Electricity to the Home 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 = 3 kW x 4 h = 12 kWh = 12 units Cost of electricity: cost = number of units x cost per unit = 12 units x 8 p / unit = 96p

43 Electric charge 19/04/2017 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)
19/04/2017 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)
19/04/2017 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 19/04/2017 4C 240C 2A 1A 5s 1.17s

47 Electrical power – calculations
Boardworks GCSE Science: Physics Electricity to the Home

48 Buying electricity – calculations
Boardworks GCSE Science: Physics Electricity to the Home

49 Quiz Quiz


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