2 Lesson 4 Revision of basics Mains Electricity AC/DC Safety Power 19/04/2017Revision of basicsMains ElectricityAC/DCSafetyPowerCost of electricityElectric Charge
3 (Words: volts, amps, ohms, voltage, ammeter, voltmeter) Basic ideas…19/04/2017Electric 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…19/04/2017If a battery is added the current will ________ because there is a greater _____ on the electronsIf a bulb is added the current will _______ because there is greater ________ in the circuit
5 Summary Current is THE SAME at any point 19/04/2017In a SERIES circuit:Current is THE SAME at any pointVoltage SPLITS UP over each componentIn 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 Ohm19/04/2017Resistance 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:VRIResistance = Voltage (in V)(in ) Current (in A)
7 Mains electricity19/04/2017Mains electricity (230 volts in the UK) is dangerous and can kill if not used safelyElectrical 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/DC19/04/2017An alternating current (a.c.) constantly changes direction. 50Hz means the current changes direction 50 times in one secondMains electricity is an a.c. supplyA direct current (d.c.) is found in battery/cell circuits. The current always flows in the same direction
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.timevoltagea.c.timevoltagepeak forwardpeak reverseThe 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 zerochanges 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 cyclea.c.timevoltagepeak forwardpeak reverseThe 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 thefrequency 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 19/04/2017Every mains circuits has a live wire and a neutral wireNeutral wire is Earthed at a substationLive wire alternates its voltage from + to – every cycleIt goes alternates between 325V (+to-)
15 230V Because mains electricity alternates from +325V to -325 V 19/04/2017Because mains electricity alternates from +325V to -325 VThis is equivalent to a DC voltage 230VWe say that the mains voltage is 230voltsChanges 50 times per second (50Hz)
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 theappliance.
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.terminalsTeacher notesThe 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 resistancecase made of insulating materialWhen a large current flows the wire becomes hot. If too much current flows, the wire overheats and melts, whichbreaks 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 19/04/2017Fuses may be 3, 5 or 13ACalculate current of device, then select a Fuse500w heater at 230V500/230 = 2.2ATherefore use 3A fuse
22 Fuses19/04/2017A 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 circuitThe fuse should be just higher than the normal working currentA circuit breaker can be reset rather than replaced
23 You choose which fuse to use! Teacher notesThis 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.
26 Earth If an appliance has a metal case it needs to be earthed. 19/04/2017If 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.
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 whichis designed to break the circuit when the current gets too high.switchelectromagnetToo 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 powerElectrical 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 19/04/2017Power = Current x VoltagePower is measure in Watts1 Watt means that 1 Joule of energy was changed in 1 secondEnergy transferred = Voltage x ChargeCharge = Current x Time
35 In other words, 1 Watt = 1 Joule per second Energy and Power19/04/2017The POWER RATING of an appliance is simply how much energy it uses every second.In other words, 1 Watt = 1 Joule per secondETPE = Energy (in joules)P = Power (in watts)T = Time (in seconds)
36 How is power calculated? Boardworks GCSE Science: PhysicsElectricity to the HomeHow 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
38 Calculating the units of electricity Boardworks GCSE Science: PhysicsElectricity to the HomeCalculating the units of electricityThe amount of electrical energy (i.e. the amount of electricity) used by an appliance depends on its power and how longthe electricity is used for.electrical energy = power x timePower 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: PhysicsElectricity to the HomeHow 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: PhysicsElectricity to the HomeThe cost of electricity is the number of units of electrical energy multiplied by the cost per unit.cost = number of units x cost per unitExample: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: PhysicsElectricity to the HomeA 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= unitsCost 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: PhysicsElectricity to the HomeAn 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 unitsCost of electricity:cost = number of units x cost per unit= 12 units x 8 p / unit= 96p
43 Electric charge19/04/2017When a device is on, electrons are forced through the device by the P.DThe P.D causes a flow of charge through the device carried by the electronsUnit of charge is a Coulomb (C).
44 Charge Q Unit is Coulomb (C) 19/04/2017Unit is Coulomb (C)Equal to the charge flow when current is 1 ampere for 1 secondCharge flow = I x tExample – 5A in 200s is 1000 C1 A is 1 coulomb per second
45 Calculations Q=I x t Calculate Q, (0.4A, 10s) 19/04/2017Q=I x tCalculate 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)