Mains Electricity Syllabus Lesson 1 Safe Electricity Lesson 2 Power Lesson 3 Heating
Mains electricity • identify common materials which are electrical conductors or insulators, including metals and plastics (P2.2) • recall the hazards of electricity including frayed cables, long cables, damaged plugs, water around sockets, and pushing metal objects into sockets (P2.3) • describe the uses of insulation, double insulation, earthing, fuses and circuit breakers in a range of domestic appliances (P2.4) • know some of the different ways in which electrical heating is used in a variety of domestic contexts (P2.5) • understand that a current in a resistor results in the electrical transfer of energy and an increase in temperature (P2.6) • recall and use the relationship power = current × voltage (P = I × V) and apply the relationship to the selection of appropriate fuses (P2.7) • use the relationship between energy transferred, current, voltage and time: energy transferred = current × voltage × time (E = I × V × t) (P2.8) • recall that mains electricity is alternating current (a.c.) and understand the difference between this and the direct current (d.c.) supplied by a cell or battery (P2.9).
Safe Electricity16/07/2014 Aim Explain the safety features in electrical equipment Connect a plug
How can electricity be used safely? • Do not overload sockets. • Wire plugs correctly, and check they are not damaged. • Never mix water and electricity. • Regularly check cables for fraying. • Do not stick anything other than a plug in a socket.
What is inside an electrical cable? copper • The wires are made of copper (a good conductor) • The wires are surrounded by plastic (an insulator) • In each cable there are two or three wires insulating plastic
What does each wire do? The live wirecarries a current that alternates between a negative and positive voltage. The earth wireis a safety wire that is needed to earth appliances with a metal case. This makes it safer to touch the appliance if it develops a fault. The neutral wirecompletes the circuit. It is kept at a zero voltage by the electricity company.
The fuse does two jobs. • It protects the wiring if something goes wrong. • It can also protect us • A fuse has a rating in amps (ie 13A) the rating of the fuse must be greater than the current used by a device
How does a circuit breaker work? 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.
What does ‘double insulated’ mean? Another way of improving the safety of electrical appliances is to make them double insulated. • Double insulatedappliances have plastic cases • Thus the case cannot become live • So even if a wire inside the case breaks the appliance is still safe symbol on double insulated appliance
Earth Wire Live Wire Fuse Neutral Wire Outer Insulation Cable Grip
Electrical Safety An electric iron is fitted with an earth and a plug containing a fuse Describe briefly how a fuse works Give two reasons why a fuse in a plug is important for the safe use of mains-operated appliance Why should the fuse in the plug be connected to the live wire? Explain how earthing the iron protects the user from receiving an electric shock A fault in an electrical circuit can cause too great a current to flow. Some circuits are switched off by a circuit breaker. One type of Circuit breaker is shown. A normal current is flowing. Explain, in detail, what happens when a current which is bigger than normal flows. Worksheet
Safe Electricityrecap Aim Connect a plug Explain the safety features in electrical equipment
Power16/07/2014 Aim explain alternating current compare ac and dc traces calculate Power Starter: a d e b f c
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1½ V 6V 230V 12 V
- + 1 ½ V
Mains Electricity Mains electricity is Alternating Current. Small Circuits (Batteries) use Direct Current This means that the current changes from positive to negative. Frequency (Hz) is number of waves per second a.c. d.c. voltage voltage time time
Power • Power is “the rate of doing work” • Or how much energy does it use a second
Calculating Power - Mechanical Method: • Whose the Most Powerful in the Class?
Power E P t • Power is “the rate of doing work” • (or how much energy it uses in a second) Energy = Power x time ( joule)(Watt ) (seconds) E = Energy (J) P = Power (W) t = time (s)
Some example questions • What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds? • What is the power of an electric fire that transfers 10,000J of energy in 5 seconds? • Farhun runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what is his power rating? • How much energy does a 150W light bulb transfer in • one second, b) one minute? • Shaun’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a physics lesson? • Damien’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day?
Calculating Power – Electrical Method - - - - - - - - - - - - - - - - • What would happen if a greater voltage was applied • What would happen if the bulb was replaced with a more powerful one?
Power and fuses P I V Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by: Power =currentxvoltage in W in A in V Peter Is a Veggie Using this equation we can work out the fuse rating for any appliance. For example, a 3kW (3000W) fire plugged into a 240V supply would need a current of _______ A, so a _______ amp fuse would be used (fuse values are usually 3, 5 or 13A).
Power and fuses P I V Copy and complete the following table:
Mains Electricityrecap Aim explain alternating current compare ac and dc traces calculate Power
Heating16/07/2014 Aim To understand the mechanism of electrical heating To name uses for resistors in homes To calculate the energy transferred by a heater
Electrical Heating • As the electrons flow through a wire they will occasionally collide with an atom. • This results in a transfer of energy to the wire causing it to heat.
Electrical Heating • Resistors are used in the home mostly for heating: • Kettle • Electric Fire • Electric Oven • Heat Bulb • Hair Dryer • Electric Radiators / Heaters
Power Equations Combining the equations: Energy = Power time Energy = voltage x current time Energy = voltage x current x time E = V x I x t (or E = V I t) (J) (V) (A) (s) Power = current x voltage Energy = Power x time
Example Question: • A 12V immersion heater draws 2 A of current for 100 seconds how much energy is transferred? • What do we know? • V = 12V • I = 2A • t = 100s • E = V x I x t E = 12 x 2 x 100 E = 2400 J
Experiment: Measuring the Energy Transferred A V Method: • Measure 250 ml of water into a 250 ml beaker • Connect up the circuit • Use a clamp to suspend the immersion heater in the beaker • Turn on the immersion heater • Measure the temperature, Voltage and Current and start your stopwatch • Take readings every minute for 10 minutes Results: Plot a graph of Energy (X) against Temperature (Y)
Heatingrecap Aim To name uses for heaters in homes To understand the mechanism of electrical heating To calculate the energy transferred by a heater