In order to get the G.C.S.E. grade you are capable of, you must make your own revision notes using your Physics notebook.

Transcription

1 In order to get the G.C.S.E. grade you are capable of, you must make your own revision notes using your Physics notebook. When summarising notes, use different colours and draw diagrams/pictures. If you do, you will find them easier to remember. Once you have made revision notes for a topic, re-visit these regularly (on the day of your examination you will not remember something you revised 4 weeks previously). Each time you re-visit a note tick the top of the page/card. This will allow you to identify any notes you have neglected. WARNING: DO NOT RELY SOLELY ON THE REVISION POWERPOINTS!

2 The electricity Board sells electricity in units known as kilowatt hours. One kilowatt hour (kwh) is the amount of electrical energy converted by a (1kW) one kilowatt appliance in one hour. The kilowatt hour is a measure of the amount of ENERGY converted. Energy = power x time = 1000 x (1x60x60) = J =3.6 x 106 J = 3.6MJ i.e. Electrical energy into other forms: 1kWh = 3.6MJ

3 The number of units or kilowatt hours used can be calculated using the formula: Number of kilowatt hours = power (kw) x time (h) The cost can be calculated using: COST = power (kw) x time (h) x cost of 1 unit or COST = number of kwh x cost of 1 kwh Always be careful to use the correct units!!!

4 D.C. or direct current flows around a circuit in ONE DIRECTION ONLY. Batteries produce D.C. Voltage or current + o - time Voltage or current A.C. or alternating current continually CHANGES DIRECTION, i.e. current flows one way and then the other. Mains voltage produces A.C. + o time

5 Live Wire: Essential for a complete circuit. It is at a high voltage and is brown in colour. It is always connected to the fuse and switch. Neutral Wire: Essential for a complete circuit. It has zero voltage and is blue in colour. Earth wire: Fuse: Used as a safety measure (see safety section). Used as a safety measure. It melts when the current becomes too high.

6 Memorize the diagram of the plug so that you can recognise any faults and/or label the diagram. Common faults: 1. Bare wire visible. 2. Wire connected to the wrong pins. 3. Wires visible below the cable grip.

7 There are two main types of switch: (A) ONE-WAY SWITCH If the switch is open no current (B) TWO-WAY SWITCH A two-way switch can direct current along one of two paths. A switch is always connected to the Live Wire.

8 Staircase The diagram shows a combination of 2 two-way switches. This is used in long hallways and staircases. It enables the light to be switched on and off at two different locations.

9 The THREE ways to reduce the dangers associated with Mains Electricity are: (A) Fuses (B) The Earth Wire (C) Double Insulation Note: these all have the sole purpose of preventing somebody getting an electric shock when using an electrical device. They do this either by isolating the device from the live connection (earth wire, fuses and circuit breakers) or by constructing the device so it isn t possible for a user to touch any live parts (double insulation)

10 (A) FUSES CIRCUIT SYMBOL: A fuse is a short piece of thin wire with a low melting point. When the current gets too high the fuse heats up and melts, breaking the circuit and preventing damage to electrical appliances and fires. The fuse is always connected to the Live wire.. There are two types of fuse: 1] The cartridge type which cannot be rewired. 2] A single piece of fuse wire which is rewireable. Choosing a fuse: Use the equation I = P/V (from P=IV). The chosen fuse must be close to this value but slightly greater so to allow the current to pass through.

11 (B) THE EARTH WIRE This is a low resistance wire connecting the case of an appliance to the Earth. If a fault develops in the appliance and the live wire touches the metal case then a large current will flow from the live wire to the low resistance earth wire and the fuse will burn out. This isolates the appliance from the live terminal If there is no Earth connection the case of the faulty appliance would be at 240V and anyone touching the live case would receive an electric shock. (C) DOUBLE INSULATION Used when an appliance does not have an earth connection, e.g. some electric drills and hair dryers. Defining Double Insulation: Symbol: The external metal parts which could become live if a fault developed are insulated from all the external parts which can be touched by the user.

12 1. Bar Magnet Put a magnet on a page and draw an outline around it. N S Dot 2. Dot 1. N S Place a compass at the north pole of the magnet and mark the ends of the needle N S Move the compass as shown and mark the position of the needle head. Repeat this until you reach the south pole of the magnet.

13 N S When this is repeated from several different points the magnetic field is shown to have the appearance opposite. Remember: 1. Field lines never touch or cross 2. Field lines point away from the north pole and towards the south pole.

14 2. Coil of wire A. Place magnets around a coil of wire. They should all point in the same direction north. B. Supply a current to the coil and note the direction in which the compasses point.

15 Conclusion S The shape of the magnetic field surrounding a current carrying solenoid (coil of wire) is similar to that which surrounds a bar magnet. N To determine which end of the solenoid is the north pole, grip the solenoid with your right hand so your fingers curl in the direction of the current. Your thumb will point to the north pole. + -

16 The strength of the magnetic field can be increased by: 1. Increasing the current 2. Increasing the turns density (number of turns per cm) 3. Placing a soft iron core through the centre of the solenoid A major advantage of an electromagnet over a bar magnet is that it can be switched on and off. Electromagnets are used in electric bells and electric relays.

17 If a conductor carries a current at right angles to a magnetic field it will experience a force. The direction of the force is reversed if: 1. The current flows in the opposite direction (see animation above) 2. The magnetic field changes direction (see animation above)

18 Using your left hand, point you first finger in the direction of the magnetic field and your second finger in the direction of the current. Your thumb will then point in the direction of the force.

19 An electric motor changes electrical energy into kinetic energy. 1. Current flows around the rectangular coil. 2. Current flows in opposite directions along sides AB and DC. 3. Sides AB and DC experience forces in opposite directions. 4. The coil turns.

20 This is the inducing (generating) of electricity in a conductor by changing the magnetic field through a conductor. The following experiments can be used to demonstrate electromagnetic (em) induction. Learn these! A current is only induced when the magnetic field through the conductor changes

21 Electromagnetic Induction is used in the following: 1) The A.C. generator. To wire is wound in the shape of a coil and rotated between the poles of a magnet. When the coil is rotated (by hand, or using a steam engine or windmill etc.) the magnetic field through it changes. A voltage is therefore induced in the coil causing an alternating current (A.C.) to flow. 2) Transformers A transformer consists of two circuits, each circuit containing a coil. The primary circuit is connected to an a.c. supply and therefore causes a voltage to be induced in the secondary circuit. (see next two slides for more detail)

22 To make the magnetic field change continuously the current through the primary coil must be alternating (a.c.). This changing magnetic field induces a current (and voltage) in the secondary coil. The coils can be linked using iron ring core. The voltage induced will depend on: (1) the number of turns in both coils and (2) the primary voltage. Where: V p = Primary voltage V s = Secondary voltage N p = Number of turns in the secondary coil N s = Number of turns in the primary coil The voltage produced can be calculated using the following equation: Vs V p N N s p

23 Transmission of Electricity to your home. In order to REDUCE the ENERGY LOST when transmitting current through overhead cables, electricity is TRANSMITTED AT A VERY HIGH VOLTAGE from the power station to our homes. Using P = IV, the higher the voltage is the lower the current will be and therefore the less energy that will be lost as heat. An A.C. VOLTAGE can easily be increased and decreased easily using a transformer. There are two types of Transformers: 1. STEP-UP TRANSFORMERS (N s >N p ) are used to increase the voltage at the POWER STATION. This is done to reduce power loss through the generation of heat. 2. STEP-DOWN TRANSFORMERS (N s <N p ) are used to reduce the voltage again at our HOMES.

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25 Cost Questions 1. At 7p per unit, how much more would it cost to run a 3kW fire for 4 hours than a 100W lamp for 100 hours? (click for solution) Cost = power x time x cost of 1 unit = 3 x 4 x 7 = 84 p Cost = power x time x cost of 1 unit = 0.1 x 100 x 7 = 70 p Difference = 14 p 2. An electric shower is rated at 7kW. How much would it cost to run for a whole year at 7 pence per unit if it was used on average for 3 hours per week? (click for solution) Cost = power x time x cost of 1 unit = 7 x (3 x 52) x 7 = 7644 p = 76.44

26 A.C./D.C. Questions 1) Draw the two graphs which represent alternating and direct current. (Click for solutions) Current Current Time Time A.C. D.C. 2) Name a source of alternating current and a source of direct current. (Click for solution) a.c. mains d.c. - battery

27 Plug Question Paper 1 June a) The diagram below shows a fused 13A plug. Five parts are labelled A, B, C, D and E. B D A C E (i) Complete the table to identify the wires connected to the three pins of the plug. Wire connected to: Live pin Neutral pin Earth pin Letter (A, B or C) C A B Click for solutions (ii) What names are given to the parts labelled D and E? D Fuse E Cable grip

28 A washing machine has a metal frame. A fault occurs so that the live wire connected to the motor is detached and makes contact with the metal frame. (iii) Explain fully how the fuse and the earth wire in the previous plug work together to prevent a person touching the frame being electrocuted. Click for solution A large current flows through the low resistance earth wire. The fuse blows. The appliance is isolated from the live terminal.

29 Switch Questions 1) Draw a circuit to show how a single light bulb can be switch on and off at the top and bottom of a staircase. (click for solution) x 2) Which wire is the switch always connected to in a mains electricity circuit? (click for solution) Live wire

30 Switch Questions Paper 2 June e) Mr Henderson is an electrician. He wires an electrical heater up to the mains supply. The incomplete circuit is shown below. Symbols: Live Fuse Heater Switch Mains supply Neutral (i) (ii) Complete the diagram by inserting its symbols shown above in the correct place. (click for solution) Placing the switch in the wrong position in mains circuit is dangerous. Explain why this is so. (click for solution) As when the switch is open the appliance will not be isolated from the live wire. Therefore, there is a risk of an electric shock.

31 Safety Questions 1) Calculate the operating current and suggest a suitable fuse for the following electrical appliances: 1) 1000W kettle operating at 240 V (click for solution) 2) 2000W washing machine operating at 240V (click for solution) Possible fuses: 1 A, 2 A, 5A, 8A and 10 A. 2) Describe the purpose of a fuse. (click for solution) 3) What is the function of double insulation and when is it not used in an appliance? (click for solution) 1) (a) I = P/V = 1000/240 = 4.17 A 5 A fuse 1) (b) I = P/V = 2000/240 = 8.33 A 10 A fuse 2) If the current exceeds the rating of the fuse the fuse melts. This isolates the appliance from the mains 3) To insulate all internal parts that can become live from the user. It is not used when there is an earth wire connected to the appliance.

32 Safety Questions 4dii) An electric drill operates at 240 V and has power of 950W. Calculate the correct fuse to fit to the plug of this drill. You should select your fuse from the following: 1A, 3A, 5A, 10A, 13A. You are advised to show clearly how you get your answer. P = VI I = P/V = 950/240 = 3.96 A Paper 2 June 2008 Therefore, a 5 A fuse should be used in the plug Safety Contents

33 Circuit Breaker Questions 1) What are the advantages of using a circuit breaker instead of a fuse? (click for solution) They don t have to be replaced They respond quicker 2) Explain how the circuit breaker below works. (click for solution) If the live wire touches the outer casing of an appliance a large current flows through the low resistance earth wire. The electromagnet in the live wire becomes strong enough to break the circuit Circuit Breaker Contents

34 EMI Question 1) A bar magnet is placed inside a coil of copper wire as shown below. The ends of the coil are joined to a sensitive centre-zero ammeter. What would be observed on the sensitive ammeter when the following actions are carried out? Record the observations by ticking ( ) in the appropriate box below. (click for solutions) Action No deflection of the pointer Pointer deflects and quickly returns to zero Steady deflection of the pointer 1 The coil and magnet are moved together to the left. 2 The coil and magnet are moved together to the right. 3 The magnet is moved quickly out of the coil to the left and held stationary. 4 The magnet is pushed, just as quickly as action 3, back into the coil and left there. 5 The coil is moved quickly away from the magnet and held stationary beyond the magnet. (in the opposite direction to 3) (in the same direction as 3)

35 EMI Question Paper 2 June c) A strong permanent magnet is placed inside a coil of insulated copper wire (Step 1). Permanent magnet (Step 1) The ends of the coil are now attached to a sensitive ammeter (Step 2). Permanent magnet (Step 2) Sensitive ammeter (i) (ii) Will an electric current be detected on the ammeter? Give a reason for your answer. No. The magnetic field through the coil does not change. Using no extra equipment, describe how you could make a current flow in the ammeter. Remove the magnet from the coil.

36 (iii) With the same equipment, what must be done to make the current larger? Move the magnet faster. (iv) Explain why the current would be larger. There would be a greater change in the magnetic field through the coil. (v) Name the process used to produce electric currents in this way. Electromagnetic induction

37 Transformers Questions 1.(a) Calculate the number of turns on the secondary coil of a step-down transformer which would enable a 12V bulb to be used with a 240V a.c. mains power if there are 480 turns on the primary. (click for solution) N s /N p = V s /V p N s / 480 = 240 / 12 N s = 9600 turns 2. Complete the following table: (Click for solutions) Primary p.d. Secondary p.d. Primary turns Secondary turns Step-up or step-down 100V 1000V Step-up 100V 10V Step-down 240V 12V Step-down 11000V Step-up

38 Transformers Question Paper 2 June d) The diagram represents a transformer with a primary coil of 90 turns and a secondary coil of 300 turns. (i) (ii) What type of operating voltage does a transformer use? Alternating What name is given to the type of transformer shown above? Step up (iii) The voltage connected to the primary coil is 12V. Calculate the output voltage of this transformer. V s /V p = N s /N p V s /12 = 300/90 V s /12 = 3.33 V s = 40 V