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Boardworks GCSE Separate Sciences 2009 Transformers

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Presentation on theme: "Boardworks GCSE Separate Sciences 2009 Transformers"— Presentation transcript:

1 Boardworks GCSE Separate Sciences 2009 Transformers

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Current and magnetism Boardworks GCSE Separate Sciences 2009 Transformers Every electric current produces a magnetic field. The shape and strength of the magnetic field depends on the shape of the wire carrying the current. A single straight wire carrying a direct current is surrounded by a circular magnetic field: Every point on an infinite wire is equivalent to every other, so the magnetic field must be the same at every point – it is made up of concentric circles. A much stronger magnetic field can be made by twisting a wire into a tight coil, or solenoid. This creates a magnetic field like that of a bar magnet.

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Field around a wire Boardworks GCSE Separate Sciences 2009 Transformers The direction of the magnetic field around a straight wire can be worked out by using the right hand grip rule. Grip a wire so that your thumb points in the direction of the conventional current (from the positive to the negative terminal of a battery). + Your fingers will curl around the wire in the direction of the magnetic field (from north to south pole).

5 Field around a solenoid
Boardworks GCSE Separate Sciences 2009 Transformers The right hand grip rule can also be used to find the orientation of the magnetic field around a solenoid: N Grip the solenoid so that your fingers follow the direction of the conventional current. + Your thumb will now point towards the north pole of the electromagnet created by the solenoid. S Teacher notes Students should be made aware that the direction of a magnetic field is usually taken as being N–S rather than S–N, but in this rule the thumb is seen as pointing towards the north pole. The electromagnet can be made stronger by increasing the number of coils, or by adding an iron core.

6 Inducing current in a coil
Boardworks GCSE Separate Sciences 2009 Transformers

7 Electromagnetic induction
Boardworks GCSE Separate Sciences 2009 Transformers What do we know so far about the relationship between current and magnetism? All currents have a magnetic field associated with them. A wire in a changing magnetic field will experience an induced current. The first of these effects is the basis of the electromagnet. The second effect is called electromagnetic induction, or the dynamo effect. It converts movement into electrical energy. This is the basis of the generator. Photo credit: © 2009 Jupiterimages Corporation

8 Electricity and magnetism
Boardworks GCSE Separate Sciences 2009 Transformers

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10 Linking circuits with magnetism
Boardworks GCSE Separate Sciences 2009 Transformers

11 Linking circuits with magnetism – results
Boardworks GCSE Separate Sciences 2009 Transformers In the experiment, a current was induced in the second circuit when the first circuit was switching on or off. In order for power to be transferred continuously between two circuits, the current in the first circuit must be changing continuously. This can be achieved by using an alternating current. In order for as much power to be transferred as possible, the two circuits must be as closely magnetically linked as possible. This can be achieved by winding the two circuits into tight coils around an iron core. This is a transformer.

12 Primary side – how it works
Boardworks GCSE Separate Sciences 2009 Transformers A transformer links two circuits together. To understand how it works, it is important to look at each side separately. The primary side is simply an electromagnet. By passing an electric current through a coil of wire, we make a magnetic field, just like the field around a bar magnet. Direct current makes one end of the iron north, and the other end south. N S +

13 Secondary side – how it works
Boardworks GCSE Separate Sciences 2009 Transformers The secondary side is not connected directly to any power supply. It is just a piece of iron with some wire wrapped around it. The secondary side works using electromagnetic induction. To make a current flow, a magnetic field needs to be changing perpendicular to the coil. When there is an alternating current in the primary side, the direction of the magnetic field around the transformer alternates. This induces a second alternating current in the secondary side.

14 How a transformer works – summary
Boardworks GCSE Separate Sciences 2009 Transformers

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Parts of a transformer Boardworks GCSE Separate Sciences 2009 Transformers

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17 Properties of transformers
Boardworks GCSE Separate Sciences 2009 Transformers Transformers transfer power between circuits. The design of a transformer determines the characteristics of the electricity flowing in its secondary circuit. The frequency of the alternating current in the secondary circuit will always match the primary circuit, but what about current and voltage? The voltage in each circuit is related to the number of coils on each side of a transformer by the following equation: primary voltage primary turns = secondary voltage secondary turns Vp Np = Vs Ns

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Step-up transformers Boardworks GCSE Separate Sciences 2009 Transformers A step-up transformer is used to increase voltage. It has more turns on its secondary side than on its primary side. But the power in the secondary circuit cannot be greater than the power in the primary circuit, or the transformer would be more than 100% efficient! What is the relationship between power, voltage and current? P = V × I A step-up transformer increases voltage, but reduces current.

19 Step-up transformer calculations
Boardworks GCSE Separate Sciences 2009 Transformers A transformer has 100 turns on its primary coil. It has an input voltage of 35 V and an output voltage of 175 V. How many turns are on the secondary coil? Vp Np Vs = Ns = × Np Vs Ns Vp Vs Ns 175 Ns = × 100 = 35 Vp Np Ns Vs = turns = Np Vp

20 Step-up transformer uses
Boardworks GCSE Separate Sciences 2009 Transformers Step-up transformers are used in the following applications: power transmission Step-up transformers are used to increase the voltage generated in power stations, so that it can be transported around the country at extremely high voltages. using European appliances in the USA Teacher notes See slide 34 for more information on why the National Grid transports electricity at very high voltages. Photo credit: © Shutterstock 2009, Walter Matheson The USA mains runs at 110 V, while the UK uses 230 V. Goods made for the UK, but used in the USA, need a transformer to increase their supply voltage.

21 Step-down transformers
Boardworks GCSE Separate Sciences 2009 Transformers A step-down transformer is used to decrease voltage. It has fewer turns on its secondary side than on its primary side. This kind of transformer can be found in many places around the home, as a lot of appliances use lower voltages than the 230 V provided by the National Grid. Photo credit: © Shutterstock 2009, jocicalek A mobile charger, for instance, contains a step-down transformer, which is why it is larger than a normal plug. 21

22 Step-down transformers calculations
Boardworks GCSE Separate Sciences 2009 Transformers A transformer has 200 turns on its primary coil and 50 turns on its secondary coil. The input voltage is 920 V. What is the output voltage? Vp Np = Vs Ns Vs Ns = Vp Np Ns Vs = × Vp Np 50 Vs = × 920 = 230 V 200

23 Isolating transformers
Boardworks GCSE Separate Sciences 2009 Transformers An isolating transformer has the same number of coils on its primary and secondary sides. A transformer has 100 turns on the primary side, and 100 turns on the secondary side. If the primary voltage is 230 V, what is the secondary voltage? Vp Np Np = Ns = = 1 Vs Ns Np = 1 Vs = Vp = 230 V Ns

24 Why use an isolating transformer?
Boardworks GCSE Separate Sciences 2009 Transformers Isolating transformers do not change the voltage of a power supply. So what are they used for? Isolating transformers are used in devices such as electric shaver sockets, to isolate an appliance from the mains. By separating a device, such as a shaver, from its mains supply, the risk of shock is much reduced. This is important in a bathroom where electrical items are at risk of getting wet. Photo credit: © Shutterstock 2009, Maverick

25 Transformers around the home
Boardworks GCSE Separate Sciences 2009 Transformers

26 Step-down transformer uses
Boardworks GCSE Separate Sciences 2009 Transformers

27 Transformers around the home
Boardworks GCSE Separate Sciences 2009 Transformers How many transformers can you find in this house? Teacher notes This illustration contains several examples of transformers in use, for example: Several electrical items have large plugs, which contain transformers, including the digital phone and printer. Other items have normal plugs, but run on small voltages and must contain internal transformers, such as the television and games console. The laptop is connected to the mains by a cable that incorporates a box transformer. In the bathroom, an electrical shaver is visible. The shaver socket contains an isolating transformer. There are also a few red herrings in the picture, such as the kettle, toaster and electric heater, which all run on mains voltage.

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29 What is the National Grid?
Boardworks GCSE Separate Sciences 2009 Transformers The National Grid is a network of power lines designed to carry mains electricity around the country, from the power stations where it is generated to the homes and factories where it is used. Transformers are an important part of the National Grid, because electricity must be transported at a much higher voltage than it is generated at or used at in homes.

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Power loss in cables Boardworks GCSE Separate Sciences 2009 Transformers When electrical energy is carried in wires, a current must flow. There is a power loss in cables which is related to the amount of current flowing: power loss = current2 × resistance P = I2 × R Power is measured in watts (W). Current is measured in amps (A). Resistance is measured in ohms (Ω).

31 Power loss in cables – example
Boardworks GCSE Separate Sciences 2009 Transformers

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Transformer power Boardworks GCSE Separate Sciences 2009 Transformers A step-up transformer may increase voltage but it cannot create energy! primary secondary In a perfect transformer the power in is equal to the power out. As power = V × I, if voltage goes up, then current must go down. Vs Vp Is Ip power in = power out Pp = Ps Vp × Ip = Vs × Is

33 Transformer power example
Boardworks GCSE Separate Sciences 2009 Transformers A transformer has a primary voltage of 1000 V and a primary current of 0.5 A. If the secondary circuit has a current of 0.01A flowing, what is the secondary voltage? primary secondary Vp × Ip = Vs × Is Vs Vp Ip Vs = Vp × Is Is Ip 0.5 = 1000 × = V 0.01

34 Step-up transformers in the National Grid
Boardworks GCSE Separate Sciences 2009 Transformers A step-up transformer is positioned near a power station. This raises the voltage of the generated electricity, ready for transmission around the country. High voltages are used because a high voltage results in a low current flowing, for a fixed power. Photo credit: © Shutterstock 2009, Arogant A low current means the wires lose less energy as heat over long distances.

35 Step-down transformers in the National Grid
Boardworks GCSE Separate Sciences 2009 Transformers Step-down transformers are positioned close to homes and factories. They are used to reduce the voltage from the very high voltages used for transmission. High voltages are useful for saving energy, but are very dangerous. Household appliances need much lower voltages, so the voltage is reduced while the current increases, for a fixed amount of power. Photo credit: © Shutterstock 2009, marilyn barbone

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The National Grid Boardworks GCSE Separate Sciences 2009 Transformers

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Glossary Boardworks GCSE Separate Sciences 2009 Transformers Glossary alternating current – A flow of electricity that switches direction periodically, such as that supplied by the National Grid. direct current – A flow of electricity in a fixed direction, such as that supplied by a battery. dynamo effect – Another term for electromagnetic induction. electromagnetic induction – The effect whereby a current is induced in a wire by a changing magnetic field. generator – A device that uses the dynamo effect, or electromagnetic induction, to convert mechanical energy into electrical energy. isolating transformer – A transformer designed to isolate a circuit from the mains for safety reasons, without altering the supplied voltage. magnetic field – The area around a magnet where its influence can be felt. mains electricity – The domestic electrical power supply with a voltage of 230 V. National Grid – The network of high-voltage power lines that carries electricity from power stations across the country. power – The rate of electrical energy transfer in a circuit, which is equal to the product of current and voltage. power station – A facility where electrical power is generated. primary coil – The coil around the primary side of a transformer. primary side – The part of a transformer that uses an alternating power supply to produce an alternating magnetic field. right hand grip rule – A rule that uses the fingers and thumb of the right hand to predict the direction of the magnetic field around a current. secondary coil – The coil around the secondary side of a transformer. secondary side – The output side of a transformer, where the changing magnetic field in the transformer’s core induces an alternating current. step-down transformer – A transformer that reduces voltage, with a smaller number of turns on its secondary coil. step-up transformer – A transformer that increases voltage, with a larger number of turns on its secondary coil. transformer – A device that transfers power between two unconnected electrical circuits.

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Anagrams Boardworks GCSE Separate Sciences 2009 Transformers

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Multiple-choice quiz Boardworks GCSE Separate Sciences 2009 Transformers Teacher notes This multiple-choice quiz could be used as a plenary activity to assess students’ understanding of transformers. The questions can be skipped through without answering by clicking “next”. Students could be asked to complete the questions in their books and the activity could be concluded by completion on the IWB.


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