Book pg. 193 194 Syllabus 6.17 6.20 www.cgrahamphysics.com
Transformers - YouTube [720p].mp4 Not me a real transformer www.cgrahamphysics.com
Understand how transformers work ALL State the func,on of step up and step down transformers MOST Explain how transformers work and perform transformer calcula,ons SOME Explain why values from equa,ons are just an approxima,on KEYWORDS: transformer, core, step up, Step down, primary, secondary, induced www.cgrahamphysics.com
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Transformers 5 of 32 Boardworks Ltd 2012 Power can be transferred between circuits using two coils wound around a soft iron core. This is called a transformer. The alternating current in the primary (input) coil produces an alternating magnetic field. This alternating magnetic field induces an alternating current in the secondary (output) coil. This is the circuit symbol for a transformer: iron core primary coil secondary coil
Primary side how it works 6 of 32 Boardworks Ltd 2012 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, it makes a magnetic field, just like the field around a bar magnet. A direct current makes one end of the iron north, and the other end south. It also causes a potential difference between the ends of the coil. What would happen with an alternating current? + N S
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Secondary side how it works 8 of 32 Boardworks Ltd 2012 The secondary side coil is not connected directly to the primary side or any power supply. The secondary side works using electromagnetic induction. When the magnetic field is perpendicular to the secondary coil and changes, a current is induced in the coil. When there is an alternating current in the primary side, the magnetic field around the transformer alternates. This induces an alternating current in the secondary side coil, and a potential difference across it. +
Parts of a transformer 9 of 32 Boardworks Ltd 2012
Investigating transformers 10 of 32 Boardworks Ltd 2012
Using a transformer to change voltage 11 of 32 Boardworks Ltd 2012 The voltage induced in the secondary (output) coil depends on the number of turns on the primary and secondary coils. A step-up transformer has more turns on the secondary coil and so increases voltage. A step-down transformer has fewer turns on the secondary coil and so decreases voltage.
Properties of transformers 12 of 32 Boardworks Ltd 2012 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 matches the primary circuit, but what about voltage? In an ideal transformer, the voltage in each circuit is related to the number of coils on each side by the following formula: primary voltage secondary voltage = primary turns secondary turns V p V s = N p N s
Step-up transformers Step-up transformers increase the voltage. The secondary coil has more turns than the primary coil. Since there are more turns, a larger potential difference is generated across the secondary coil. + + V s = N S N P V p ratio greater than 1 13 of 32 Boardworks Ltd 2012
Step-down transformers Step-down transformers decrease the voltage. The secondary coil has fewer turns than the primary coil. Since there are fewer turns, a smaller potential difference is generated across the secondary coil. + + V s = N S N P V p ratio less than 1 14 of 32 Boardworks Ltd 2012
LO: Understand how transformers work 15 of 32 Boardworks Ltd 2012 Example question A transformer has 400 turns on the primary coil and 200 on the secondary. If the primary voltage is 300v, what will the secondary voltage be? Given V S =? V P =300V N S =200 N P =400 Step down transformer Solution: V S = 200/400 300=150V
LO: Understand how transformers work 16 of 32 Boardworks Ltd 2012 Example question A transformer has 175 turns on the primary coil and 525 on the secondary. If the secondary voltage is 300v, what will the primary voltage be? Given: V S = 300V P =? S =525 P =175 Solution V P / V S = N P / N S V P = N P / N S x V S V P = 175/525 x300=100v Step up transformer
LO: Understand how transformers work 17 of 32 Boardworks Ltd 2012 How transformers work 1. The alternating current in the primary coil makes the iron core into an electromagnetic 2. As the current is alternating, the magnetic field moves and also changes direction 3. This moving magnetic field causes a current to be induced in the secondary coil Make a flow diagram of how a transformer works in your book and draw a diagram to go with each stage 4. If the number of coils on the secondary is higher, the potential difference will increase and it is a step-up transformer 5. If the number of coils on the secondary is lower, the potential difference will decrease and it is a stepdown transformer
Transformer power 18 of 32 Boardworks Ltd 2012 Transformers not only change voltage; they change current. For example, if the voltage increases, such as in a step-up transformer, the current decreases. This is because for any transformer: and: power in = power out power = voltage current this gives: V p I p = V s I s V s V p + + I p I s
Transformer power example 19 of 32 Boardworks Ltd 2012 A transformer has a primary voltage of 1,000 V and a primary current of 0.5 A. If the secondary circuit has a current of 0.01 A flowing, what is the secondary voltage? Vp Ip = Vs Is Vs = Vp Ip Is = 1,000 V = 50,000 V 0.5 A 0.01 A V s V p + + I p I s
LO: Understand how transformers work Example question A transformer has a primary voltage of 30v and a primary current of 2A. If the secondary voltage is 15v, what will the secondary current be? Given: V P =30V V S =15V I P =2A I S =? More current More or less current? Solution I S = V P x I P / V S I S = 30x2/15 =4A 20 of 32 Boardworks Ltd 2012
LO: Understand how transformers work 21 of 32 Boardworks Ltd 2012 Example question A transformer has a primary current of 10A. If the secondary voltage is 2v and the secondary current is 20A, what will the primary voltage be? Step up or step down transformer? Given: V P =? V S =2V I P =10A I S =20A Solution V P = V S x I S / I P V P = 2 20/10 =4V Current increases à step-down transformer
Power loss in wires 22 of 32 Boardworks Ltd 2012 Wires have resistance, so there is a power lost in the form of heat when current flows through them: power loss = current 2 resistance P = I 2 R l power is measured in watts (W) l current is measured in amps (A) l resistance is measured in ohms (Ω). When transmitting electricity, how does a transformer help to minimize the power lost?
Transformers and the supply chain 23 of 32 Boardworks Ltd 2012
Plenary: Why use high voltages for distribution? 24 of 32 Boardworks Ltd 2012
Plenary - Calculating voltage 25 of 32 Boardworks Ltd 2012