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Published byTheodora Davidson Modified over 8 years ago
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Do now! Can you finish the questions you started last lesson?
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Last lesson - Electromagnetic induction If a magnet is moved inside a coil an electric current is induced (produced) COPY this please!
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Electromagnetic induction A electric current is induced because the magnetic field around the coil is changing.
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Generator/dynamo A generator works by rotating a coil in a magnetic field (or rotating a magnet in a coil). A slip-ring commutator produces alternating current (a.c.) Can you stick this in please?
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COPY this please!
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Today’s lesson - Transformers
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Stand up!
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☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺
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☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ cell energy electron lamp
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Coulomb of charge (electrons) Think of it as a “bag of electrons” (containing 6000000000000000000 electrons!) ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺
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Current The number of Coulombs flowing past a point in the circuit every second. ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ A I’m counting how many coulombs of electrons go past me every second 1 Amp = 1 coulomb per second
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In a series circuit Current is the same at any point in the circuit 2.5 A
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In a parallel circuit The current splits (total current stays the same) 2.5 A 1.25 A
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Voltage(emf) ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ V I’m checking the difference in energy (per coulomb) between the 2 red arrows 1 Volt = 1 Joule per coulomb
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Voltage (p.d.) ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ V I’m checking the difference in energy (per coulomb) before and after the lamp 1 Volt = 1 Joule per coulomb
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In a series circuit The sum of the voltages across the lamps equals the voltage across the cells 9 V 3 V
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In a parallel circuit In a simple parallel circuit, voltage across each lamp equals the voltage across the cells 5 V
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Resistance Measures how difficult it is for current to flow. Measured in Ohms (Ω) VA Resistance = voltage/currentR = V/I
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Ohm’s Law V = IR V R I X ☺ Can you copy this please?
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Power The amount of energy changed/transformed by a device per second, measured in Watts (Joules per second) VA Power = voltage x currentP = VI
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Today’s lesson - Transformers Can you gather around Mr Porter?
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Transformers A transformer consists of 3 main parts
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Transformers A changing current in the primary coil produces a changing magnetic field in the core. This changing magnetic field induces a changing current in the secondary coil.
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Changing voltages The ratio of the number of turns on the coils equals the ratio of the voltages V p /V s = N p /N s # of turns = N s # of turns = N p
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Changing voltages A transformer where the voltage rises is called a step-up transformer
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Changing voltages A transformer where the voltage falls is called a step-down transformer
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No energy for nothing However, no new energy is made
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No energy for nothing Power in = power out V p I p = V s I s
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a.c. only Because transformers rely on induction (changing fields), they only work with alternating current (a.c.)
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Why do we need transformers?
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Current flowing through wires causes them to get hot and energy is lost.
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Why do we need transformers? Keeping the current low means electricity can be transported long distances without losing too much energy.
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Why do we need transformers? However, in order to send a large quantity of energy, high voltages are needed.
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Transmission of Electricity Step-up Step-down
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Let’s try some questions! Page 221 Question 3 Page 223 Questions 2 & 3 Page 225 Questions 3, 6 & 7 Mr Porter wants you to finish these for homework PLUS the questions he is now giving you. Due Tuesday 20 th Oct.
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