Do now! Can you finish the questions you started last lesson?

Last lesson - Electromagnetic induction If a magnet is moved inside a coil an electric current is induced (produced) COPY this please!

Electromagnetic induction A electric current is induced because the magnetic field around the coil is changing.

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?

COPY this please!

Today’s lesson - Transformers

Stand up!

☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺

☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ cell energy electron lamp

Coulomb of charge (electrons) Think of it as a “bag of electrons” (containing electrons!) ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺

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

In a series circuit Current is the same at any point in the circuit 2.5 A

In a parallel circuit The current splits (total current stays the same) 2.5 A 1.25 A

Voltage(emf) ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ V I’m checking the difference in energy (per coulomb) between the 2 red arrows 1 Volt = 1 Joule per coulomb

Voltage (p.d.) ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ V I’m checking the difference in energy (per coulomb) before and after the lamp 1 Volt = 1 Joule per coulomb

In a series circuit The sum of the voltages across the lamps equals the voltage across the cells 9 V 3 V

In a parallel circuit In a simple parallel circuit, voltage across each lamp equals the voltage across the cells 5 V

Resistance Measures how difficult it is for current to flow. Measured in Ohms (Ω) VA Resistance = voltage/currentR = V/I

Ohm’s Law V = IR V R I X ☺ Can you copy this please?

Power The amount of energy changed/transformed by a device per second, measured in Watts (Joules per second) VA Power = voltage x currentP = VI

Today’s lesson - Transformers Can you gather around Mr Porter?

Transformers A transformer consists of 3 main parts

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.

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

Changing voltages A transformer where the voltage rises is called a step-up transformer

Changing voltages A transformer where the voltage falls is called a step-down transformer

No energy for nothing However, no new energy is made

No energy for nothing Power in = power out V p I p = V s I s

a.c. only Because transformers rely on induction (changing fields), they only work with alternating current (a.c.)

Why do we need transformers?

Current flowing through wires causes them to get hot and energy is lost.

Why do we need transformers? Keeping the current low means electricity can be transported long distances without losing too much energy.

Why do we need transformers? However, in order to send a large quantity of energy, high voltages are needed.

Transmission of Electricity Step-up Step-down

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.