#### Transcript Do now! - MrSimonPorter

```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)
Electromagnetic induction
A electric current is induced because the
magnetic field around the coil is changing.
Can you stick this in please?
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.)
Today’s lesson - Transformers
Stand up!
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cell
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energy
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electron
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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
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I’m counting
how many
coulombs of
electrons go
past me every
second
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The number of
Coulombs flowing
A
past a point in the
circuit every second.
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1 Amp = 1 coulomb per second
In a series circuit
Current is the same at any point in the circuit
2.5 A
2.5 A
2.5 A
2.5 A
In a parallel circuit
The current splits (total current stays the
same)
2.5 A
2.5 A
1.25 A
1.25 A
Voltage(emf)
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V
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I’m checking the
difference in
energy (per
coulomb) between
the 2 red arrows
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1 Volt = 1 Joule per coulomb
Voltage (p.d.)
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I’m checking the
difference in
energy (per
coulomb) before
and after the lamp
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V
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1 Volt = 1 Joule per coulomb
In a series circuit
The sum of the voltages across the lamps
equals the voltage across the cells
9V
3V
3V
3V
In a parallel circuit
In a simple parallel circuit, voltage across
each lamp equals the voltage across the
cells
5V
5V
5V
Resistance
Measures how difficult it is for current to
flow. Measured in Ohms (Ω)
V
Resistance = voltage/current
A
R = V/I
Ohm’s Law
• V = IR
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R
Can you copy
Power
The amount of energy changed/transformed by
a device per second, measured in Watts (Joules
per second)
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V
Power = voltage x current
P = 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
# of turns = Np
# of turns = Ns
Vp/Vs = Np/Ns
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
VpIp = VsIs
a.c. only
Because transformers rely on induction
(changing fields), they only work with
alternating current (a.c.)
Why do we need transformers?
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 20th Oct.
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