#### Transcript Do now! - MrSimonPorter

```Do now!
Can you stick
the sheet in
your book
please?
Last lesson - Transformers
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
Today’s lesson
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Field around a wire
Field around a coil (solenoid)
Electromagnets
Bells
Relays
Can you gather around Mr Porter?
Moving charges (currents)
Moving charges (electric currents) also
produce a magnetic field
Conventional current –
electrons flow in the
opposite direction
http://www.sciencebuddies.org
Magnetic field around a straight wire
You can use the right
hand screw rule to
decide which direction
the field goes
Stronger field closer to
wire
Magnetic field around a straight wire
You can use the right
hand screw rule to
decide which direction
the field goes
Stronger field closer to
wire
Can you
copy this?
Shape around a coil (solenoid)
Can you gather
around Mr
Porter?
Field around a solenoid (coil)
Electromagnets
To make an electromagnet
stronger?
To make an electromagnet
stronger
The 3 C’s
• More Coils
• More Current
• Use an iron Core
Can you
copy this?
Electric bells
• Can you gat
Electric bells
• B Bell
• H Hammer
• S Screw
• A Armature
• E Electromagnet
• K Switch
• U Cell
The Electric bell
Can you stick the sheet in and complete
please?
Relays
Relay circuit
Relay circuit
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Circuit 1 is a simple electromagnet which requires only
a small current. When the switch is closed, current flows
and the iron rocker arm is attracted to the electromagnet.
The arm rotates about the central pivot and pushes the contacts
together.
Circuit 2 is now switched on. Circuit 2 may have a large
current flowing through it to operate a powerful motor or very
bright lights.
When the switch is opened the electromagnet releases the rocker
arm and the spring moves the contacts apart. Circuit 2 is
now switched off.
The advantage of using a relay is that a small current (circuit 1)
can be used to switch on and off a circuit with a large
current (circuit 2).
This is useful for two reasons:
(i) circuit 1 may contain a component such as an LDR,
which only uses small currents,
(ii) only the high current circuit
needs to be made from thick wire.
A relay is used to operate the starter motor in cars
and the heating circuit in diesel engines.
```