Transcript GCSE Physics - Physics4IGCSE

GCSE Physics
Magnetism and Electromagnetism
1
Lesson 6 - Transformers
Aims:
•To recall the structure of a transformer, and understand
that a transformer changes the size of an alternating
voltage by having different numbers of turns on the
input and output sides.
•To know and be able to use the relationship between
input (primary) and output (secondary) voltages and the
turns ratio for a transformer :
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What is a transformer?
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What is a transformer?
A device used to increase or
decrease voltage.
Where are transformers used?
In the national grid and
household appliances.
What do we call a transformer that increases voltage?
A step-up transformer.
What do we call a transformer that decreases voltage?
A step-down transformer.
4
Transformers
•A transformer has two electromagnetic coils.
•When the electricity changes in the first coil
an electric voltage is created across the second
coil.
•Transformers are used to change the voltage
and current of the electricity that comes from
BELCO.
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How does it work?
6
Transformers
•A transformer can change the voltage and current of
an electrical supply.
•Some devices need low voltage and some need some
need high voltage.
•Only a.c. voltage can be transformed from one
voltage to another, this is why mains electricity needs
to be a.c.
•To understand a transformer we need to learn more
about coils and cores.
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Remember this?
An electric voltage is only induced when the magnet is
moving.
We need a changing magnetism to make electricity.
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Bar magnet
have a similar
magnetic field
patter to a
solenoid with
current.
Input a.c.
voltage
Input or
primary
coil
What is what ?
A soft iron ‘O’
core
Output a.c.
voltage
Output or
secondary
coil
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What is what ?
The input
or primary
voltage
Number of
primary
coils
Number of
secondary
coils
The
output or
secondary
voltage
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Input side
The primary voltage
creates a magnetic field
when its current passes
through the coil.
The a.c. voltage means
that the magnetism
through the coil is
always changing.
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Output side
The magnetism from the
primary coil is passed
through the iron to the
secondary coil.
Because the magnetism
is changing a voltage is
induced in the secondary
coil.
The size of the output
voltage depends on the
size of the two coils.
Changing the
magnetism is just like
moving a magnet, only
easier!
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Soft iron core
• Iron is a soft magnetic material, this means
that it is very easy to magnetize and very easy
to demagnetize.
•In a transformer the direction of electricity is
changing many times each second.
•Every fraction of a second the core needs to
change its magnetism. We use iron because it
can change its magnetism from one direction to
another very easily.
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Lamp demonstration
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Transformers
•Transformers change the voltage and current of
an a.c. electrical supply.
•In the following experiment mains electricity at
220 volts enters the circuit.
•After leaving the transformer the voltage has
been reduced to approximately 11 volts.
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17
Input circuit
The voltage enters the
circuit through the
thick 220 volt wire
and passes through the
coil with 3600 turns.
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Output circuit
The voltage is induced
in the smaller part of
the transformer that
only has 300 turns of
wire. It then travels
through the thinner
wires to the 11 volt
lamp.
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The lamp is off because the magnetism is not
strong enough to go from one coil the other.
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The ‘C’ core increases the magnetism.
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The ‘O’ core is even stronger.
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Step up and Step down
… and an equation to learn!
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Is this a step-up or a step-down transformer?
secondary
primary
coil
coil
This a step-down transformer because there are
less turns in the secondary coil than the primary coil.
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Is this a step-up or a step-down transformer?
secondary
primary
coil
coil
This a step-up transformer because there are
more turns in the secondary coil than the primary coil.
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Less turns = less voltage
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Transformer formula
The formula for calculating voltages and coils in a
transformer has four variables!
Secondary _ voltage Turns _ on _ sec ondary _ coil

Pr imary _ voltage
Turns _ on _ primary _ coil
V 2 N2

V 1 N1
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Which way up?
The formula is about variables – it does not
matter which way up you remember the symbols.
V 2 N2

V 1 N1
V1
V2
=
N1
N2
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Transformer example 1
A step-down transformer is required to transform 240 V
a.c. to 12 V a.c. for a model railway. If the primary coil
has 1000 turns, how many turns should the secondary
have?
V2 / V1 = T2 / T1
T2 = T1 × (V2 / V1)
T2 = 1000 × (12/240)
T2 = 50 turns
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Transformer example 2
A transformer is designed to have 2500 primary turns
and 5000 secondary turns. What is the output voltage
if the input voltage is 120 V ?
V2 / V1 = T2 / T1
V2 = V1 × (T2 / T1)
V2 = 120V × (5000/2500)
V2 = 240 V
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Transformer example 3
A transformer has 200 turns on its primary coil and 50
turns on its secondary coil. The input voltage is 920 V.
What is the output voltage?
V2
V1
=
V2
=
V2
=
=
N2
N1
N2
x V1
N1
50
x 920
200
230 V
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Transformer example 4
A transformer has 100 turns on its primary coil. It has an
input voltage of 35V and an output voltage of 175 V. How
many turns are on the secondary coil?
N2
N1
=
N2
=
N2
=
=
V2
V1
V2
x N1
V1
175
x 100
35
500 turns
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Transformers
Transformers are used to _____ __ or step down
_______. They only work on AC because an ________
current in the primary coil causes a constantly alternating
_______ ______. This will “_____” an alternating
current in the secondary coil.
Words – alternating, magnetic field, induce, step up, voltage
We can work out how much a transformer will step up or
step down a voltage:
Voltage across primary (Vp)
No. of turns on primary (Np)
Voltage across secondary (Vs)
No. of turns on secondary (Ns)
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• If a transformer is 100% efficient, the power
produced in the secondary coil should equal
the power input of the primary coil.
Np
Vp
Vs

Np
Ns
I s Vp


N s I P VS
Power Transmission
long transmission line
home
appliance
power station
Rwire
looks like:
Rload
Rwire
Power Dissipated in an Electricity Distribution System
10km
120 Watt
Light bulb
Power Plant
on Colorado River
12 Volt
Connection Box
We can figure out the current required by a single
bulb using P = VI so
I = P/V = 120 Watts/12 Volts = 10 Amps (!)
Estimate resistance of power lines:
0.001 Ω/m  2105 m = 20 Ohms
Power dissipate/waste in transmission a line is
P = I2R = 102  20 = 2,000 Watts!!
“Efficiency” is
e = 120 Watts/2120 Watts = 5.6%!!!
What could we change in order to do better?
The Tradeoff
• The thing that kills us most is the high current
through the (fixed resistance) transmission
lines
• Need less current
– it’s that square in I2R that has the most dramatic
effect
• But our appliance needs a certain amount of
power
– P = VI so less current demands higher voltage
• Solution is high voltage transmission
– Repeating the above calculation with 12,000 Volts
delivered to the house draws only
–
I = 120 Watts/12 kV = 0.01 Amps for one bulb,
–
–
–
–
giving
P = I2R = (0.01)220 = 2010-4 Watts,
so
P = 0.002 Watts of power dissipated in transmission
line
– Efficiency in this case is e = 120 Watts/120.004 =
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99.996%
Example
• An average of 120 kW is delivered to
a suburb 10 km away. The
transmission lines have a total
resistance of 0.40 Ω. Calculate the
power loss if the transmission voltage
is:
1. 240 V
2. 24000V
P = IV
1
Power loss:
1
2
2
DANGER!
• But having high voltage in each household
is a recipe for disaster
– sparks every time you plug something in
– risk of fire
– not cat-friendly
• Need a way to step-up/step-down voltage at
will
– can’t do this with DC, so go to AC
A way to provide high efficiency, safe low voltage:
step-up to 500,000 V
step-down,
back to 5,000 V
~5,000 Volts
step-down to 220 V
High Voltage Transmission Lines
Low Voltage to Consumers
Power Transmission
• Electric power is usually transmitted
over high voltage power lines.
• Copper wire has a resistance and over
long runs some energy will be lost to the
surroundings as heat.
• A low current (high voltage) minimizes
this loss.