Transformers Date _______
In the last lecture we talked about different ways to produce electricity.
Electrical energy is transmitted great distances at high voltages and low current to minimize energy loss to heat.
Transformers are used to increase or decrease the voltage using electromagnetic induction
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The simplest alternating current (ac) transformer consists of 2 coils of wire wound around a core of iron.
The coil on the left is the primary coil and it is attached to the input ac voltage, Vp. It has a certain number of turns, Np.
The coil on the right is the secondary coil and it is attached to a device which uses the output ac voltage, Vs. It has a certain number of turns, Ns.
For example, you plug a laptop
into a 120 V input ac voltage. There is a transformer to convert
the voltage to 19.5 V.
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The primary coil is attached to
an input alternating current
source.
The input alternating current supplies a changing electric
field which creates a changing
magnetic field in the iron core.
The changing magnetic field in
the iron core results in a
changing electric field in the
secondary coil.
The changing electric field in the secondary coil creates a potential difference and induces current to flow.
It is similar to moving a magnet in and out of the secondary coil to induce a current.
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The transformer changes the voltage from the primary coil into a different value in the secondary coil. When the voltage changes the current also changes. The transformer transfers energy from one coil to the other.
The rate at which the energy is transferred is POWER.
The power used in the secondary coil is supplied by the primary coil.
The law of the conservation of energy is obeyed since the primary coil gives no more power that the secondary coil uses.
Power into primary coil = Power out of secondary coil
Pin = Pout
since P == IV
IpVP = ISVS
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Step Up Transformer
If the secondary coil has more turns
(number of wire loops) that the
primary coil, it is a step up transformer.
The voltage will increase and the current
will _____________.
Step Down Transformer
If the secondary coil has less turns
than the primary coil is it a step down transformer.
Step down
transformer
The voltage will decrease and the current
will ____________.
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Transformer Equation
The number of turns in the primary and secondary coils can be related to the voltages in the primary and secondary coils.
or it can be written as
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Practice Problems
1) The voltage across the input terminals of a transform is 120 volts. The primary coil has 150 turns and the secondary coil has 50 turns.
Is this a step up or step down transformer?
2) What voltage does the transformer put out?
3) If the resistance in the secondary coil is 100 ohms, what current flows through the secondary coil?
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4) What is the power output from the secondary coil?
5) What is the power input into the primary coil?
6) If the voltage from my outlet at home is 120 volts, but I need 10 volts to start by hair dryer, what is the ratio of the number of turns in the primary coil to the number of turns in the secondary coil for the transformer in my hair dryer cord?
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