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Transformer

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Transformer

Electricity pylons are an iconic symbol of the electricity system in the UK. They are used to transmit electricity all over the country. However, if you attempted to charge your phone from the cables running over pylons, it would explode! This is because electricity flows through the cables at extremely high voltages. To use electricity in our homes, the voltage needs to be reduced to a level suitable for household appliances. This is done by the use of transformers.

Transformer definition

Transformers are electrical devices that transfer electrical energy between alternating current circuits and can be used to either increase or decrease the voltage from one circuit to the next. They are based on the fact that an alternating current in one circuit generates a magnetic field, which can induce a voltage in a second circuit.

Transformer diagram


Transformers transformer diagram StudySmarterThe green dotted line around the iron core shows how the alternating magnetic field is transported from the primary coil to the secondary coil.

There are three main parts that make up all transformers:

  • The primary coil - this is connected to the first alternating current (AC) circuit and the voltage across it is equal to the incoming voltage from the circuit. This voltage is calledVp, with the subscript referring to 'primary'.
  • The secondary coil - this is connected to the second AC circuit and it provides the voltage to the second circuit. This voltage is calledVs, with the subscript referring to 'secondary'.
  • The iron core - this links the two coils, which are wrapped around opposite sides. There is no electrical connection between the two coils, they are only joined by the iron core.

How does a transformer work?

The circuit must have alternating current flowing through it. The reason for this can be understood by considering how a transformer functions in steps:

  1. The primary coil has the alternating current from the original circuit running through it. The coil is a metal so it becomes an electromagnet and produces an alternating magnetic field. An alternating magnetic field constantly changes direction.
  2. The changing magnetic field is carried around the iron core to the secondary coil, as can be seen in the figure above.
  3. When the alternating magnetic field reaches the secondary coil, a current is induced in it as the coil acts as a conductor.

The above points show why direct current cannot be used in the first circuit. It will produce a magnetic field that is not alternating. As the magnetic field is not changing, no current and hence no voltage is induced in the secondary coil.

The dotted green lines in the diagram above represent the magnetic field lines running through the iron core. The energy from the primary coil is transferred to the secondary coil by the magnetic field - there are no electrical connections. This means that the magnetic field must be able to be transferred very efficiently through the core. For this to be the case, a magnetically "soft" material should be used, such as an alloy of iron with silicon. However, some of the initial energy is always lost in real-life situations due to the resistance in the wires and the iron core resisting the changing of the magnetic field.

Wasted energy is a big problem when working with transformers, In electrical power stations, extremely large transformers are used and they are often contained in huge tanks so that the core and the coils are completely enclosed. A very small amount of energy is lost in the form of heat and cooling fluid is continuously pumped around the tanks in order to remove the heat.

Transformer formula

The voltage leaving the secondary coil to the external circuit is related to the ratio of the number of turns on the coils of the transformer. Using the symbols for the incoming and outgoing voltage as stated in the earlier diagram, a simple formula relates the number of turns on the two coils to the output voltage:

VpVs=NpNS

Np represents the number of turns on the primary coil and Ns the same for the secondary coil. The equation is very simple and is just saying that the ratio of the number of turns on the secondary coil to the number on the primary coil is equal to the ratio of the output voltage to the incoming voltage.

The voltage of the electricity running through a pylon is 23000 V. The voltage needs to be decreased to 230 V so that it is a suitable level for the mains electricity supply in a house. This can be done by using a large transformer. The specific transformer used in this case has 1000 turns on the primary coil. How many turns should it have on the secondary coil?

For this question we can use the transformer equation from above:

VpVs=NpNS.

It can be rearranged to give:

Ns=NpVsVp.

The values given in the question can then be plugged in to find the number of turns on the secondary coil:

Ns=1000·23023000=10,

So there are 10 turns on the secondary coil.

Types of transformer

There are two types of transformers; step-up and step-down transformers. The names give you a clue about what they do - step-up transformers increase the voltage from one alternating circuit to the next whereas step-down transformers decrease the voltage.

Transformers different types of transformer StudySmarterStep-down transformers decrease the voltage and step-up transformers increase the voltage.

For a transformer to increase (step-up) the voltage from the first circuit to the next, the number of turns on the secondary coil must be greater than that on the primary coil. Conversely, to decrease (step-down) the voltage from one circuit to the next, the turns on the secondary coil must be less than on the primary coil.

Examples of transformers

Transformers have many useful applications. Both step-up and step-down transformers are used in different situations.

The national grid

Electrical power is supplied across the UK from power stations that send the electricity along high-voltage power cables. The national grid is the network of cables that connect all the places in the UK that need electricity. The reason that high voltages are used to transmit electrical power is that it results in a lower current flow through cables, which means that the energy lost will be less. Energy is lost because the resistance in the wires is fighting against the current and they become warm resulting in a loss of thermal energy to the surroundings.

Transformers the national grid StudySmarterElectricity produced at power stations is sent to our homes through the use of transformers.

High voltages are very dangerous and this is why the cables that transport the electrical power are high above the ground on pylons, as shown in the diagram above.

Power stations usually generate electricity at 25 kV. This is increased by step-up transformers to a voltage of 250 kV to be transmitted along the pylons, and this voltage is then decreased to around 230 V once the high-voltage cables reach our local sub-station.

Household appliances

Radios will often run off the mains voltage supply. However, their operating voltage is normally quite a lot lower than230 V. There are step-down transformers built into radios to reduce the voltage to a suitable level. On the other hand, the operating voltage of microwave ovens is higher than the mains voltage for a household so they also have a step-up transformer inside to increase the voltage.

Transformer - Key takeaways

  • Transformers are used to increase or decrease an input voltage by a known amount.
  • The ratio of the number of turns on the primary and secondary coils is equal to the ratio of their voltages.
  • There are no electrical connections between the primary coil and the secondary coil. They are only connected by the iron core and the energy is transferred between them through alternating magnetic fields.
  • Step-up transformers increase voltage and step-down transformers decrease voltage.
  • Transformers are used in the national grid to raise the voltage and improve the efficiency of transferring electricity from power stations around the country.

Frequently Asked Questions about Transformer

The current through the primary coil induces as an alternating magnetic field in the magnet, which induces a current in the secondary coil. The ratios of the turns on each coil determines the output voltage from the secondary coil.

A transformer is an electrical device that can increase or decrease the voltage from one circuit to another.

There are step-up transformers and step-down transformers. Step-up transformers increase the voltage, while step-down decrease it.

A transformer circuit is a circuit in which the input voltage is increased or decreased by the use of the transformer.

A transformer is based on the fact that an alternating current in one circuit generates a magnetic field, which can induce a voltage in a second circuit. By varying the number of coils on each side of a transformer, the voltage can be increased or decreased between the two circuits.

Final Transformer Quiz

Question

What is DC Current?

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Answer

DC Current is a form of current that remains traveling in the same direction at all times.

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Question

What is AC Current?

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Answer

AC Current is a form of current that will change direction at periodic intervals.

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For an AC current, what is the most common form of wave?

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Answer

A sine wave.

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Question

What is the amount of times an AC current runs through a complete cycle known as?

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Answer

Frequency.

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Question

Name an example of how AC current is used.

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Answer

  • AC current is used in the mains supply.
  • AC current is used in transformers to transmit electricity over long distances.

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Question

Name an example of how DC current is used.

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Answer

  • DC current is used in a DC Motor.
  • DC current is used in electromagnets and in the generation of magnetic fields.

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Question

Describe what frequency is, and how it applies to an AC current.

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Answer

Frequency is the measure of how many times a repeating event occurs per second. AC current is measured with frequency as it changes direction or value a certain amount of times a second, every half of a time period.

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Question

What voltage does the mains supply run at in the UK?

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Answer

The mains supply in the UK runs at 230V.

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What frequency does the mains supply run at in the UK?

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Answer

The mains supply in the UK runs at 50Hz.

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Question

What is the name for a complete cycle of a sine wave?

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Answer

The time period.

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Question

Describe how to create an electromagnet using DC current.

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Answer

To make an electromagnet using DC current, you need to coil a solenoid around a magnetic core, and run a DC current through the solenoid to produce a magnetic field.

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Question

What conditions increase the magnetic field strength of an electromagnet?

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Answer

The value of the DC current running through the solenoid, and the number of coils around the magnetic core.

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Question

What are the two types of transformer?

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Answer

Step-up and step-down.

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Question

Do step-up transformers increase or decrease the input voltage?

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Answer

Increase.

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Question

If you wanted to increase the voltage of a circuit by using a transformer, would you need more turns on the primary or the secondary coil?

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Answer

The secondary coil.

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What connects the two coils in a transformer?

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Answer

The iron core.

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Why is iron used as the material for the core in a transformer?

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Answer

It is a magnetically soft material, so magnetic energy can transfer through it easily.

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Question

What might cause energy loss in a transformer circuit? Give one example.

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Answer

Resistance in the wires, resistance in the iron core.

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Question

Why is a high voltage used for transmitting electrical energy in the national grid?

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Answer

A high voltage results in a low current which means that less energy will be lost.

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Question

What type of current must be present for a transformer to work?

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Answer

An alternating current.

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Question

Why would a transformer not work if there was direct current running through the primary coil?

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Answer

Direct current would result in a non-changing magnetic field, which would not induce a current in the secondary coil.

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Question

Give a practical use of transformers.

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Answer

In the national grid, radios, microwave ovens etc

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Question

There is 1 V across the primary coil of a transformer, the primary coil has 50 turns and the secondary coil has 100 turns. What is the voltage across the secondary coil?

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Answer

2 V

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Question

In a transformer circuit, the voltage across the secondary coil is twice that across the primary cool. The secondary coil has 40 turns, how many turns does the primary coil have?

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Answer

20 turns.

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Question

In the process of applying electricity to our homes, in what order must transformers be used if the electricity is sent through pylons?

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Answer

First, a step-up transformer must be used to increase the voltage while the electricity goes through the pylons. Then a step-down transformer must be used to decrease the voltage to the right level for homes.

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Question

Which type of transformer will decrease the input voltage?

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Answer

A step-down transformer.

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Question

What is the national grid system?

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Answer

The national grid is a system of power stations, power lines, transformers, and electricity users that serves the United Kingdom.

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How is electricity transmitted by the national grid?

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Answer

Overhead powerlines

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Question

What is the mains voltage in the UK?

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Answer

230 V.

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What does the national grid do?

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Answer

The national grid transports electricity from producer to consumer. It also manages power production in the United Kingdom to ensure that production always meets or exceeds demand.

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Question

What is the operating voltage in the powerlines of the national grid?

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Answer

400,000 V.

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Question

How does the national grid minimise the amount of energy lost to heat in an overhead transmission cable?

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Answer

By using a high voltage, overhead cables can use a very low current, minimising the amount of waste heat energy.

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Question

What does a step-up transformer do in the national grid?

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Answer

It increases the voltage leaving a power station.

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Why would we want to use a step-up transformer in the national grid?

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Answer

To reduce the amount of energy wasted as heat.

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Question

What does a step-down transformer do in the national grid?

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Answer

It decreases the voltage from the overhead transmission cables.

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Why would we want to use a step-down transformer in the national grid?

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Answer

To make sure that the electricity reaching consumers' houses is safe.

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Question

What is the type of current used by the national grid?

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Answer

AC (Alternating Current).

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Question

If cables transmit a certain fixed amount of electrical power, and we double the voltage, what happens to the current?

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Answer

The current will halve.

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Question

What do the 'up' and 'down' in 'step-up' and 'step-down' transformers refer to?

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Answer

The voltage.

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Question

Generally speaking, what is the voltage of electricity leaving a power station?

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Answer

25,000 V.

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Question

What kind of current outputs from a battery?

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Answer

DC Current.

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