 A magnet can be used to induce a potential difference at the ends of a coil of wire.

# Electricity - Transformers

This GCSE Physics Electricity quiz will see how much you know about transformers. Transformers are simple devices that convert voltage from high to low and vice-versa. They are used to increase the voltage from power stations to the high voltages needed by the National Grid to transmit electricity around the country efficiently. Transformers are also found in electricity substations where they reduce the high voltage from the National Grid to lower voltages suitable for homes, offices and factories. Many home appliances do not require the 230 V which is provided by mains electricity. To overcome this problem, transformers are used to convert the voltage supplied to the required level for the appliance. Transformers that increase the voltage are referred to as step-up transformers whilst those that reduce the voltage are known as step-down transformers.

Transformers are also used in chargers for electronic appliances such as smartphones and laptops. These appliances require direct current, rather than the alternating current of the mains. Transformers work only with AC and their output is also AC, so for chargers and small electronic devices like computers, they are fitted with a rectifier circuit. This is a system that uses diodes to ensure that the output current from the transformer always flows in the same direction i.e. is direct current.

Transformers are very simple electrical devices. They consist of two separate coils of wire wrapped round opposite sides of a core. The core is made of iron which is why transformers are usually quite heavy for their size. The core is laminated to make it as efficient as possible. They work because of the principle of electromagnetic induction. When a magnet is moved next to a wire, a potential difference is created (more scientifically we say the current has been induced) between the ends of the wire. This potential difference can be used to drive a current - think about a generator. The opposite is true, if a current passes through a wire, a magnetic field can be detected around the wire. The important thing to remember is that the magnet needs to be moving or the current needs to be changing for electromagnetic induction to work.

So back to the coils of wire on the same iron core. If you pass a direct current through the first coil, in the fraction of a second that the current builds up from zero to the full flow, there will be a magnetic field, then the field will die away because there is no change. If you then swap the DC for AC, the current is constantly changing and so the magnetic field will last for as long as the current flows, changing direction as the current changes direction. The iron core concentrates and focusses the magnetic field and so it passes through the second coil. Because the current is constantly changing direction, electromagnetic induction creates a potential difference in the second coil. This can be used to push current round a circuit. The crucial thing to remember here is that no electricity passes directly between the two coils.

If the two transformer coils have the same number of turns around the core, the output voltage would be the same as the input voltage which would be pointless, so transformers all have different coils on the input and output sides. The voltage produced by electromagnetic induction depends on several factors, including the number of turns on the coil - more turns will give a greater potential difference if all other factors are constant. If the input (primary) coil of a transformer has a greater number of turns than the output (secondary) coil, the potential difference induced in the secondary coil will be lower than that of the primary coil and vice versa. The relationship between the potential difference across the ends of each coil is simple; if one coil has twice as many turns as the other, its potential difference will be twice that of the other one and so on, the two are directly proportional.

1.
What is induced across the ends of an electrical conductor which moves through a magnetic field?
A potential difference
A resistance
A current
A capacitance
Potential difference is often referred to as voltage
2.
What can be used to induce a potential difference at the ends of a coil of wire?
Heat
A vacuum
Magnet
Moving a magnet inside the coil produces a changing magnetic field, causing a potential difference to build. A potential difference is only induced for as long as the magnet is moving
3.
Considering the basic structure of a transformer, what does an alternating current in a primary coil produce in the iron core which it surrounds?
A static magnetic field
A changing magnetic field
A static electric field
A changing electric field
The iron core concentrates the magnetic field and ensures that it passes directly through the secondary coil
4.
In a step-up transformer, is the potential difference created in the secondary coil more, less or equal to the potential difference in the primary coil?
More
Less
Equal
Values for the coils are required to calculate this
Knowing that the transformer is a step-up transformer is enough to know that the potential difference will be greater in the secondary coil than the first
5.
In a step-down transformer, is the potential difference created in the secondary coil more, less or equal to the potential difference in the primary coil?
More
Less
Equal
Values for the coils are required to calculate this
Step-down (and step-up) refers to voltage, remembering that small fact will help you in transformer and perhaps National Grid exam questions
6.
Which equation correctly relates the potential difference across the primary and secondary coils of a transformer?
VpVs = npns
VpVs = nsnp
VsVp = npns
VpVp = npns
If a transformer was assumed to be 100% efficient, the electrical power output would equal the electrical power input. Thus the following formula would also be true: Vp x Ip = Vs x Is
7.
Which types of transformers operate at high frequency?
Laminated core
Variac
Stray field
Switch mode transformers
These are useful in phone, tablet and laptop chargers
8.
What is an advantage of using switch mode transformers over a traditional transformer when working from a 50 Hz mains supply?
They change the electricity at a quicker rate
They are lighter and smaller
They are more robust
They are more efficient
They are therefore more portable and can easily slip into your luggage when going on a school trip or holiday
9.
What is another advantage of using switch mode transformers when no load is applied?
They use little power
They use lots of power
They store electricity
There is no advantage offered by switch mode transformers when no load is applied
One of the issues with normal transformers is that when an appliance is switched on but in standby mode, the transformer is still operating at almost full power, which is a waste of energy
10.
A primary coil within a transformer has 48 turns around its coil and has a potential difference applied to it of 12 V. If the secondary coil has 60 turns around its coil, what is the potential difference produced at the ends of the secondary coil?
12 V
14 V
15 V
9.6 V
Either use the transformer equation or simple proportion to arrive at the correct answer. Because the secondary coil is larger than the primary, you should already know the potential difference should be greater which immediately eliminates two of the answers
Author:  Martin Moore