1 of 40© Boardworks Ltd 2009. 2 of 40© Boardworks Ltd 2009

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Current and magnetism

Every electric current produces a magnetic field. The shape and strength of the magnetic field depends on the shape of the wire carrying the current. A single straight wire carrying a direct current is surrounded by a circular magnetic field:

Every point on an infinite wire is equivalent to every other, so the magnetic field must be the same at every point – it is made up of concentric circles.

A much stronger magnetic field can be made by twisting a wire into a tight coil, or solenoid. This creates a magnetic field like that of a bar magnet.


Field around a wire

The direction of the magnetic field around a straight wire can be worked out by using the right hand grip rule.

Your fingers will curl around the wire in the direction of the magnetic field (from north to south pole).

Grip a wire so that your thumb points in the direction of the conventional current (from the positive to the negative terminal of a battery).

+

–


Field around a solenoid

The right hand grip rule can also be used to find the orientation of the magnetic field around a solenoid:

Grip the solenoid so that your fingers follow the direction of the conventional current.

Your thumb will now point towards the north pole of the electromagnet created by the solenoid.

N

S+

–

The electromagnet can be made stronger by increasing the number of coils, or by adding an iron core.


Inducing current in a coil


Electromagnetic induction

What do we know so far about the relationship between current and magnetism?

All currents have a magnetic field associated with them.

A wire in a changing magnetic field will experience an induced current.

The second effect is called electromagnetic induction, or the dynamo effect. It converts movement into electrical energy. This is the basis of the generator.

The first of these effects is the basis of the electromagnet.


Electricity and magnetism



Linking circuits with magnetism


Linking circuits with magnetism – results

In the experiment, a current was induced in the second circuit when the first circuit was switching on or off. In order for power to be transferred continuously between two circuits, the current in the first circuit must be changing continuously.

This can be achieved by using an alternating current.

In order for as much power to be transferred as possible, the two circuits must be as closely magnetically linked as possible.

This can be achieved by winding the two circuits into tight coils around an iron core. This is a transformer.


Primary side – how it works

A transformer links two circuits together. To understand how it works, it is important to look at each side separately.

The primary side is simply an electromagnet. By passing an electric current through a coil of wire, we make a magnetic field, just like the field around a bar magnet.

Direct current makes one end of the iron north, and the other end south.

N

S+

–


Secondary side – how it works

The secondary side is not connected directly to any power supply. It is just a piece of iron with some wire wrapped around it.

The secondary side works using electromagnetic induction. To make a current flow, a magnetic field needs to be changing perpendicular to the coil.

When there is an alternating current in the primary side, the direction of the magnetic field around the transformer alternates. This induces a second alternating current in the secondary side.


How a transformer works – summary


Parts of a transformer



Properties of transformers

Transformers transfer power between circuits.

The design of a transformer determines the characteristics of the electricity flowing in its secondary circuit. The frequency of the alternating current in the secondary circuit will always match the primary circuit, but what about current and voltage?

The voltage in each circuit is related to the number of coils on each side of a transformer by the following equation:

primary voltage

secondary voltage

primary turns

secondary turns=

Vp

Vs

Np

Ns

=


Step-up transformers

A step-up transformer is used to increase voltage. It has more turns on its secondary side than on its primary side.

But the power in the secondary circuit cannot be greater than the power in the primary circuit, or the transformer would be more than 100% efficient!

What is the relationship between power, voltage and current?

P = V × I

A step-up transformer increases voltage, but reduces current.


Step-up transformer calculations

A transformer has 100 turns on its primary coil. It has an input voltage of 35 V and an output voltage of 175 V.

= 500 turns

=Vs=Ns

175=Ns

Vp Np

Vs Ns

× NpVp

× 10035

How many turns are on the secondary coil?

=Vs Ns

Vp Np

=Ns Vs

Np Vp


Step-up transformer uses

Step-up transformers are used in the following applications:

power transmission

using European appliances in the USA

The USA mains runs at 110 V, while the UK uses 230 V. Goods made for the UK, but used in the USA, need a transformer to increase their supply voltage.

Step-up transformers are used to increase the voltage generated in power stations, so that it can be transported around the country at extremely high voltages.


Step-down transformers

A step-down transformer is used to decrease voltage. It has fewer turns on its secondary side than on its primary side.

This kind of transformer can be found in many places around the home, as a lot of appliances use lower voltages than the 230 V provided by the National Grid.

A mobile charger, for instance, contains a step-down transformer, which is why it is larger than a normal plug.


Step-down transformers calculations

= 230 V

=

Ns=Vs

50=Vs

Vp Np

Vs Ns

× VpNp

× 920200

=Vs Ns

Vp Np

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?


Isolating transformers

An isolating transformer has the same number of coils on its primary and secondary sides.

A transformer has 100 turns on the primary side, and 100 turns on the secondary side. If the primary voltage is 230 V, what is the secondary voltage?

Vp Np

Vs Ns

=

Vs Vp= = 230 V

Np = Ns

Np

Ns

= 1

= 1


Why use an isolating transformer?

Isolating transformers do not change the voltage of a power supply. So what are they used for?

Isolating transformers are used in devices such as electric shaver sockets, to isolate an appliance from the mains.

By separating a device, such as a shaver, from its mains supply, the risk of shock is much reduced. This is important in a bathroom where electrical items are at risk of getting wet.


Transformers around the home


Step-down transformer uses


Transformers around the home

How many transformers can you find in this house?



What is the National Grid?

The National Grid is a network of power lines designed to carry mains electricity around the country, from the power stations where it is generated to the homes and factories where it is used.

Transformers are an important part of the National Grid, because electricity must be transported at a much higher voltage than it is generated at or used at in homes.


Power loss in cables

When electrical energy is carried in wires, a current must flow.

power loss = current2 × resistance

Power is measured in watts (W).

Current is measured in amps (A).

Resistance is measured in ohms (Ω).

There is a power loss in cables which is related to the amount of current flowing:

P = I2 × R


Power loss in cables – example


Transformer power

A step-up transformer may increase voltage but it cannot create energy!

Vp × Ip = Vs × Is

primary secondary

Vp

Vs

Ip

Is

In a perfect transformer the power in is equal to the power out. As power = V × I, if voltage goes up, then current must go down.

power in = power out

Pp = Ps


Transformer power example

primary secondary

Vp

Vs

A transformer has a primary voltage of 1000 V and a primary current of 0.5 A.

Vp × Ip = Vs × Is

Ip

Is

If the secondary circuit has a current of 0.01A flowing, what is the secondary voltage?

= 50000 V0.01

0.51000 ×=

Is IpVp ×Vs =


Step-up transformers in the National Grid

A step-up transformer is positioned near a power station.

High voltages are used because a high voltage results in a low current flowing, for a fixed power.

This raises the voltage of the generated electricity, ready for transmission around the country.

A low current means the wires lose less energy as heat over long distances.


Step-down transformers in the National Grid

Step-down transformers are positioned close to homes and factories.

High voltages are useful for saving energy, but are very dangerous. Household appliances need much lower voltages, so the voltage is reduced while the current increases, for a fixed amount of power.

They are used to reduce the voltage from the very high voltages used for transmission.


The National Grid



Glossary


Anagrams


Multiple-choice quiz

Documents

1 of 40© Boardworks Ltd 2009. 2 of 40© Boardworks Ltd 2009