Solenoid

In 1820, the Danish physicist Hans Christian Oersted set up a demonstration for his class of students. He used a battery prototype to look at the effect that an electric current would have on a compass. He did not have time to prepare his demonstration beforehand and so did not know that the current would cause the compass magnet to deflect! By generating the electric current, Oersted created the world's first electromagnet - a temporary magnet. At their most simple, electromagnets consist of a solenoid - a coil of wire - with an electric current passing through. Nowadays, solenoids are used everywhere, from particle accelerators to simple household appliances.

Definition of solenoid

Fig. 1 - A solenoid that was used by the famous experimental physicist Michael Faraday, who did ground-breaking work on electromagnetism

When an electric current flows through the wire of a solenoid, an electromagnet is made.

The material making up an electromagnet does not have to be magnetic. The electric current is what produces the magnetic field, not the material. Solenoids are usually made out of copper. Other metals could also be used but copper has a relatively low resistance, allowing for a greater current flow, which leads to a stronger magnetic field. Copper is also cheap, abundant, and malleable compared to other conducting materials.

Characteristics of solenoids

When the wires at the ends of a solenoid are connected across a potential difference, a current flows through it and a magnetic field is produced. The shape of the magnetic field is shown below. Why does the field take this shape? What effect does this field have on other objects?

Magnetic field lines

The direction of the field lines in a magnetic field shows the direction of the force that would act on a magnetic north pole placed at that point. This can be best understood through bar magnets, which you may have come across before. They consist of both a north pole and a south pole. Opposite poles attract and like poles repel, as shown in Fig. 3.

The reason bar magnets behave in this way can be explained by their magnetic fields. The magnetic field lines for a bar magnet are shown in Fig. 4. Near the magnet's north pole, the arrows on the field lines point away from the magnet, showing that a different north pole would be repelled at this point. Near the south pole, the magnetic field lines point towards the magnet, showing where a different north pole magnet would be attracted.

You may have noticed that the field lines for a bar magnet are very similar to those for a solenoid! They both have the same kind of curved field lines around the outside. For this reason, we can say that solenoids have a polarity. The side of the solenoid that the magnetic field lines point away from is the north pole and the side that they point toward is the south pole.

You can work out the polarity by using the clock rule. When looking directly through the centre of a solenoid, if the current on the face is flowing in an anticlockwise direction, then the face has north polarity. If the current is flowing clockwise then the face has a south polarity. To help you visualise this please refer to Fig. 6 above.

Magnetic field due to a current-carrying wire

The reason why the magnetic field around a solenoid is like that around a bar magnet can be understood by considering the magnetic field produced by a single current-carrying wire. When an electric current runs through a straight wire, a magnetic field is produced around it. The field lines are circular and the distance between the individual lines increases at greater distances from the wire. Magnetic field strength is represented by the density of magnetic field lines so you can observe the magnetic field strength decreasing with distance from the wire.

The direction of the magnetic field lines around a current-carrying wire can be found by using the right-hand curl rule. To use this rule, point your right-hand thumb in the direction of the current and curl your fingers up. Your fingers should follow the direction of the magnetic field around the wire!

A solenoid is made by winding up a single wire into a coil. This is a way of increasing the overall magnetic field strength while maintaining the same intensity of current relative to a straight single wire. All of the magnetic fields of the individual wire loops add together inside the solenoid, producing a very strong, uniform field through the centre of the solenoid. On the other hand, outside the coil, the field lines of the wire loops cancel each other out and the resulting magnetic field is very weak.

Difference between a solenoid and an electromagnet

As mentioned above, when current flows through a solenoid, it becomes an electromagnet itself, so confusion can arise on the difference between a solenoid and an electromagnet. The key point to remember is that a solenoid is still a solenoid when there is no current flowing through it, but when the current stops it is no longer an electromagnet as the magnetic field dissipates.

Electromagnets (and solenoids with current flowing through them) are types of temporary magnets.

When the term 'electromagnet' is used, it often refers to solenoids with an iron core inside. An iron core is just a piece of iron, such as the iron nail shown in Fig. 7, and is used to increase the magnetic field strength of the coil wrapped around it. The field strength increases because iron is ferromagnetic and therefore easily magnetised and demagnetised.

Permanent magnets

Permanent magnets are always magnetic - they do not rely on an electric current as electromagnets do. An object will be a permanent magnet if it is made up of a ferromagnetic material. These are materials that produce a magnetic field due to their internal structure. Ferromagnetic materials include iron, nickel and cobalt. Examples of real-life applications of permanent magnets include compasses, bar magnets and fridge magnets.

There are several advantages and disadvantages of using temporary magnets over permanent magnets in different situations:

Advantages

• The magnetic field for a temporary magnet can be switched on and off. The magnetic field of a permanent magnet cannot.
• The magnetic poles of a temporary magnet can be reversed. A permanent magnet always retains the same polarity.
• The strength of a temporary magnet can be varied. A permanent magnet has a constant strength.

Disadvantages

• A temporary magnet requires electrical power to act as a magnet. A permanent magnet is always magnetic.
• The attractive properties of a temporary magnet vary depending on the electric current. The attractive properties of a permanent magnet do not change.

Example of a solenoid in science

Solenoids are useful in science because the strength of their magnetic fields can be varied by a number of different factors:

• Increasing the current flowing through a solenoid increases the strength of its magnetic field.
• Increasing the number of turns of wire making up a solenoid increases the strength of its magnetic field.
• Inserting an iron core inside a solenoid increases the strength of its magnetic field.

You can perform a simple experiment to see how the strength of a solenoid varies. In this experiment, you should fit an iron core into the centre of a solenoid to create an electromagnet. Then connect the electromagnet to across a potential difference so that current flows through the electromagnet, as shown in Fig. 7.

Firstly, record the current originally flowing through the circuit with an ammeter. Once this is noted down, stick as many paper clips as you can on the electromagnet! There will be a point where no more paper clips will stick. Note down how many paper clips stay on. Next, reduce the current (by adding a resistor to the circuit for example) and repeat the process. You should see that fewer paper clips can be stuck on the electromagnet when the current decreases! This is because the strength of the electromagnet's magnetic field decreases.

You can perform a similar experiment to this by testing how many paper clips will stick to electromagnets with different numbers of turns on their coil. You should keep the current the same throughout the experiment in this case.

Application of solenoids

There are a vast number of applications of solenoids in science, medicine and industry. In all of the following applications, the solenoid is used as an electromagnet.

MRI scan

An MRI scanner uses electromagnets to produce a strong magnetic field. When a patient is inside the machine, the field causes changes in the behaviour of the cells inside their body. Furthermore, MRI uses electromagnets to produce a super-strong magnetic field. This field causes changes in the behaviour of cells, which can be detected by sensors once the electromagnet is switched off. A computer is then used to form an image of the tissue from the signals.

Fig. 9 - Electromagnets are used in MRI scans.

Scrap yards

Objects made of magnetic materials such as iron and steel are attracted by magnets. In scrap yards, huge electromagnets with strong magnetic fields are used to lift large amounts of these materials.

Fig. 10 - Huge electromagnets are used in scrap yards to pick up and move large amounts of magnetic material.

Particle accelerators

Electromagnets are also used in particle accelerators. The accelerator tubes are lined with them so that the magnetic fields steer the beams of particles as they travel through the vacuum tube at velocities close to the speed of light.

Fig. 11 - Electromagnets are used in the Fermilab particle accelerator in Illinois.