1. Do not leave the electromagnet connected to the battery for a long time, because the circuit can heat up extensively.
2. Carefully strip the wire ends using a wire stripper or a scissors. This step should be performed by adults only.

Did you know that you can built a magnet from scratch with household materials? The magnets you can build using household magnets aren’t like the usual ‘boring’ permanent magnets like the ones you can stick on a fridge, but can be controlled using electricity. We call this type of magnet an electromagnet – ‘electro’ means electricity, so it is a magnet controlled by electricity. In fact,  we can switch the magnet on and off whenever we want to by turning the electric circuit on and off!

If we spread some paperclips on the table you can see that the nail behaves as usual and none of the paperclips are attracted to it. However, if we tightly wrap an insulated wire around the nail and pass a current through the wire by connecting it to the battery, the nail suddenly becomes magnetic and the paperclips are attracted to the nail.

Can anyone guess what is going on here? Can you tell me how we can utilize this property of having a controllable magnet?

Why are the paperclips attracted to the nail?
When a current is passed through the solenoid a magnetic field is produced which is amplified by the iron nail core.

Can the magnetic force produced be varied?
Yes, either by changing the current or by changing the turn density of the solenoid.

Can any material be magnetized?
No. Only ferromagnetic materials can be magnetized. For example, iron is a ferromagnetic material but aluminium is not.

Why does current pass through the wire?
Current is simply the flow of electrons within the wire. When a circuit is complete, the battery drives the electrons at a flow depending on the potential difference (voltage rating) of the particular battery.

When the current is switched off, does the electromagnet retains some of its magnetic properties?
This depends on the core making the electromagnet. In the case of an iron core (a good ferromagnetic material) the electromagnetic remains slightly magnetic even when the current is switched off – this occurs because some dipoles remain aligned after the current is switched off.

The wire wrapped around the nail is referred to as a solenoid. When a current is passed through the wire, a magnetic field is produced around the wire, making the solenoid behave just like a magnet. In electromagnetism, electric and magnetic fields are deeply related to each other. While the flow of electricity in the wire generates a magnetic field, the specific shape of the solenoid allows the field to behave exactly like that of a permanent magnet. Any ferromagnetic materials (such as paperclips) in the vicinity of the magnetic field are attracted to the electromagnet. Refer to the Figure below.

If the solenoid were wound without the nail in the middle, the magnetic field generated within it would not be significantly strong and would not attract the paperclips. However, in the presence of an iron core (i.e. the nail) the magnetic field is greatly strengthened .

Diagram showing the magnetic effect when applying a current through a solenoid.

Passing a current through a solenoid generates a magnetic field having a direction that satisfies the right-hand screw rule. The magnetic field is uniformly concentrated in the center of the solenoid whilst the outside field is weak and divergent.

The magnetic field produced can be approximated using the equation,

                                                                               B=μnI                                                                             [1]

Where B is the magnetic field, μ is the permeability, n is turn density and I  is the current supplied to the solenoid.

Equation [1] indicates that the magnet can be made stronger by increasing the number of turns of the solenoid and/or increasing the current supplied. is a property of the material and can therefore not be changed.

The ability of a material to be magnetized in an applied magnetic field is measured by the permeability of the material. The permeability of free space is given as 0=1.257 10^-6 N/A² while that of iron is =5000N/A², indicating that iron is much more readily magnetizable than air. This is because iron is made up of tiny magnets called dipoles which are randomly aligned. In the presence of a magnetic field these dipoles align in the direction of the field making the whole magnet stronger. The large difference in permeability between air and iron explains why an iron core greatly increases the magnetic properties of the solenoid.

The ideal materials used as the core of the electromagnet are called ferromagnetic materials (such as iron) as their dipoles naturally line up with an external magnetic field making a much stronger resultant magnetic field.

Application
Electromagnets are used to maneuver large and heavy magnetic metals in scrapyards. Electromagnets are ideal  since their magnetic properties can be controlled with a flick of a switch: when the electromagnet is turned on, it can be used to extract magnetic materials from debris which may include non-magnetic materials like plastics and glass, and when it is turned off, the scrap metal is immediately deposited at its destination. The electromagnets are usually attached to cranes which can move the debris from one area of the scrapyard to the other.

Powerful electromagnets are also used in new transportation technologies. In particular, electromagnetic force is used to levitate trains above tracks in maglev trains. Such trains were developed in Japan and are known as Bullet Trains. Electromagnets allow the train to levitate on the tracks, providing a smoother ride. Levitation also minimises friction allowing the train to reach speeds as high as 320 km/hr.

Electromagnets are also used in musical apparatus and instruments such as loudspeakers and bells. For example, a loudspeaker is essentially made up of a permanent magnet inside a wire coil. When electricity is passed through the wire coil, the coil itself becomes an electromagnet. The magnetized

coil is attached to the loudspeaker drum. As the magnetized  coil is attracted to the permanent magnet and repelled from it, it moves back and forth. These vibrations are transmitted to the drum of the loudspeaker. As the drum moves back and forth, it creates sound waves. Similar mechanisms are found in electric bells and musical instruments such as the electric guitar.  

Research
Busy rail networks all over Europe require extensive maintenance to ensure maximum safety. In the United Kingdom, research is being carried out to further develop inspection techniques for train axles. The main goal of this research project is to develop more efficient inspection techniques that are more convenient to train operators. Electromagnetic testing is on the forefront of this project. This novel technique helps to detect transverse cracking in sensitive parts of the rail track.

• Alter the number of turns in the coil of wire and observe how the strength of the magnetic field varies. The strength of the magnetic field generated can be tested simply by counting the maximum number of paperclips that can be attracted.
• Repeat the experiment using nails of different lengths and widths and observe how the magnetic field strength varies.
• Investigate the effects of using different batteries on the field strength.
• Try using an AA battery or a C battery, for example. Try using wires of different thicknesses to make the coil.
• Test how the tightness of the turns in the wire affects the field strength.