How do Electromagnets work?

An Electromagnet is that magnet which attracts metals only when it is connected to an electric supply. It is made up of a solenoid (a coil of insulated copper wire wound on soft iron).  When an electric current flows through a wire, it produces magnetic flux around the wire.

Magnetic flux produced by current. Magnetic flux is clockwise 

when current is moving away through the wire (from right to left).

Since the coil has many turns, that is 500 or more, the magnetic fluxes in each turn join each other to form larger lengths of  magnetic fluxes.These magnetize the iron which also produces fluxes due to its ferromagnetic property. As a result, the soft iron and the solenoid wire together produce very strong magnetic fluxes. But it should be noted that the soft iron will quickly lose its magnetism if the electric current supply is switched off.

According to Ohm's law R = V/I. Therefore in order to harvest magnetic flux, resistance (R) must be greater than Current (I) and whenever resistance increases voltage also increases. This means if a solenoid of 500 turns is connected in series with a battery of 12 Volts, one will need to connect a solenoid of about 700 turns if the battery voltage is increased to 15 Volts. If the number of turns are not increased at 15 volts, then the solenoid will just heat up and it will not produce magnetic flux.

You might also need to read about Relationship between the Resistance and Dimensions of a Conductor

Who invented Electromagnets?
When William Sturgeon, a British scientist, was trying to magnetize soft iron permanently he found out that the iron could only be strongly magnetized when current was flowing through the solenoid wire. Therefore he discovered/invented an Electromagnet.

History of Magnets

In 600 B.C, the lodestone also known as Magnetite was already known to the Greek. It is an iron ore which has the property of attracting metals especially small pieces of iron. Chemically, a lodestone is made up of iron oxide with the formula Fe3O4. The place where magnetic iron ore was first discovered is called Magnesia.

The word lodestone is got from an old English word way, this refers to the property, of the stone, of being able to show the direction of the earth's North pole and South pole. During middle ages, navigational compasses were made by joining a piece of  lodestone to a wooden splint then this was made to float on water in a small container. These two could point in N-S direction.

Uses of Electromagnets

Electromagnets are due to the magnetic effect of current in a wire or a conductor.   There are quite a number of applications of magnetic effect of current in a conductor and among these are:

• Electric Bells
• Lifting magnets
• Solenoid switch for the car starter motor
• Magnetic circuits of generators and motors

Curious about Electric Motors?

Electric Motors work by following a rule called Fleming's Left Hand Rule. But before that rule, when an electric current flows through a conductor, it produces magnetic flux around the conductor.

Picture 1.

Right-hand grip rule, a rule concerned with current through a conductor. It says, Imagine  the wire to be held firmly in the right hand with the thumb pointing along the wire in the direction of the current. The direction of the fingers will give the direction of the magnetic flux. Like in the illustration above.

Note these symbols

Picture 2.

they mean...

Picture 3.

When a conductor currying current is placed in a magnetic field created by some other sources than its self, it experiences a force. The fields of the conductor carrying current tend to repel those of the permanent magnet and this produce a turning couple. See this in the picture below.

Picture 4. Magnetic field pattern of permanent magnet and coil of wire (conductor).

Picture 5. Simple direct current motor. Blocks labelled N and S are permanent magnets with a conductor carrying current (coil) placed between them.

Picture 6. A sketch to show Fleming's Left-hand rule

Fleming's Left-hand Rule states that , 'if the thumb (T) and the first two fingers of usually left hand are mutually at right angles with the first finger pointing in the direction of the field (B) and the second finger pointing in the direction of the current (I)  then the thumb (T) predicts the direction of the force or motion.

Picture 7. The coil of wire as shown in picture 5 above. Observe the direction of the flux when current is flowing through it.

By applying  Fleming's left-hand rule, the side where flux is anticlockwise will experience an upward force and the side where flux is clockwise will experience a downward force. These two forces cause the coil to rotate with its momentum supporting it.

The magnitude of the force acting on the conductor depends on:

1. The strength of magnetic field (B)
2. The length of the conductor cutting the magnetic flux (L),
3. The magnitude of current(I) flowing in the conductor
4. The angle between the magnetic field and the current

Picture 8. The length of the conductor as mentioned in the list above, that means the number of turns of the coil should be suitable enough for the motor to be powerful.

 

Picture 9. Electronic DC motors

Picture 10. Electromagnet

For better results, a number of coils should be wound on a soft iron armature made up of soft iron discs with slots. The coils are wound in slots. The armature, when magnetized, adds its magnetic flux to that of the coils. Also the commutator is multi-segmented. Electronic DC motors are often constructed this way.

Picture 11.

Picture 12. Permanent magnets inside

In larger motors, the magnetic field in which the armature rotates is produced by an electromagnet. Now let's get the difference, the coils of the electromagnet are called field coils and the coils of the armature are called armature coils. An example of an electromagnet is shown in picture 10.