• ## Magnetic field

Electric motors convert electrical energy into mechanical energy. They are found in the home in various appliances such as washing machines, clothes dryers, refrigerators, vacuum cleaners, CD players…

Some motors operate on direct current (DC) while others require alternating current (AC)

Now we start exploring the principle of electric motors.

• ## Magnetic field around a magnet

Observation: opposite poles of magnets attract each other and like poles of magnets repel each other. Close to a magnet, a compass free to rotate takes a particular direction.

Interpretation:

Magnets are surrounded by magnetic fields. The direction of the magnetic field at a par­ticular point is given by the direction of a compass (south to north) placed at this particular point.

Representation of a magnetic field:

Magnetic fields are represented in diagrams using lines. These show the direction and strength of the field. The density of the field lines represents the strength of the magnetic field. The closer the lines are together, the stronger the field.

Arrows on the magnetic lines show the direction of a compass (South-North) placed in the magnetic field.

Magnetic field lines never cross. When a region is influenced by the magnetic fields of two or more magnets or devices, the magnetic field lines show the strength and direction of the resultant magnetic field acting in the region. They show the combined effect of the individual magnetic fields.

The spacing of the magnetic field lines represents the strength of the magnetic field. It follows that field lines that are an equal distance apart represent a uniform magnetic field.

Magnetic field lines leave the N pole of a magnet and enter the S pole.

• ## Magnetic field around a straight current-carrying conductor

Observation: a current in a straight current-carrying conductor produces a circular magnetic field around the conductor.

• ## The right-hand grip rule

When to use it?

When the direction of current is known and we want to find the direction of the magnetic field.

How to use it?

The direction of the magnetic field around a straight current-carrying conductor is found using the right-hand grip rule. When the right hand grips the conductor with the thumb pointing in the direction of conventional current, the curl of the fingers gives the direction of the magnetic field around the conductor.

• ## Method 1

#### The right-hand grip rule

The right hand grips the solenoid with the fingers pointing in the same direction as the conventional current flowing in the loops of wire and the thumb points to the end of the solenoid that acts like the N pole of a bar magnet; that is, the end of the solenoid from which the magnetic field lines emerge.

• ## Method 2

Draw a diagram of the ends of the solenoid, and mark in the direction of the conventional current around the solenoid. Then mark on the diagram the letter N or S that has the ends of the letter pointing in the same direction as the current. N is for an anticlockwise current, S is for a clockwise current.

Definition: an electromagnet is a solenoid that has a soft iron core. When a current flows through the solenoid, the iron core becomes a magnet. The polarity of the iron core is the same as the polarity of the solenoid. The core produces a much stronger magnetic field than is pro­duced by the solenoid alone. In Figure 3.1.1 (11-12) the magnetic field of a permanent magnet is compared to that of an electromagnet.

The strength of an electromagnet can be increased by:

– increasing the current through the solenoid

– adding more turns of wire per unit length for a long solenoid

– increasing the amount of soft iron in the core.