# Using Magnetic Fields to Keep Things Moving

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ELECTROMAGNETS

The ends of a magnet are called magnetic poles.  There is a north pole at one end and a south pole at the other end.  The region around the magnet in which a magnetic object will be attracted to it is called the magnetic field.  Iron filings placed near a magnet will form a pattern of lines that will loop from one pole to the other.  These are magnetic field lines.  A plotting compass placed in the magnetic field field will always point along a magnetic field line.  If two magnets are brought close to each other with like poles, they will repel each other.  If two magnets are brought together with opposite poles, they will attract each other.

When a current flows through a wire, a magnetic field is produced around the wire.  An electromagnet is made by wrapping insulated wire around a piece of iron called the core.  When a current flows through the wire, the iron becomes strongly magnetised.  When the current is switched off, the iron loses its magnetism.  This temporary magnetism makes electromagnets very useful. Electromagnets are used in devices such as scrapyard cranes, circuit breakers, electric bells and relays.

THE MOTOR EFFECT

When we place a wire carrying an electric current in a magnetic field, it may experience a force.  This is called the motor effect.  The force is a maximum if the wire is at an angle of 90 degrees to the magnetic field, and zero if the wire is parallel to the magnetic field.  Fleming's left hand rule can be used to determine the direction of the force.  THUMB - force, INDEX FINGER - magnetic field (north to south), MIDDLE FINGER - current (positive to negative).  The size of the force can be increased by increasing the strength of the magnetic field and increasing the size of the current.  The direction of the force on the wire is reversed if either the direction of the magnetic field or the direction of the current is reversed.  The motor effect is used in different appliances.

The speed of the motor can be increased by increasing the size of the current.  When a current passes through a coil, the coil spins because a force acts on each side of the coil