1. Magnetism – Magnetic Fields
Like electric and gravitational force, magnetic force is a field force. The
definition of a magnetic field is similar to that of an electric field.
Magnetic fields can be represented by magnetic lines of force ‘magnetic flux’.
All magnets have two poles ‘north pole and south pole’. Like poles repel and
unlike poles attract.
‘A magnetic circuit is all the space occupied by a magnetic field (flux)’
Magnetic field around a bar
magnet
N S

2. Magnetism – Magnetic Fields
At any point in a magnetic field, the direction of a line of force is the
direction of the magnetic force that the field would exert on a north
pole at that point.
Magnetic lines of force always form complete closed loops.
Magnetic lines of force tend to be as short as possible.
Magnetic lines of force repel one another and cannot intersect.
Magnetic lines of force
between unlike poles
N S N S
Magnetic lines of force
between like poles
N S NS

3. Magnetic Fields - Current Carrying Conductor
When an electric current passes through a conductor a magnetic field is set up
around the conductor.
The magnetic lines of force form concentric circles perpendicular to the axis of
the conductor.
As we move outward from the centre of the conductor, these circular lines of
force are spaced further apart and the strength of the magnetic field decreases.
Conventional current
flow
Conventional current
flow
Although the magnetic field around a straight conductor has no poles, the lines
of force do have direction depending on the direction of current flow.

4. Magnetic Fields – The Right Hand Rule
We can determine the direction of the magnetic field around a straight
current-carrying conductor by the right-hand rule.
When the thumb of a right hand points in the direction of conventional current
through a conductor, the fingers point in the direction of the magnetic lines of
force when grasping the conductor.
Direction of
fluxRight hand
Conventional
current

5. Magnetic Fields – Direction of Field
We can determine the direction of the magnetic field around a straight
current-carrying conductor by the right-hand rule.
When the thumb of a right hand points in the direction of conventional current
through a conductor, the fingers point in the direction of the magnetic lines of
force when grasping the conductor.
forces conductors apart forces conductors together

6. Magnetic Fields – The Right Hand Rule for a Solenoid
We can concentrate the magnetic field of the conductor by winding the
conductor around an iron core to form a solenoid.
When the fingers of the right hand circling a solenoid point in the direction of
conventional current, the thumb points in the direction of the magnetic lines of
force through the solenoid.
Right handDirection of
flux
Conventional
current+ -

7. Magnetic Fields – Force on a Current Carrying Conductor
When current carrying conductor is placed within a magnetic field a force is
exerted on the conductor in the directions shown.
When the direction of magnetic flux lines are in opposition they tend to cancel out
and bend away from the conductor, weakening the magnetic field.
When the direction of magnetic flux lines are the same the flux density increases
and bend the lines of flux away from the conductor.
Since the lines of flux tend to become as short as possible the magnetic field
exerts a force on the conductor.
N S
Direction of force on
conductor
N SX
Direction of force on
conductor
NN

8. Magnetic Fields – Summary
• Lines of force always form closed loops.
• All magnets have a north and south pole.
• Unlike poles attract, like poles repel.
• A current carrying conductor produces a magnetic field.
• Fleming’s ‘Righthand’ rule can be used to determine the direction of a magnetic
field.
•Fleming’s ‘Lefthand’ rule can be used to determine the direction of a force on a
conductor in a magnetic field.