Section 5:
Magnetic Fields
Magnetic Flux Density
Magnetic fields:
• A magnetic field is like a gravitational field or electric field. It is a region in which a force would
act on or the direction that a small north
pole would take.
Magnetic fields around a wire:
When a current flow in a wire or any other long straight conductor, a magnetic field is induced around
the wire. The field lines are concentric circles centred on the wire.
• The direction of the magnetic field around a
current carrying wire can be worked out with the
right-hand rule:
1. Curl your right hand into a fist and stick your
thumb up
2. Point your thumb in the direction of the current
through the wire
3. Your curled fingers will then show the direction of
the field
Solenoids:
Looping a current-carrying wire into a coil in one plane,
the surrounding magnetic field is doughnut shaped, while
a coil with length (a solenoid) forms a field like a bar
magnet.
Forces on a current-carrying wire:
• If a current-carrying wire is put into an external magnetic field (e.g. between two magnets), the
field around the wire and the field from the magnets are added together.
o This causes a resultant field – the lines closer together show where the magnetic field is
stronger. The bunched-up lines cause a pushing force on the wire.
Magnetic Fields
Magnetic Flux Density
Magnetic fields:
• A magnetic field is like a gravitational field or electric field. It is a region in which a force would
act on or the direction that a small north
pole would take.
Magnetic fields around a wire:
When a current flow in a wire or any other long straight conductor, a magnetic field is induced around
the wire. The field lines are concentric circles centred on the wire.
• The direction of the magnetic field around a
current carrying wire can be worked out with the
right-hand rule:
1. Curl your right hand into a fist and stick your
thumb up
2. Point your thumb in the direction of the current
through the wire
3. Your curled fingers will then show the direction of
the field
Solenoids:
Looping a current-carrying wire into a coil in one plane,
the surrounding magnetic field is doughnut shaped, while
a coil with length (a solenoid) forms a field like a bar
magnet.
Forces on a current-carrying wire:
• If a current-carrying wire is put into an external magnetic field (e.g. between two magnets), the
field around the wire and the field from the magnets are added together.
o This causes a resultant field – the lines closer together show where the magnetic field is
stronger. The bunched-up lines cause a pushing force on the wire.
