Magnetic Field Definition
- A magnetic field is a field of force that is created either by:
- Moving electric charge
- Permanent magnets
- Permanent magnets are materials that produce a magnetic field
- A stationary charge will not produce a magnetic field
- A magnetic field is sometimes referred to as a B-field
- A magnetic field is created around a current carrying wire due to the movement of electrons
- Although magnetic fields are invisible, they can be observed by the force that pulls on magnetic materials, such as iron or the movement of a needle in a plotting compass
Representing Magnetic Fields
- Magnetic fields are represented by magnetic field lines
- These can be shown using iron filings or plotting compasses
- Field lines are best represented on bar magnets, which consist of a north pole on one end and south pole on the other
- The magnetic field is produced on a bar magnet by the movement of electrons within the atoms of the magnet
- This is a result of the electrons circulating around the atoms, representing a tiny current and hence setting up a magnetic field
- The direction of a magnetic field on a bar magnet is always from north to south
- When two bar magnets are pushed together, they either attract or repel each other:
- Two like poles (north and north or south and south) repel each other
- Two opposite poles (north and south) attract each other
The key aspects of drawing magnetic field lines:
- The lines come out from the north poles and into the south poles
- The direction of the field line shows the direction of the force that a free magnetic north pole would experience at that point
- The field lines are stronger the closer the lines are together
- The field lines are weaker the further apart the lines are
- Magnetic field lines never cross since the magnetic field is unique at any point
- Magnetic field lines are continuous
- A uniform magnetic field is where the magnetic field strength is the same at all points
- This is represented by equally spaced parallel lines, just like electric fields
- Magnetic fields can be represented in 3D by using the following symbols:
- Dots represent the magnetic field directed out of the plane of the page
- Crosses represent the magnetic field directed into the plane of the page
Exam Tip
The best way to remember which way around to draw magnetic fields in $3 D$ is by imagining an arrow coming towards or away from you
- When the head of an arrow is coming towards you, you see the tip as a dot representing the arrow coming ‘out’ of the page
- When an arrow is travelling away from you, you see the cross at the back of the arrow representing the arrow going ‘into’ the page