# 25.1.1 Representing Magnetic Fields

### 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 Bfield
• 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 3D 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

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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