Magnetic Fields (SL IB Physics)

• A magnetic field is a region of space in which a magnetic pole will experience a force
• A magnetic field 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

Magnetic Flux Density

• The strength of a magnetic field can be described by the density of its field lines
• The magnetic flux density B of a field is defined as

The number of magnetic field lines passing through a region of space per unit area

• Magnetic flux density is measured in teslas (T) 
• One tesla, 1 T, is defined as

The flux density that causes a force of 1 N on a 1 m wire carrying a current of 1 A at right angles to the field

• The higher the flux density, the stronger the magnetic field i.e. regions where field lines are closer together
• The lower the flux density, the weaker the magnetic field i.e. regions where field lines are further apart 

Representing Magnetic Fields

• Like with electric fields, field lines are used to represent the direction and magnitude of a magnetic field
• In a magnetic field, field lines are always directed from the north pole to the south pole

The magnetic field lines around a bar magnet show the field is strongest at the two poles

• The simplest representation of magnetic field lines can be seen around bar magnets
  • These can be mapped using iron filings or plotting compasses

• The key aspects of drawing magnetic field lines are:
  • Arrows point out of a north pole and into a south pole
  • 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

Magnetic Field Between Two Bar Magnets

• 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

Two opposite poles attract each other and two like poles repel each other

Uniform Magnetic Fields

• In a uniform magnetic field, the strength of the magnetic field is the same at all points
• This is represented by equally spaced parallel lines, just like electric fields

A uniform magnetic field has equally spaced field lines and is created when two opposite poles are held close together

The Earth's Magnetic Field

• On Earth, in the absence of any magnet or magnetic materials, a magnetic compass will always point north
• This is because the north pole of the compass is attracted to the Earth's magnetic south pole (which is the geographic north pole)

The Earth's magnetic field acts in a similar way to a bar magnet. A compass points to the Earth's magnetic south pole which is the geographic north pole

• Magnetic fields are formed wherever a current flow, such as in:
  • long straight wires
  • long solenoids
  • flat circular coils

Magnetic Field around a Current-Carrying Wire

• Magnetic field lines in a current-carrying wire are circular rings, centred on the wire
• The field lines are closer together near the wire, where the field is strongest
• The field lines become further apart with distance from the wire as the field becomes weaker
• Reversing the current reverses the direction of the field

The direction of the field around a current-carrying wire can be determined using the right-hand grip rule

• The field lines are clockwise or anticlockwise around the wire, depending on the direction of the current
• The direction of the magnetic field can be determined using the right-hand grip rule
  
  • This is determined by pointing the right-hand thumb in the direction of the current in the wire and curling the fingers onto the palm
  • The direction of the curled fingers represents the direction of the magnetic field around the wire
  • For example, if the current is travelling vertically upwards, the magnetic field lines will be directed anticlockwise, as seen from directly above the wire

• Note: the direction of the current is taken to be the conventional current i.e. from positive to negative, not the direction of electron flow

Magnetic Field around a Solenoid

• As seen from a current-carrying wire, an electric current produces a magnetic field
• An electromagnet utilises this by using a coil of wire called a solenoid
  • This increases the magnetic flux density by adding more turns of wire into a smaller region of space
• One end of the solenoid becomes a north pole and the other becomes the south pole

The magnetic field lines around a solenoid are similar to a bar magnet

• As a result, the field lines around a solenoid are similar to a bar magnet
  • The field lines emerge from the north pole
  • The field lines return to the south pole

• The poles of the solenoid can be determined using the right-hand grip rule
  • The curled fingers represent the direction of the current flow around the coil
  • The thumb points in the direction of the field inside the coil, towards the north pole

In a solenoid, the north pole forms at the end where the current flows anti-clockwise, and the south pole at the end where the current flows clockwise

Magnetic Field around a Flat Circular Coil

• A flat circular coil is equivalent to one of the coils of a solenoid
• The field lines emerge through one side of the circle (north pole) and enter through the other (south pole)
• As with a solenoid, the direction of the magnetic field depends on the direction of the current
  • This can be determined using the right-hand grip rule
  • It is easier to find the direction of the magnetic field on the straight part of the circular coil to determine which direction the field lines are passing through

Magnetic field lines of many individual circular coils can be combined to make a solenoid