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Summary OCR-A A2 Level Magnetic Fields Notes
A summary of magnetic fields, corresponding to Chapter 22 of the OCR Physics A textbook
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Magnetic FieldsMagnetic field patterns
● A magnetic field is a field surrounding a permanent magnet or a
current-carrying conductor in which magnetic objects experience
a force
○ You can detect the presence of a magnetic field with a small
plotting compass
○ The needle will deflect in the presence of a magnetic field
● Magnetic field lines map magnetic field patterns around magnetics
and current-carrying conductors
○ Magnetic field patterns are visual representations that help
us to interpret the direction and strength of the magnetic
fields
○ The arrow on a magnetic field points from north to south
○ Equally spaced and parallel magnetic field lines represent a
uniform field, so the strength of the field does not vary
○ The magnetic field is stronger when the magnetic field lines are closer
○ Like poles repel and unlike poles attract
Electromagnetism
● When a wire carries a current, a
magnetic field is created around the
wire
○ The field is created by the
electrons moving within the
wire
● Any charged particle that moves
creates a magnetic field in the space
around it
● For a current-carrying wire, the
magnetic field lines are concentric
circles centered on the wire and
perpendicular to it
○ The direction of the magnetic field can be determined using the
right-hand screw rule
● Both the coil and the solenoid produce north and south poles at their opposite
faces (Fig.5)
○ At the center of the core of the solenoid, it is uniform
, Magnetic fields and forces
● Magnets are made of alloys of a rare earth element, neodymium
● The strength of magnets, and magnetic fields, is measured in
Tesla
● A current-carrying conductor is surrounded by its own
magnetic field
○ When the conductor is placed in an external magnetic
field, the 2 fields interact like the fields of 2 permanent
magnets
○ The 2 magnets experience equal and opposite forces
● The direction of the force experienced by a current-carrying
conductor placed perpendicular to the external magnetic field
can be determined using Fleming’s left-hand rule
○ First finger gives the direction of the external magnetic
field
○ Second finger gives the direction of the conventional
current
○ Thumb gives the direction of motion (force) of the wire
Magnetic flux density
● The magnitude of the force experienced by a
wire in an external magnetic field depends on a
number of factors
○ The force is a maximum when the wire is
perpendicular to the field
○ The force is 0 when the wire is parallel to
the magnetic field
● The magnitude of the force experienced by the
wire is directly proportional to the:
○ Current
○ Length of the wire in the magnetic field
○ sin 𝛉, where 𝛉 is the angle between the
magnetic field and the current direction
○ The strength of the magnetic field
○ Therefore, F = BILsin𝛉
● B is the magnetic flux density - the strength of
the field - and is a vector quantity
○ The SI unit is the Tesla
● The magnetic flux density is 1T when a wire
carrying a current of 1A placed perpendicular to
the magnetic field experiences a force of 1N per meter of its length
● When the wire is perpendicular to the magnetic field, 𝛉 = 90° and sin 𝛉 = 1
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