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Summary Magnetic Fields Physics A-Level
Summary of Magnetic Fields for Physics
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B-fieldsSpecification
(a) predict the direction of the force on a current-carrying conductor in a magnetic field,
(b) define magnetic field B by considering the force on a current carrying conductor in a
magnetic field; recall and use F = BIlsinθ,
(c) use F Bqvsinθfor a moving charge in a magnetic field;
(e) understand the processes involved in the production of a Hall voltage and understand that
VH B for constant I.
(f) describe how to investigate steady magnetic fields with a Hall probe,
(g) sketch the magnetic fields due to a current in (i) a long straight wire, (ii) a long solenoid,
(h) use the equations for the field strengths due to a long straight wire and in a long solenoid,
(i) know that adding an iron core increases the field strength in a solenoid,
(j) explain why current-carrying conductors exert a force on each other and predict the
directions of the forces,
(k) understand how the equation for the force between two currents in straight wires leads to
the definition of the ampere,
The force on a wire carrying current in a magnetic field
Wires carrying a current at an angle to a magnetic field will experience a force (this is
called the motor effect). The force is given by the equation: F = BIlsinθ.
B is the magnetic flux density (or B-field). It is equivalent to the field strength and it
is related to the density of field lines. It is a vector quantity and so the magnetic flux
density at a given point is the vector sum of the magnetic flux density at that point.
Unit = Tesla, T (large unit like C). Field lines point from north to south.
I is the current.
l is the length of the wire in the B-field (see below for complex example of l)
θ is the angle between the wire (carrying the current) and the magnetic field (Isinθ is
the component of the current perpendicular to the field). To obtain the maximum
force (for a given field, wire and current) you need sinθ = 1. The angle θ should be
90°, i.e. the wire should be at right angles to the magnetic field (B-field). In the 3D
diagram shown, the wire passes through the B-field perpendicular to the field so you
can simplify the equation (in this case) to: F = BIl.
, Sometimes circular magnets are used so that at every point on the coil, it is
perpendicular to the magnetic field and so the whole coil feels a force and so the
overall force on the coil is maximised.
Assuming the coil has diameter d and has N turns, in the equation, l = Nπd. This
shows you will sometimes need to adjust l to account for different circumstances.
Fleming’s left-hand rule (FLHR)
The direction of the force can be found using Fleming’s left-hand rule:
NB “current” refers to the component of the current perpendicular to the field.
Sometimes the magnetic field will not be represented by field lines but rather by
arrow heads or arrow tails (remember O = out of page):
Motor
When the coil is vertical, the split ring commutator is no longer in contact with the
brush contacts and so the coil carries no current.
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