Comparing Fields

A LEVEL
PHYSICS
MAGNETISM
GRAVITAIONAL FIELD
Vs MAGNETIC FIELD
 Gravitational Fields
 Determined only by the mass ( or mass-energy) of a body.
Charged and uncharged massive particles produce the same
gravitational field.
 Gravitational fields have no polarity at all.
 At large distances, gravitational fields diminish as the inverse
square of distance from their source.
 Gravitational fields can be detected by using anything to
measure a change in velocity.
 Magnetic Fields
 Produced by charged particles in motion, and depend on the
charge and velocity of these particles, but not on their mass.
 Magnetic fields are 'polar' fields with a North and South
polarity.
 Magnetic fields at large distances from their source, decrease
as the inverse cube of the distance.
 We can only detect magnetic fields by using charged particles
to measure their deflection.
COMPARING ELECTRIC, MAGNETIC
AND GRAVITAIONAL FIELDS
GRAVITAIONAL
FIELD ELECTRIC FIELD MAGNETIC FIELD
SOURCE Point mass Point charge No point source,
dipole required
Field intensity M / r2 Q / r2 n / a because no
proportional to point source
Range Long range Short range Short range
Strength Weak Strong Strong
Depends on the
direction of the
motion of a charge
Direction of field Towards center of Towards center of generating the
lines mass charge magnetic field lines.
otherwise they go
from north to south.
MAGNETIC FLUX
 DEFINITION: Product of the magnetic flux and
area normal to the lines of flux.
For a uniform magnetic field of flux density B
which makes an angle Ө with an area A, the
magnetic flux (ф) is given by the expression
Ф = BA sinӨ
 SI unit: Weber (Wb)

 1 Wb = 1 Tesla meter2 (Tm2)
ELECTROMAGNETIC
INDUCTION
 Henry and Faraday demonstrated that an emf could be
induced by a magnetic field. This effect was called
electromagnetic induction.
 Electromagnetic induction can be identified easily with
the help of a galvanometer.
 GALVANOMETER: An instrument that detects very small
currents and also shows the direction of the current
induced.
 An emf is induced when
 The wire is moved through the magnetic field.
 The magnet is moved so that the wire passes across the face
of the poles.
 An emf is not induced when
 the wire is held stationary between the poles.
 The magnet is moved so that the poles move along the length
of the wire.
 The wire is moved length wise so that the position between
the poles is not changed.
ELECTROMAGNETIC
INDUCTION
 Conclusions
 The magnitude of the emf
 Increases as the speed at which the wire is moved increases.
 Increases as the speed at which the magnet is moved increases.
 Increases if the wire made into loop with several turns.
 Increases as the number of turns n the loop increases.
 Magnitude of induced emf depends on the

rate at which the flux lines are cut by a
single wire or by different number of turns.
 Important factors for magnetic flux
linkage(Nф)
 Change in magnetic flux
 Number of turns
Change in magnetic flux linkage Δ(Nф) = NΔф

FARADAY’S LAW
 DEFINITION: the emf induced is proportional to
the rate of change of magnetic flux linkage.
 This rule helps in identifying direction of
current, magnetic field and force.
When thumb, Index finger and middle
finger are held perpendicular to each other
then,
Thumb indicates FORCE
Index Finger indicates MAGNETIC FIELD
Middle Finger indicates DIRECTION OF
CURRENT
MOTOR EFFECT (RECAP)
 The direction of the induced emf can be found by
reference to the motor effect and conservation of
energy.
 Since the wire is in the form of continuous loop,
induced emf gives raise to current.
 By motor effect, there will be a force on the wire.
Note: An electric current is a form of energy

and this energy must have been converted
from other form.
o Movement of wire against the
electromagnetic force means that work
has been done in overcoming this force
and it is this work which is seen as
electrical energy.
LENZ LAW
 DEFINITION: The direction of the induced emf is such as to
cause effects to oppose the change producing it.
E = d(Nф)/dt
 E is the induced emf by the rate of change of flux linkage

of d(Nф)/dt.
 The minus sign indicates that the induced emf causes
effects to oppose the change producing it.
 For a small change Δ(Nф) in flux linkage that occurs in time
Δt (or flux linkage changes with time linearly), then induced
emf is given by
E = -Δ(Nф)/Δt

Comparing Fields

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A LEVEL

 SI unit: Weber (Wb)

 Magnitude of induced emf depends on the

Change in magnetic flux linkage Δ(Nф) = NΔф

Note: An electric current is a form of energy

 E is the induced emf by the rate of change of flux linkage

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