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Electric Currents and
Magnetic Fields
History
Lodestones were discovered 2000 years
ago and were magnetic. They were named
after Magnesia which is a region in Greece
where they were found.
 The Chinese used them for navigating
ships in the 12th century. We know they
contained iron ore which is called
magnetite.

Moving Charges Create Magnetic
Fields

In 1820 a Danish scientist Hans Christian
Oersted noticed that a wire containing
current would deflect a compass needle.
 This indicated the relationship between electricity
and magnetism.
 Moving charges create a magnetic field.
 Current running through a wire will create a
magnetic field.
 The direction of magnetic field can be determined
by the right hand rule.
Magnetic field lines

Bar Magnet
Wire with current
Earth’s Magnetic Field

The earth is basically one huge magnet.
However, the north and south poles are
not located exactly at the locations we call
the north and south poles.
Earth’s Magnetic Field Changes?
The angle between the geographic (true)
North Pole according to the earth’s magnetic
field and the magnetic north that compasses
point to is called magnetic declination.
 Magnetic north has wandered throughout
geologic time. More than 20 reversals have
been recorded in the orientation of the
dipoles in rocks on earth (over 5 billion
years!)

Compasses Align in the direction of
the field
Magnetic fields created by wires
Charged Particles in Magnetic fields

Moving charges create a magnetic field.
– When charged particles move through an
external magnetic field their magnetic fields
interact and the charge experiences a force.
– The force the charge experiences is always
perpendicular to the charge’s velocity and the
external magnetic field.
Right Hand Rule

Use your right hand:
– The palm of your hand pushes in the direction
of the force the charge experiences.
– Your outstretched fingers point in the
direction of the external magnetic field.
– Your thumb points in the direction of the
current or the velocity of a negative charge
charge moving through the external field.
Right Hand Rule for charges
entering a magnetic field
Right Hand rule is used to determine the
direction in which the metal rod is deflected
To calculate the force a charge experiences
when traveling in an external magnetic field

Fmagnetic = Bvqsinθ
– Fmagnetic magnetic force exerted on the charge by the external
magnetic field.
 Unit - Newtons
– q the magnitude of the charge traveling in the magnetic field.
 Unit - Coulombs
– v the velocity of the charge traveling in the magnetic field
 Unit - m/s
– B The magnitude the the external magnetic field.
 Unit - Tesla
 One Tesla is equal to 1 C of charge is moving 1 m/s perpendicular
to the external magnetic field and experiencing 1 N of force.

sinθ angle between v and B

Degree of force the charge experiences
depends on the direction of motion
relative to the external magnetic field.
– The force has its maximum value when it is
traveling perpendicular to the magnetic field.
– The force decreases when the charge moves
at angles to the magnetic field.
– The charge will experience zero force when it
is moving parallel to the magnetic field lines.
– When a charge is traveling perpendicular to a
uniform magnetic field the particle will make a
circular path.
Right Hand Rule for current Flowing
through a wire.
Force exerted on a wire carrying charge
perpendicular to an external magnetic field
 Fmagnetic
= B l I sinθ
– Fmagnetic Force wire experiences due to the external
magnetic field.
–B
– l (L)
–I
– sinθ
 Unit – Newtons
Magnetic Field
 Unit – Tesla
Length of the wire
 Unit – meters
Current in the wire
 Unit – amperes
angle between I and B
Devices Utilize Electromagnetism
Electromagnets
 Galvanometer
 DC Motors
 Generators
 Transformers

Electromagnets
A coil of wire attached to a battery creates a
magnetic field inside the coil.
 The overlapping magnetic field lines intensify the
field inside the coil.
 If a piece of metal is placed inside the coil, the
domains of the metal become aligned with the
magnetic field inside the coil and strengthens
the field.
 Change the direction of current and you change
the direction of the magnetic field.

Galvanometers

Device that measures current using
magnetic force.
– A coil of wire is wrapped around a soft iron
core that is mounted on a pivot attached to a
needle.
– When current is run through the wire the
magnetic field created by the current interacts
with an external magnetic field.
– The degree of deflection is related to the
amount of current in the coil.
Electric Motors

Motors use electricity to create mechanical
motion.
– Motors use direct current.
– A coil attached to a battery is rotated in the presence
of a permanent magnet.
– Current flows in opposite directions in the upper and
lower part of the loop.
– As the current moves through the upper part of the
loop it is deflected by the permanent magnetic field.
And rotates half a turn.
– The direction of current is switched every half rotation
by contacts at the end of the shaft which keeps the
loop rotating in the same direction.
DC Motor
DC Motor
• http://www.walterfendt.de/ph14e/electricmotor.htm
Electromagnetic Induction

Electric current can create a magnetic field but
can a magnetic field create current?
– British scientist Michael Faraday and American
scientist John Henry in 1831 independently
discovered the answer is yes.
– They found by moving a magnet in and out of a coil
of wire electric current is induced.
– A changing magnetic field can create current.
– The amount of current induced is dependent on:
 The rate in which the magnet s moved in and out of the coli
of wire.
 The number of coils in the wire.
Generator Simulation
• http://www.walterfendt.de/ph14e/generator_e.htm
• http://www.physclips.unsw.edu.au/jw/electr
icmotors.html#mandg