Electricity & Magnetism

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Fields and forces
 The concept of a field is used to describe any
quantity that has a value for all points in space.
 You can think of the field as the way forces are
transmitted between objects.
 Charge creates an electric field that creates forces
on other charges.
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Fields and forces
 Gravitational forces are far weaker than electric
forces.

the electric field

EMF – Electric & Magnetic
Fields
 Electricity produces two types of fields; an electric
field and a magnetic field called electromagnetic
fields or EMF.
 Electric fields are created by the presence of electric
charges and are measured in volts per meter (V/m).
 An electric field is associated with any device or
wire that is connected to a source of electricity,
even when a current is not flowing.

Electric fields and
electric force
 On the Earth’s surface, the gravitational field
creates 9.8 N of force on each kilogram of mass.
 With gravity, the strength of the field is in Newton
per kilogram (N/kg) because the field describes the
amount of force per kilogram of mass.

Electric fields and
electric force
• With the electric field, the strength is in Newton
per coulomb (N/C).
• The electric field describes the amount of force per
coulomb of charge.

What is a magnet?
• If a material is magnetic, it has the ability to
exert forces on magnets or other magnetic
materials.
• A permanent magnet is a material that keeps its
magnetic properties even when it is NOT close to
other magnets.

Magnetism is the properties and interactions
of magnets
Magnets produce magnetic forces and have
magnetic field lines
magnetism

magnetism
 The magnetic field of a coil is identical to the field of
a disk-shaped permanent magnet.

The force between two
magnets
 The strength of the force
between magnets depends
on the distance between
them.
 The magnetic force
decreases with distance
much faster than does
either gravity or the
electric force.

The magnetic field
 All magnets create a magnetic
field in the space around them,
and the magnetic field creates
forces on other magnets.
 The number of field lines in a
certain area indicates the
relative strength of the magnetic
field in that area.
 The closer the lines are
together, the stronger the field.
 The arrows on the field line
syndicate the direction of the
force

The Magnetic Field of the
Earth
 When you use a compass, the
north-pointing end of the
needle points toward a spot
near (but not exactly at) the
Earth’s geographic north pole.
 The Earth’s magnetic poles
are defined by the planet’s
magnetic field.
 That means the south
magnetic pole of the planet is
near the north geographic
pole.

The Magnetic Field of the
Earth
 Depending on where you are, a compass will point
slightly east or west of true north.
 The difference between the direction a compass
points and the direction of true north is called
magnetic declination.
After correcting for the
declination, you rotate the
whole compass until the
north-pointing end of the
needle lines up with zero
degrees on the ring.
The large arrow points in the
direction you want to go.

Electric Current and
Magnetism
 Two wires carrying electric current exert force on
each other, just like two magnets.
 The forces can be attractive or repulsive
depending on the direction of current in both
wires.

Magnetism
 The magnetic field around a single wire is too
small to be of much use.
 There are two techniques to make strong
magnetic fields from current flowing in wires:
1. Many wires are bundled together, allowing the
same current to create many times the
magnetic field of a single wire.
2. Bundled wires are made into coils which
concentrate the magnetic field in their center.

Magnetism
 The electrons moving
around the nucleus carry
electric charge.
 Moving charge makes
electric current so the
electrons around the
nucleus create currents
within an atom.
 These currents create the
magnetic fields that
determine the magnetic
properties of atoms.

Magnetic force on a
moving charge
 The magnetic force on a wire is really due to force
acting on moving charges in the wire.
 A charge moving in a magnetic field feels a force
perpendicular to both the magnetic field and to
the direction of motion of the charge.

RIGHT HAND RULE
 It’s a method of determining
the direction of Force (F),
Current (I) or Magnetic
Field (B)
The direction of the force can
be deduced from the right-
hand rule.
If you bend the fingers of your
right hand as shown, your
thumb, index, and middle
finger indicate the directions
of the force, current and
magnetic field.

RIGHT HAND RULE

Magnetic force on a
moving charge
 A magnetic field that has a strength of 1 tesla (1 T)
creates a force of 1 Newton (1 N) on a charge of 1
coulomb (1 C) moving at 1 meter per second.
 This relationship is how the unit of magnetic field
is defined.

Magnetic force on a
moving charge
 A charge moving perpendicular to a magnetic field
moves in a circular orbit.
 A charge moving at an angle to a magnetic field
moves in a spiral.

Magnetic field near a wire
The field of a straight wire is proportional to the
current in the wire and inversely proportional to the
radius from the wire.
Magnetic field
(T)
Radius (m)
Current (amps)
B = 2x10-7 I
r

Magnetic fields in a coil
The magnetic field at the center of a coil comes from
the whole circumference of the coil.
Magnetic
field
(T)
Radius
of coil (m)
Current
(amps)
No. of turns of
wire
B = 2 x10-7 NI
r

Electromagnets and the
Electric Motor
 Electromagnets are magnets
that are created when electric
current flows in a coil of wire.
 A simple electromagnet is a
coil of wire wrapped around a
rod of iron or steel.
 Because iron is magnetic, it
concentrates and amplifies
the magnetic field created by
the current in the coil.

Electromagnets and the
Electric Motor
The right-hand rule:
 When your fingers curl in
the direction of current,
your thumb points
toward the magnet’s
north pole.

The principle of the
electric motor
 An electric motor uses electromagnets to convert
electrical energy into mechanical energy.
 The disk is called the rotor because it can rotate.
 The disk will keep spinning as long as the external
magnet is reversed every time the next magnet in
the disk passes by.
 One or more stationary magnets reverse their
poles to push and pull on a rotating assembly of
magnets.

Electric Motors
 If you take apart an electric motor that runs on
batteries, the same three mechanisms are
there; the difference is in the arrangement of
the electromagnets and permanent magnets.

Electric motors
 The rotating part of the
motor, including the
electromagnets, is
called the armature.
 This diagram shows a
small battery-powered
electric motor and
what it looks like
inside with one end of
the motor case
removed.

Electric motors
 The permanent magnets
are on the outside, and
they stay fixed in place.
 The wires from each of the
three coils are attached to
three metal plates at the
end of the armature.
commutator

Electric Motors
 As the motor spins, the three plates come into
contact with the positive and negative brushes.
 Electric current flows through the brushes into
the coils.

 As the motor turns, the plates rotate past the brushes,
switching the electromagnets from north to south by
reversing the positive and negative connections to the
coils.
 The turning electromagnets are attracted and repelled
by the permanent magnets and the motor turn
Electric Motors
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