Download Magnetism and Electromagnetic Induction

Magnetism
What can electricity tell us?
Similarities to Electricity
 No touching force
 N/S is analogous to +/ Create field lines to help us keep track
Important Differences
 N/S cannot be separated!
 Measured in Teslas (special unit) not N/C
(a derived unit)
 More than just magnetic poles
experience a force
Just who does
experience a force?
Electricity:
 Charged particles
Magnetism:
 Magnetic poles
 Moving charged
particles
There is an obvious connection between electricity and
magnetism that shows up here!
Magnetic Field Lines
Electricity:
 Force points in direction
of field, always tangent
to the field line
 Closer lines = stronger
field
 Leave positive and land
on negative
Magnetism:
 Force points in direction
of field, always tangent
to the field line
 Closer lines = stronger
field (called flux)
 Leave North and land on
South
The Connection Continues…
 Who creates magnetic fields?
 Magnetic Poles
 Moving Charged Particles
 Electricity!
What do magnetic field
lines look like?
 Magnetic Dipole:
 Near Current Carrying Wires:
Mapping other field lines…
 Perform the procedure to create your
own magnetic field lines around two sets
of magnets.
 Does the magnetic force follow an
inverse square law?
The Direction of the
Magnetic Field
 The Force, Field and Motion are
ALWAYS orthogonal!
 Motion can be a moving individual particle or
a current carrying wire
 Leads us to two equations:
Current Carrying Wire
F = I l B sin θ
 F = Force
 I = current
 l = length of wire in the field
 B = Magnetic Field (Teslas)
 Θ = the angle between the field and
current
Moving Charged Particle
F = q v B sin θ
 F = Force
 q = charge
 v = velocity of particle
 B = Magnetic Field (Teslas)
 Θ = the angle between the field and
velocity
The Right Hand Rule
 Since thumb, palm and fingers are
orthogonal, they serve as a mnemonic
device.
Field Lines Around a Wire
 Grab the wire with your thumb point in
the direction of the conventional current.
 Fingers point in the direction of the
magnetic field.
Forces on Wires
and Particles
 Point Fingers in direction of the current or
velocity.
 Point Palm in the direction of the
magnetic field.
 Thumb points in the direction of the force.
Review
 Moving charged particles create
magnetic fields.
 Moving charged particles in a magnetic
field experience a force.
 Charged particles in an electric field
experience a force.
 There must be some connection between
them!
Michael Faraday
 Faraday did many experiments that lead
to the connections between electricity
and magnetism.
 Many of them were about
electromagnetic induction.
Electromagnetic Induction
 If electricity (moving charged particles)
creates magnetic fields, then maybe the
opposite can be found to be true?
 Moving magnetic fields can create
electricity; called electromagnetic
induction.
Magnetic Flux
 Flux is defined as the number of
magnetic field lines that perpendicularly
intersect an area in space.
FB = B A cos Q
Flux = magnetic field times the
perpendicular area.
Faraday’s Law
 A changing magnetic flux causes
electricity.
E = -N (D FB/ Dt)
The induced emf (voltage) is related to the
number of loops of wire times the rate in
change of the magnetic flux.
How does the
flux change?
 Since FB = B A cos Q, then:
E = -N (D (B A cos Q) / Dt)
 So one of three things must be changing
to induce a voltage:
 Magnetic Field (B)
 Area (A)
 Angle (Q)
Change the Magnetic Field
 Move in and out of a uniform magnetic
field
 Move away or toward a magnetic pole
 Increase or decrease a current through a
wire
Changing the Area
 Increase the size of the hoop
 Decrease the size of the hoop
Changing the Angle
 Rotate the hoop near a uniform magnetic
field
 Rotate a hoop near a magnetic dipole
Magnetic Fields Due to
Loops of Wire
 Already learned that current carrying
wires create magnetic fields.
 Loops of wire with current create
magnetic fields.
 The fields intersect the area of the loop
and create flux.
 Use the right hand rule to determine the
direction of the flux.
Faraday’s Law Revisited
 E = -N (D (B A cos Q) / Dt)
 This voltage induces a current in the
wire: this is Faraday’s Law
 But what direction will the current move?
Lenz’s Law
 The direction of an induced current is
such that the magnetic flux created by
the current compensates for the flux
change that created the current.
 It is the negative sign in Faraday’s Law
 E = -N (D (B A cos Q) / Dt)