Electromagnetic Induction
If north side being moved into page, current moving
anticlockwise, and vice versa. If south side being moved into
page, current moving clockwise.
Introducing a current (magnetic flux)
Equal to amount of m field passing thru area.
Dependant on B and A. Flux thru coil is
A*perpendicular B (𝜑 = 𝐵𝐴 in Wbm-2)
B MUST BE PERPENDICULAR
Potential difference (faraday’s law of induction)
States that changing magnetic flux thru a circuit induces emf in
circuit. The magnitude of induced emf depends on rate of
𝑛∆𝜑𝐵
change of magnetic flux thru a circuit.
𝜀=−
∆𝑡
When calculating direction of current
flow, it is opposite of the emf (i.e. if ∆𝜑𝐵 = 𝜑𝑓𝑖𝑛𝑎𝑙 − 𝜑𝑖𝑛𝑖𝑡𝑖𝑎𝑙
emf is into page, induced current is out of page)
Eddy currents
Created
when there is relative movement between magnetic field
`
and metal object. Area of magnetic field does not cover whole of
metal object. Eddy currents also created when conducting
material is in presence of changing magnetic field. Eddy currents
increase temp of metal objects due to collisions between moving
charges and atoms of metal. Well known example this include
induction cookers.
Movement of metal
Eddy current loop
Eddy current loop can be
explained in terms of right
hand rule
Insulation layer
Iron layer
Increasing current
Lenz’s law
States that induced emf always gives rise to current that creates
magnetic field that opposes original change in flux thru circuit.
In the Lenz’s law diagram:
a) No change in inducing flux, current and induced flux
b) Inducing flux increases, current counter clockwise,
induced flux in opposite direction as B1
c) Inducing flux decreases, current clockwise, induced
flux in same direction as B1
d) Inducing flux increases, current clockwise, induced
flux in opposite direction as B1
e) Inducing flux decreases, current counter clockwise,
induced flux in same direction as B1
The right hand rule for finding the
direction of a magnetic field created
by a current I. The current moves to
the direction of your thumb and the
magnetic field curves to the direction
of the fingers
Transformers
Devices that convert AC input voltage signal to higher or lower
AC output voltage. Consists of primary and secondary coil,
linked by iron core. (𝑉𝑝 = 𝑛𝑝 𝑎𝑛𝑑 𝑉 𝐼 = 𝑉 𝐼 )
𝑉𝑠
Note that: 𝑉𝑠 =
Direction of
force on particles
Positive particles Direction of
current flow
Direction of
external
magnetic field
(N-S)
Energy distribution & power loss
Losses in powerlines calculated using 𝑃𝑙𝑜𝑠𝑠 = 𝐼 2 𝑅
Power losses in powerlines reduced with use of step up and step
down transformers to increase voltage which decreases current
𝑅=
𝜌𝑙
𝐴
(p is resistivity) 𝑉𝑑𝑟𝑜𝑝 = 𝐼𝑅 𝑃 = 𝑉𝐼 𝑃𝑙𝑜𝑠𝑠 = 𝐼 2 𝑅
Notes:
- Another way to find direction of
current in a solenoid (coil of wire)
- If an object being removed from
o and x emf, current is
anticlockwise and clockwise
(respectively)
- If an object is being added into o
an x emf, current is clockwise and anticlockwise respectively
𝑛𝑠 ∆𝜑
∆𝑡
𝑝 𝑝
𝑛𝑠
and 𝑉𝑝 =
𝑠 𝑠
𝑛𝑝 ∆𝜑
∆𝑡
If ns is greater than np, output voltage will be greater than input.
This is a step-up transformer If vice versa, it is a step-down
transformer.
Iron core
Flux
Primary coil
Secondary coil
Iron layers separated by insulator
Limitations of ideal transformer:
- flux leakage
- heat production
- eddy currents produced
- Electromagnetic induction is the generation of an emf
and/or electric current with the use of a magnetic field.
- Side of the eddy current loop inside magnetic field
experiences force due to magnetic field. Direction of force on
eddy current determined using the right-hand push rule and is
always opposite to direction of motion of sheet.