Electromagnetic Induction
In a electric motor because the induced emf is proportional to the speed of rotation of the motor, the current changes as the motor speed changes
1) At low speed, the current is high because the opposing induced emf is small 2) At high speed, the current is low because the induced emf is high
What three parts of a tranfromers design makes transformers efficient?
1) Low - resistance winding to reduce power wasted due to the heating effect of the current 2) a laminated core that consists of layers of iron separated by layers of insulator. Induced current inside the core itself refferd to as eddy currents, are reduced in this way so the magnetic flux is a high as possible and the heating effect is reduced 3) A core of soft iron which is easily maganatised and demagnatised. This reduces power wasted through repeated magnatism ans demaganatims of the core
In a solenoid giev the direction of current at the north and south pole
1) current passes aNticlockwise round the North pole end 2) current passes clockwise round the south pole end
when is the induced emf at a maximum and zero on a alternating current generator
1) the induced emf is maximum when the sides of the coil cut at right angles across the feild lines 2) The induced emf is zero when the sides of the coil move parallel to the feild lines. At this possition the rate of change of lux is zero and the sides of the coil do not cut feild lines
Emf is induced in a wire if ...
1) there is relative motion between a magnet and a wire 2) The wire cuts the feild lines of the magnectic feild
step- up transformers at the power station increase the alternationg voltage to
400kV or more for long distance transmission via the grid system.
Each power station generates alternating current at a precise frequency of
50Hz and 25kV
Magnetic flux Φ
A measure of the strength of a magnetic field over a given area/ number of field lines. =BA (weber)
what does a trnasformer consist of
A transormer consits of two coils: the primary coil and the secondary coil. The two coils share the same iron core.
Induced emf ε =
BIv
In a step up transformer, the volatge is stepped up and the current is stepped
DOWN
Magnetic flux linkage
N x Φ = BAN cosθ. The product of the magnetic field strength, the area of a loop, the number of turns of wire in the loop and the cosine of the angle between the area normal and B. (scalar)
The Turns Rule is:
Ns/ Np = Vs/ Vp Ns = number of turns on the secondary coil Np = number of turns on the primary coil Vs = voltage across the secondary coil Vp = voltage across the primary coil
electromagnetic induction
Process by which electrical current is induced in a wire when it is moved through a magnetic field
back emf
The EMF that arises in an electric motor or similar device as the motor starts to turn, and which opposes the externally supplied EMF that is responsible for that turning. This is the back EMF, must act in the opposite direction to the applied EMF from the battery or power supply that caused the coil to move.
Magnetic flux linkage NΦ
The product of the magnetic flux and the number of turns in a given coil.
Why is transmission of electrical power over long distances much more effcient at high voltage than at low voltage
This is because the current needed to deiliver a certain amount of power is reduced if the voltage is increased. So the power wasted due to the heating effect of the current through that cables is reduced
what is the unit for Magnetic flux linkage NΦ
Tm^2 = Weber (Wb)
what is the unit for magnetic flux Φ
Tm^2 = Weber (Wb)
in a step down transformer, the voltage is steped down and the current is steped
UP
Emf equals the change in flux per second therfore the volt V per second =
Weber (Wb) per second
How does a trnasformer work
When the primary coil is connected to a source of altenrating pd and alternating magnetic field is produced in te core. Thfeild passes through the secondary coild, casuing an alternating emf to be induced in the secondary coil by the changing magnetic feild
A magnetic feild is produced in and around a coil when it is connected to
a batery and a current passes through it
solenoid
a coil of wire that produces a magnetic field when carrying an electric current
when does an alternating current generator alternate
because as the rectangular coil spins the fluck linkage changes consantly, as the angle to the feild lines changes. Remember N x Φ = BAN cosθ
Why must lenz's law hold true
because energy is never created or destroyed. The incduced current could never be in a direction to help the change that causes it, this would invole producing electrical energy from nowhere
How can a DC generator be made
by replacing the two slip rings of the ac generator with a split ring. The connections between the split ring and the brushes reverse every half cycle
transformers only work is the magnetic flux through them is
changing, they wouldnt work with steady AC
A step down transformer has
fewer turns on the secondary coil than the primar coil. So the seconday volatge is stepped down compared with the primary volatge Ns < Np so Vs < Vp
How are homes supplied in the grid system
homes are supplied via a local transformer sub- station witha single phase ac at 230V
If you change the number of turns in the coils (primary/ secondary) in a transformer you change the
induced emf. This allows you to change (transform) the voltage from the primary to the secondary coil.
A step up transformer has
more turns on the secondary coil than on the primary coil. So the secondary voltage is stepped up compared with the primary coil Ns>Np so Vs>Vp
Lenz's law
states that the direction of the induced current is always such as to oppose the change that causes the current
how do step down transformers operate
step down transfomrer operate in stages. Factoiers are supplied with all three phases at either 33kV or 11kV.
what is the unit for Magnetic flux density, B
tesla T = 1Nm^-1A^-1
electromotive force
the amount of energy derived from an electric source; the force that results from differences in potential that cause an electric current, commonly measured in volts
Magnetic flux density, B
the amount of magnetic flux in a unit area perpendicular to the direction of magnetic flow
When relative motion between a wire and magnet ceases
the emf becomes zero
Magnetic flux density
the magnetic force per unit length, per unit current on a current carrying conductor at right angles to the feild lines. (telsa T0
Faraday's law of electromagnetic induction
the magnitude of the emf induced in a circuit is equal to the rate of change of the magnetic flux linkage through the circuit | ε(avg)| = - N |ΔΦ / Δt| N: number of loops in a coil - minus represents that fact the induced emf acts in such a direction as to opposite the change that causes it
The effecicney of a transformer in words is
the power delivers by the secondary coil/ power supplied to the primary coil
An emf is induced in a rod containing free elecrons because
when there is relative motion each electron will experance a force at right rangles to the direction of motion of the rod. This forces electrons to one end of the rod so that one end becomes negative and the other end possitive. Hence, an emf is induced
For an alternating current generator ε=
εo sin 2 π f t where f is the frequency of rotation of the coil and εo is the peak emf.