Module 6 Electromagnetism Flashcards
When is a current or EMF induced in a wire?
when there is relative motion between conductor and a magnetic field
When inducing an EMF in a wire by moving it through a magnetic field, which way is the force?
opposite of movement of the wire
When moving a wire through a magnetic field, what does the force act on?
on the charge carriers (electrons) in the wire
Explain the conservation of energy when an EMF is induced into a wire
moving in a magnetic field?
Force is induced in the opposite direction to movement of charges
Mechanical work must be done against this force to induce an EMF and current
Work done = gain in electrical energy
Explain how you find the direction of current when the north of a magnet is moved towards a solenoid
Current produced forms a magnetic field in the solenoid acting against the motion of the permanent magnet
Use the right hand grip rule on solenoid - where the thumb is direction of north
Curl of fingers gives direction of current in solenoid
What are the four factors affecting the magnitude of induced EM
Magnetic flux density of magnet
Area of turns in the coil
Number of turns on the coil
Speed of motion of magnet
Give the word equation for induced EMF using an electromagnet
minus rate of change of magnetic flux linkage
What is the symbol for magnetic flux?
Φ
Define magnetic flux in words
magnetic flux density x area when the field is at 90º to area
What are the units for magnetic flux?
webers (Wb)
How to differentiate between magnetic flux and magnetic flux density?
flux is magnetic flux density x area
Φ = B x A
Define magnetic flux linkage
The product of the magnetic flux Φ and the number of turns of the coil N
Units for magnetic flux linkage
Wb turns/Weber turns
State Lenz’s law
When an electric current is induced, it will always flow in a direction to oppose the change producing it
Describe how a simple AC generator can be established
B field
With a coil who’s face turns through the field
In an AC generator, when is magnetic flux linkage maximum?
when at 0 degrees, cos= 1 so Φ = B A cos (angle) = B A so maximum
In an AC generator, when is the EMF at a maximum
When magnetic flux linkage is 0 (so maximum rate of change of flux linkage)
Or when gradient of flux linkage time graph is the highest
What does AC stand for?
alternating current
What does AC mean?
alternating current! Changes from +ve to -ve
What is a transformer
a device that changes high alternating voltage and low current to a high current and low alternating voltage and vice versa
Describe how a transformer functions
A laminated core with a primary and secondary coil
Primary coil has an input AC and the secondary has an output AC
Current in the primary voltage produces a magnetic field which magnetises the iron core
(Which directs fields lines to the secondary coil)
Current is alternating, so magnetic flux is alternating and so there’s a rate of change of magnetic flux linkage in the secondary coil
Inducing an EMF
What is the function of the an iron core in a transformer?
directs magnetic field lines to from the primary coil to the secondary coil
What is a useful equation which is not given when using transformers?
power in = power out so Vp x Ip = Vs x Is
What is a useful way of knowing whether you’re working with a step up or step down transformer?
Number of turns in secondary coil is proportional to induced EMF in the secondary coil
If there are more turns in secondary coil then there it’s a step up
