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
When do you know if it’s a step up or a step down transformer?
step up if there are more turns on the secondary coil and step down if less
Describe and explain how transformers are constructed in order to observe maximum efficiency
Iron core to direct magnetic fields
Core is laminated using thin strips of insulator which reduces induced current
By limiting where current can go
What is the function of lamination in iron cores?
plastic insulator strips help reduce currents by limiting the possible paths current can take
Explain why a DC cannot be used in transformers
No rate of change of magnetic flux linkage
Induced EMF will be zero
(there will only be a short burst of induced emf in the secondary coil when the current is switched off or on)
How can you convert from AC to DC?
Use a diode to remove -ve EMF
Use a capacitor which is constantly charged and discharged to smooth out remaining curve
What kind of instrument can be used in order to measure Vs of a transformer?
CRO (cathode ray oscilloscope)
What things must be kept constant when carrying out investigations on Vs by changing Ns?
Np Vp
Frequency from signal generator
Same magnitude of current
Describe how the structure of a search coil is related to its function
Small area so when moved it can detect small magnetic flux densities
Many number of turns which maximises N, change in flux linkage, and induced EMF
Connected to a CRO which displays the induced EMF
Describe and explain the variation in EMF of a magnet falling through a coil of wire
Falling so there is a rate of change of flux and thus an EMF is induced
Magnet leaves in the opposite direction
But at a faster rate due to acceleration
Greater EMF produced but this time opposite sign
Lasts shorter time
Why does the velocity of a falling magnet decrease as it moves out of the coil?
Lenz’s law states that when an electric current is induced it will always flow in a direction to oppose the change producing it
Current produces a magnetic field in the coil to oppose the motion of the magnet
And thus reduces acceleration
What instrument is used to detect an alternating pd?
AC voltmeter or CRO
