Magnetic Fields and Electromagnetism Flashcards
What is a magnetic field?
A region of space which will cause a magnetic pole to experience a force
What direction does the magnetic field travel?
North to south
What are magnetic field lines called?
Magnetic flux
What is the strength of the magnetic field called?
Magnetic flux density
What is the unit for magnetic flux density?
Tesla
If the magnetic field lines are close together, is the field weak or strong?
Strong
What is the unit for magnetic flux?
Weber
What is the motor effect?
If a wire is placed in a magnetic field and a current is paced through it, the wire will feel a force
When is the motor effect at its greatest?
When the wire and the field are perpendicular
Do electric currents passing through a wire cause a magnetic field?
Yes, the magnetic field is produced around the wire
Is magnetic flux density a scalar or vector quantity?
Vector
Why does an electron moving in a magnetic field travel in a circle?
The force is perpendicular to the magnetic field and the direction in which the electron is moving. Acts as a centripetal force
What is Faraday’s law?
The induced e.m.f is proportional to the rate of change of flux linkage
What is Lenz’s law?
The direction of an induced e.m.f is such as to oppose the change creating it
What can Flemming’s right-hand rule be used for?
To find the direction of the induced e.m.f
Explain what electromagnetic induction is
If you move a magnet near a wire, the electrons in that wire will feel a force tending them to move through the wire. This is an e.m.f
What is a solenoid?
A long spring-like coil
What do transformers do?
Increase or decrease the voltage of A.C power supplies
Will a transformer work with a D.C supply? Why?
No. Because a D.C will not produce an alternating magnetic field
What is the transformer equation for an ideal transformer?
Vs/Vp = Ns/Np
How can you show a transformer is 100% efficient?
IsVs = IpVp
If there are more turns on the secondary coil, will it be a step-up or step-down transformer?
Step-up
Why does the motor turn when a current is passed through it?
The current travels in opposite directions on opposite sides of the coil. This causes a force in opposite directions to twist the coil (Flemmings left hand rule)
Why is it necessary for the molecules to be ionised in a mass-spectrometer?
Only charged particles can be accelerated to produce a beam
State the condition under which this equation applies. F=BIL
The magnetic field must be at right angles to the current
When will induced e.m.f be a maximum? (2)
When the coil is parallel to the M field
Greatest rate of change of flux is when the angle = 90
2 electric cables connect to a p.d and there is a coil. How does this work? (4)
Cables produce an alternating M field
Coil rotates, cuts the M field
Induces e.m.f
Induces current
What is a neutral point?
Where the fields from 2 magnets cancel out and the resultant field strength is zero
What is magnetic flux density?
The quantity of flux passing through a unit area, at each point in the field
How can you increase the size of the induced e.m.f across a coil? (4)
Use a stronger magnet
Use a coil with more turns
Use a coil with a greater area
Move the magnet faster
What is Lenz’s law a result of?
Conservation of energy
What is an advantage and disadvantage of using electro magnetics rather than permanent magnetics for building a motor?
Advantage: the motor power can be altered for different applications
Disadvantage: it is more complex
When will Flemings right hand rule be used?
To find the direction of an induced e.m.f
Explain the shape of the graph of a magnet falling through a coil (5)
The e.m.f increases to a level then decreases
When the middle of the magnet falls though the middle of the coil no lines of flux is being cut -> zero e.m.f
Second peak is greater due to the magnetic accelerating
The areas of each peak are equal
First peak has longer time period
When a magnet falls through a coil, when does maximum e.m.f occur? Why?
When the magnetic leaves the coil, because the magnetic accelerates as it falls -> fastest rate of flux being cut
Explain why a coil turns and why it continues to rotate?
Vertical forces on each side of the coil
Force on one side acts up, rotates clockwise
Force on other side acts down, rotates anti-clockwise
Split ring commutator switches current direction
Explain the role of a magnetic field in a cyclotron
Force on particles is at right angles to the motion
Force acts as a centripetal, keeps it in circular motion
