Electromagnetism Flashcards
What is a magnetic field?
A region where a force is exerted on magnetic materials
How is a magnetic field represented?
flux lines
What direction do flux lines go?
north to south
How is the strength of a magnetic field shown?
How close together the flux lines are
How is the strength of a magnetic field shown?
How close together the flux lines are
How do you know if a magnetic field is uniform?
If flux lines are equally spaced and in the same direction
What happens when current flows through a wire (or any long straight conductor)?
A magnetic field is PRODUCED around the wire
Describe the flux lines around a current carrying wire?
Concentric circles centred on the wire, direction can be found using the right hand rule
What is the shape of the field around a loop of wire?
doughnut shaped
What is the shape of the field around a coil of wire?
Forms a field like a bar magnet
What does Flemming’s left hand rule show?
thumb - direction of motion (the force)
index - direction of flux lines (magnetic field)
middle - current
FBI
What causes an electromagnetic force?
Flux lines contracting and straightening
(change in flux/flux linkage)
What happens when you put a current carrying wire into an external magnetic field?
The field around the wire and the field from the magnets (external magnetic field) are added together which changes the shape of the resultant flux lines meaning they contract and straighten which causes an electromagnetic force that pushes on the wire
What happens when a current carrying wire is parallel to an external magnetic field?
Wire won’t experience a force because no component of the fields are perpendicular so the flux lines won’t change
What direction is the force compared to the current and the magnetic field?
Perpendicular
What is the force on a wire proportional to?
flux density
current
length of wire
(F=BIL)
What is the equation for the magnetic force on a wire?
F=BIL
B = magnetic flux density
I = current
L = length of wire
What is magnetic flux density and units?
The fore on one metre of wire carrying a current of one amp at right angles to the magnetic field (define using F=BIL)
(measure of the strength of a magnetic field)
Tesla or Wb m-2
Is magnetic flux density scalar or a vector?
vector
How do you find flux density from an experiment?
Set up a piece of wire between two magnets and above a balance so that the electromagnetic force acts downwards
Connect the wire to a variable resistor so that the current can be varied, then record the mass for a given current (do repeats and find averages)
Then convert the mass readings to force (F=mg) and plot force against current.
Gradient = BL so divide by the length of the wire to find the flux density
How do you vary the current in a wire?
Connect it to a variable resistor
What direction must the magnetic field and wire be when investigating flux density?
perpendicular
What must you do when recording the mass in the flux density experiment?
zero it and then turn the power supply on
What face should the poles of the magnets being used in flux density experiment be on?
Largest face
Why does charge accumulate at one end of a conducting rod when moving perpendicular to a magnetic field?
The electrons in the rod will experience a force which causes them to accumulate on one end
When does a conducting rod experience an electromagnetic force?
When moving perpendicular to a magnetic field
Why does a rod moving perpendicular to a magnetic field induce an electromotive force (emf)?
The electrons accumulating at one end of the rod indues an emf across the ends exactly as connecting a battery to it would
What happens if a rod with an emf is connected to a complete circuit?
An induced current will flow through it
What is the process of inducing an emf called?
Electromagnetic induction
What induces an emf?
Lines of flux being cut
(when there is relative motion between a conductor and magnetic flux)
When does flux cutting induce a current as well as an emf?
If the circuit is complete
What is Faraday’s law?
The induced emf is directly proportional to the rate of change of flux linkage
How can Faraday’s law be written?
ε = - flux linkage / time taken
ε = - d(ϕN) / dt
ε = induced emf
ϕN = flux linkage
ϕ = flux
How do you calculate total magnetic flux?
ϕ = BA
(magnetic flux density x area)
What does the size of the emf induced depend on?
Magnetic flux passing through the coil
Number of turns on the coil
How quickly conductor and flux are moving relative to one another
What is flux linkage?
The product of magnetic flux and no. turns (total flux)
Nϕ