Magnetic Feilds Flashcards
Which fingers represent what in Flemings LHR and explain the symbol to show that current is either out or into the page
First finger= mag feild
2nd= current
Thumb = force
Each finger shld be at 90 degrees to eachother
A circle with a cross through means current is into the page
A circle with a dot = current out of the page
Name 4 magnetic metals
Iron, steel, cobalt, nickel
State 2 methods for producing a uniform magnetic feild
With 2 opposite bar magnets
And a solenoid ( multiple coils on a wire packed together )
A solenoid produces a magnetic feild traveling from A to B what will happen to the magnetic feild pattern if current is both increased and reversed
The feild would be from B to A and the magnetic feild would get stronger therfore feild lines are closer together
A student placed a permenant magnet ( has. A north and South Pole permanently fixed together) on a top pan balance. He clamped a straight peice of wire between the magnet so that it was suspended in the magnetic feild
When a current is passed through the wire the reading on the balance increased why?
(Current is out of page, feild is to the right)
The current creates a magnetic feild in the wire (if it was a rod , the feild would be around the rod) which interacts with the magnetic feild from the permanent magnet
Flemings LHR says the force on the wire is upwards so the force on the permanent magnet is downwards (N3) hence reading increased.
Define magnetic flux density
The magnetic flux per unit area
Define a tesla
The magnetic flux density that would give a 1N force on a 1m wire carrying a 1 A current, perpendicular to the feild
Describe an experiment to show the validity of the equation F=BIL
Place a wire between a permenatn magnet and place the magnet on top of a balance. Connnect the wire to an ammeter in series and a variable resistor in series and a voltage supply.
Measure the length of the wire in the magnetic feild with a ruler
Use a set square to make sure that the current is entering the field at 90 degrees , camp the wire
Pass current through the wire and use an ammeter to measure the current
Measure mass using a balance F=mg
Change the current using a variable resistor and test 6 values, then record the reading on the balance, repeat 3 times and find a mean
Do this 6 times
Make sure balance is on 0 before starting
Then plot. A graph of F against I, straight line through origin
a student uses a permanent mag and a top pan balance and current carrying wire, the current causes the reading on the balance to go from 85g to 85.4g, the poles on the mag are reversed ,what would be the new reading on the balance
change in mass due to current=+0 .4 g
therfore if poles are reversed new change in current= - 0.4g
therfore new reading=85-0.4 = 84.6g
what is the meaning of the uniform in the phrase uniform magnetic feild
the mag feild lines are parallel and equally spaced
what happens to the force prooduced on a wire in the magnetic feild if the poles wee placed further apart
the force will decrease as the magnetic feild strength decreases
define a magnetic feild
a region of space in which moving charge particles are subject to a magnetic force
what is a permanent magnet
an object made from magnetized material that creates its own persistent magnetic feild
What direction do you use in Flemings left hand rule for the direction an electron is travelling
2nd finger point in opposite direction to path
Describe the motion of an electron traveling perpendicularly into a uniform magnetic feild where the feild is into the page
Use flemmings LHR, current in opposit direction to electrons path, therfore the force acts perpendicular to to the electrons velocity at all times therfore has a circular path
Derive an equation for the raduis of an electrons path in a magnetic feild (travels perp to feild)
Therefor explain what happens to radius as velocity increases
Circulator path therfore
Mv^2/r=Bev
Rearrange r=mv/Be
Radius will also increase if v increases
Derive F=Bqv for an electron traveling perpendicular to a uniform magnetic feild
F=BIL
Sin theta=1 (sin 90 = 1)
L=Vt
I=Q/t
therefore F=Bqv
Describe what a mass spectrometer does
It separates ions by mass
Ions are fed through a velocity selectors which uses electric and magnetic fields to separate particles by their velocities, and only the particles which satisfy Eq=bqv will make it through a straight line and reach the magnetic feild, (the slower and faster electrons will be deflected) which they are then separated by mass, as the velocity is now constant the haveier particles will have a larger radius and follow a circular motion until it hits a detector which will detect which particle hit it.
A uniform extrication feild is produced with a p.d accross them, the plates are separated by 5cm and are 10 cm long , an electron with the horizontal velocity v passes through the feild , how can we find the horizontal distance at which it hits the bottom plate
First use E=V/d to find the electric feil
Then use E=F/Q to use the electrical force
Now use F=ma for acceleration
Then find time spent in the feild by using the hor distance and hor velocity as this is constant for the vertical and hor
Then use Suvat where u=0 to fins s
how can we induce an emf through a wire (generator effect)
A generator converts kinetic energy into electrical energy which can then be used to power
electrical appliances. a coil of wire placed in a constant
uniform magnetic field, The coil is then rotated and so the area perpendicular to the
magnetic field is constantly changing . This changing magnetic flux linkage causes an alternating current to be induced
in the wire.
define magnetic flux
the product of the cross-sectional area of the coil and the component of the magnetic flux density which is perpendicular to this area
if the coil is placed at 90 degrees to the flux lines what angle do we use
and what theta do we use when coil is parallel to the lines (cos)
1)theta=0
2)theta=90 therfore flux=0
define magnetic flux linkage
the product of the magnetic flux and the no. of turns on the coil
when is an emf generated across a conductor
when there is a change in magnetic flux linkage with the coil
what will happen to emf if you hold a magnet still in a coil
nothing as the magnetic flux linkage does not change
define a weber
the amount of flux when a flux density of 1 Tesla passes at right angles through an area of 1m^2
an aeroplane with a wing span of 40m moves at 200ms^-1 horizontally through an area where the vertical component of the earths magnetic flux density is 5x10^-5 T what is the emf induced across the wing tips
the area = 40x200 when t=1, as the plane moves a length of 200m in 1s therfore use this to calc E
state faraday’s law
the magnitude of the induced emf is proportional to the rate of change of flux linkage
state Lenz’s law
the direction of the induced emf is such that it opposes the charge that caused it
how can we use search coils to measure the magnetic flux density of a magnet
hold the coil in the mag field ,perp to the field lines, then connect an oscilloscope across it to measure the emf then remove it quickly and use the divisions in the oscilloscope to find the time period and emf and use equation
what is the phase difference between the emf of a generator and the flux linkage graphs
pi/2
at what points on a flux linkage graph for a generator show when the coil is flat or perp to flux lines
when at peak there is max flux linkage therefore coil is perp to lines, when flux linkage =0 coil is flat
how can we tell when the emf graph of a generator is max or min using the graph for the flux linkage graph
when at peak or trough of the fl graph the rate of change of flux linkage=0 therefore E=0 due to faradays law
when there is an inflection point the gradient is max therefore the emf is also max (when flux linkage =0 there is a peak in emf)
describe the graph of emf against time for a bar magnet passing through a vertical coil
when magnet enters coil emf is induced due to the change in flux linkage with the coil, the emf is +ve as the mag is entering the coil therefore graph is a +ve and a curve
when the magnet is exiting the coil (middle of coil) the direction of emf changes therefore there is a point of inflection
when mag leaves coil, the m,ag accelerates trough coil therefore emf is larger than when mag enter coil, the graph is under the x axis and the emf is induced for a shorter time as mag leaves coil quicker
How is an output voltage induced in a transformer
An alternating current flows through the primary coil. Therfore an alternating magnetic field is induced in the iron core
Causing a continuous change in the magnetic field flux linkage through the secondary coil
Due to faradays law an emf is induced accross the secondary coil
What would happen if a DC was used for a transformer
There would be no change in flux linkage within secondary coil theory and emf is only induced in the secondary coil for a split second
The secondary coil has double the turns as the primary what factor is the secondary coil than the primary
Double
What is a temp up transformer
When the secondary coil has more turns than primary opposite of step down
