electromagnetism Flashcards
magnetic field line
path which a North Pole would take when placed in a magnetic field
field lines for from north to south
magnetic filed
- region of space in which moving charged particles are subject to a magnetic force
- force is caused by the interaction of two magnetic fields (there is a filed around the moving charged particles which interacts with the existing magnetic field they are passing through)
how can you map field lines around a magent
- place iron filings on a piece of paper and then put the magnet on the paper and the filings will align to the field
- use a plotting compass and place it in various positions around the magnet, mark direction of the needle at each point and connect them
how can you represent the strength of a magnetic field on a diagram
represented by how close together the field lines are - the closer they are the stronger the field
magnetic field density
force per unit current per unit length on a current carrying conductor placed in a magnetic field perpendicular to the field lines (magnetic flux per unit area)
what is the unit of magnetic flux density
Tesla
1T = 1 Nm^-1A^-1
why does a compass point to the North Pole of the earth
the earths geographic north pole is actually the magnetic South Pole, so the north pole of the compass magnet lines up with earths field and points to the magnetic south which is what we call geographic north
how do you work out the shape of the field around a current carrying wire
right hand rule
how do you work out the shape of field around a solenoid
current is going anticlockwise around the coil is the North Pole. at the South Pole, the current goes clockwise the shape of the field is then similar to a bar magnet
motor effect
when a current-carrying conductor is placed within a magnetic field, it experiences a force perpendicular to the flow of a current and the field lines which pushes it out of the field
how can you predict which direction the force will push the conductor
left hand rule
describe an experiment to measure flux density
- place horseshoe magnet on a digital balance and zero it
- connect rigid piece of straight wire to DC supply, variable resistor and ammeter (in series)
- align the wire so the force on it acts upwards (so there will be a downward force on the magnet - newtons third law)
- measure the length of the wire in the field
- record extra mass on the balance and use this to calc force, f=mg
- plot graph of current against mass
- gradient gives BL/g
- since L and g are both known B can be calculated
derivation of F = BQv
F = BIL for a magnetic force on a conductor at 90 to field lines
then use I = Q/t and L = vt
F = BQvt/t
the ts cancel out leaving F = BQv
why do charged particles move in a circular orbit in a magnetic field
- force always perpendicular to the velocity of the particle so they end up being forced in a circular orbit
- particles undergo centripetal acceleration with the centripetal force being the magnetic force
how do you derive the formula for the radius of the circular orbit
equate centripetal force and magnetic force
mv^2/r = BQv
rearrange for r
using r = mv/BQ explain how changing the mass, velocity, flux density and charge affects the radius of the orbit
- increasing mass or velocity will increase the radius
- increasing the flux density or charge will decrease the radius
purpose of a velocity selector
they isolate particles of a specific velocity, this is useful for things like mass spectrometry
how does a velocity selector work
- electric plates above and below so the force acts upwards
- magnetic field passing through sideways the magnetic force acts downwards
- in order for the particles to pass through undeflected the electric and magnetic fields must be balanced so BQv = EQ
from this you can derive v = E/B
- if velocity is too fast or too slow the particle will be deflected and not pass through
magnetic flux
product of magnetic flux density and the area perpendicular to the field lines
unit for magnetic flux
weber
magnetic flux linkage
magnetic flux of an entire coil of wire, this is the product of the magnetic flux and the number of turns on the coil
Lenz’s law
induced emf is always in a direction so as to oppose that change that caused it
explain Lenz’s law in terms of energy
- follows the principle of the conservation of energy
- if the induced emf was in a direction that aided the change which caused it, it would be creating electrical energy from nowhere
faraday’s law
induced emf in a circuit is proportional to the rate of change of flux linkage throughout the circuit
formula that links faradays law and Lenz’s law
e = - (change N x phi) / change in time
- e = induced emf
- change N phi = change in flux linkage
- change in time
- negative because emf acts in the opposite direction to the charge that caused it
search coil
- flat coil of insulated wire connected to a galvanometer
- can be used to determine magnetic flux density from the current induced in the coil when it is withdrawn from a magnetic field
how can you measure magnetic flux using a search coil
- place the coil in a magnetic field of known strength and pull it out again. since Max is proportional to B you can calc constant of proportionality
- place coil in the field that is to be measures and withdraw it. use the value for k and the current to calc the flux density
- calc magnetic flux from flux density using
phi = BAcostheta
structure if simple AC generator
rectangular coil which spins in a uniform magnetic field
how does a simple AC generator work
flux linkage in the coil changes continuously inducing an alternating current in the coil
how does emf and flux linkage vary in a simple AC generator
emf is +- maximum when the coil is parallel to the field lines where Nphi is +- maximum when the coil is 90 to the field lines
how can the peak emf of an AC generator be increased
- increase the speed of rotation
- increase the magnetic flux density for the coil
- increase the cross sectional area of coil
- increase the number of turns on the coil
propose of a transformer
- change the peak value of an alternating PD to a different value.
- set up transformers increase it
- step down trasnsformers decrease it
describe the structure of a single transformer
- two coils, primary and secondary
- wrapped around the two sides of a laminated iron ring
- sep up there are more turns on the secondary
- step down there are more turns on the primary
how does a transformer work
alternating current is run through primary coil, which induces an alternating magnetic field in the iron core this in turn induces an alternating emf in the secondary coil
formula that relates number of turns with the potential difference of each coil
Vs/Vp = Ns/Np
v = potential difference
N = number of turns
for ideal transformer (100% efficient) five the formula relating potential difference and current in both coils
if efficiency = 100%, power in the primary coil = power in secondary coil
this means IpVp = IsVs
rearranging gives
Ip/Is = Vs/Vp
what role do transformers play in the national grid
- step up transformers are used to increase the voltage and decrease current before the electricity travels long distances
- this is to reduce energy lost as heat due to resistance in the wires as the electricity passes through
experiment to investigate the relationship Vs/Vp = Ns/Np
- vary number of turns on each coil and measure peak potential difference of each one every time
- then use the values to prove the relationship between potential difference and number of turns
- better to use oscilloscope than a voltmeter for an alternating pd since it os easier to ee the peak value
