magnetic fields Flashcards
where do field lines go in a magnetic field and whats another name for them?
they go from north to south
also called flux lines
when will a magnetic field be induced around a wire?
when current flows in a wire or any other long straight conductor
how can you work out the direction of a magnetic field around a current-carrying wire?
use the right hand rule, thumbs up
what’s a solenoid?
a coil with length
what’s Flemings left hand rule?
thumb= direction of force
first finger= magnetic field
second finger= current
all perpendicular
how could you make a wire through a magnetic field vibrate?
pass an ac current through it
magnetic flux density definition
the force on 1 metre of wire carrying a current of one amp at right angles to the magnetic field
describe an experiment to investigate the effect of current on the force exerted on a current-carrying wire in a magnetic field (RP10)
- set up a top pan balance with a square loop of wire fixed to it, so that it is standing up and that the top of the loop passes through a magnetic field, perpendicular to it
- connect the wire in a circuit with a variable resistor, ammeter and dc power supply. zero the top balance when no current is flowing
- vary the current using the variable resistor. at each current value, record the current and mass. repeat three times and average
- convert into force using F=mg
- plot a graph of force-current and draw line of best fit
- gradient is B x length
what happens to the radius of a charged particle moving in a magnetic field if the mass is increased?
increases
what happens to the radius of a charged particle moving in a magnetic field if the velocity is increased?
increases
what happens to the radius of a charged particle moving in a magnetic field if the magnetic flux density is increased?
decreases
what happens to the radius of a charged particle moving in a magnetic field if the charge is increased?
decreases
what are some uses of cyclotrons?
producing radioactive tracers
producing high-energy beams for radiotherapy
describe the structure of a cyclotron
two hollow semi-circular electrodes with alternating pd
they have a slight gap between them
a uniform magnetic field is applied perpendicular to the plane of the electrodes
how does a cyclotron work?
-charged particles are fired to one electrode
-the magnetic field causes it to flow a semi-circular path and return to the gap between electrodes
-the pd between them creates an electric field which accelerates them across the gap
-the velocity is now higher so the particle takes a path with a larger radius
-particle continues spiralling out until it exits
what is magnetic flux density?
the measure of the strength of a magnetic field
B- symbol
T- units
what happens when a conductor is moved in a magnetic field?
if it cuts through field lines, an emf is induced in the conductor
the electrons in the rod will experience a force which causes them to accumulate at one end of the rod- this induces an emf across the ends of the rod= electromagnetic induction
what is electromagnetic induction?
when an emf is induced in a conductor that cuts through magnetic field lines
how can you induce an emf in a flat coil or solenoid?
moving the coil towards or away from the poles of the magnet
moving a magnet towards or away from the coil
how can you investigate flux linkage with a search coil? RP11
- the stretched metal spring acts as a solenoid when connected to an alternating power supply
- the alternating supply means the magnetic field of the solenoid is constantly changing- meaning the flux through the search coil is changing which can induce an emf
- make sure the peak of the ac voltage from the ac power supply stays the same throughout the experiment
- the search coil should have a known area and a set number of loops of fine wire. it’s connected to the oscilloscope to record induced emf in the coil
- set up the oscillator so it only shows the amplitude of the emf as a vertical line (turn off base time)
- a protractor is used to measure the orientation of the normal to the area of the search coil as an angle from the line of the magnetic field
- position the search coil so its about halfway along the solenoid and within the inside of the solenoid but not touching it
- then record induced emf in the search coil from the amplitude of the oscilloscope trace
- rotate the search coil so its angle to the solenoid and the magnetic flux lines changes by 10 degrees
- record the induced emf and repeat until rotated 90 degrees
where’s the magnetic field of the solenoid the strongest?
inside the solenoid
what should happen to the emf as the search coil is turned and why? RP11
as you turn the search coil, emf should decrease
because the search coil is cutting fewer flux lines as the component of the magnetic field perpendicular to the area of the coil gets lower so magnetic flux linkage experienced by the coil is lower
emf should be maximum at 0 degrees
and 0 at 90 degrees
what is faraday’s law?
induced emf is directly proportional to the rate of change of flux linkage
what does the symbol ø represent in magnetic fields?
magnetic flux (Wb)
what is Lenz’s law?
the induced emf is always in such a direction as to oppose the change that caused it
what produces an induced current?
the energy used to pull a conductor through a magnetic field, against the resistance caused by the magnetic attraction
what is flemings left hand rule?
thumb= force of resistance
first finger= field direction
second finger= induced emf direction
how is emf induced in a rotating coil?
when a coil rotates uniformly in a magnetic field, the coil cuts the flux and an alternating emf is induced
how can the shape of the graph of induced emf be altered?
changing speed of rotation or the size of the magnetic field
how does increasing speed of rotation effect frequency and emf
increased frequency and increase the maximum emf
how does increasing magnetic flux density effect frequency and emf
frequency stays the same, increase the maximum emf
how do generators work?
they convert kinetic energy into electrical energy- they induce an electric current by rotating a coil in a magnetic field
what is alternating current?
one that changes direction with time
the voltage across a resistance gap goes up and down in a regular pattern
how can you measure an alternating current with an oscilloscope?
-you can use an oscilloscope to display the voltage of an alternating current
-the trace you see is made by an electron beam moving across the screen
-the vertical height of the trace at any point shows the input voltage at that time
-you can select how many volts per division you want
-an alternating current source gives a regularly repeating sinusoidal waveform
what are the three basic pieces of information you can get from an ac oscilloscope trace?
the time period
the peak voltage
the peak-to-peak voltage
symbol for peak voltage
V zero
how do you work out average power for an ac supply?
average power= I rms x V rms
what is the UK mains electricity voltage?
230V
what are transformers?
devices that make use of electromagnetic induction to change the size of the voltage for an alternating current
what are some important uses of transformers?
transfer of electricity from power stations to the nation and in many electrical equipment
how do transformers work?
-an alternating current flows in the primary coil causing the core to magnetise, demagnetise and remagnetise continuously in opposite directions
-this produces a rapidly changing magnetic flux across the core so a magnetically soft material is needed eg. iron
-the rapidly changing magnetic flux in the iron core passes through the secondary coil where it induces an alternating voltage of the same frequency
what are step-up transformers?
increase the voltage by having more turns on the secondary coil then the primary
what are step-down transformers?
reduce the voltage by having fewer turns on the secondary coil
why is a transformer not 100% efficient?
- the metallic core is being cut by the continuously changing flux
- heat generated by resistance in coils
- energy needed to magnetise and demagnetise the core is wasted as it heats the core
how does the metallic core being continuously cut by changing flux cause energy losses? transformers
induces an emf in the core which causes currents called eddy currents. these cause it to heat up and energy to be lost
what are eddy currents?
looping currents induced by the changing magnetic flux in the core
they create a magnetic field that acts against the field that induced them, reducing the magnetic field
also dissipate energy by generating heat
how can effects of eddy currents be reduced?
by laminating the core- having layers of the core separated out by thin layers of insulator so a current can’t flow
how to reduce heat generated by resistance in the coils of transformers?
use wires with low resistance
eg. thick copper wires
how to reduce the energy needed to magnetise and demagnetise the core for transformers?
use a magnetically soft material that magnetises and demagnetises easily eg. iron
how can we reduce magnetic losses in transformers?
a core design in which the coils are as close as possible can be used
how do you work out the energy lost in a transformer?
E=Pt
is the national grid high or low current and why?
lowest possible current
to reduce energy losses due to less heating in the cables
how can you investigate the relationship between the number of turns and the voltage across the coils of a transformer?
what would you need to add to investigate the relationship between current and voltage for a given number of turns?
put two C-cores together and wrap wire around each to make the coils
begin with 5 turns in primary and 10 in secondary coil
turn on the ac supply to the primary coil.
use a low voltage as transformers increase voltage
record the voltage across each cell
keeping primary voltage the same, repeat with different number of turns
add a variable resistor to the primary coil and an ammeter to both circuits
Define the tesla
The tesla is the (strength) of the magnetic field/flux density that produces a force of 1 newton in a wire of length 1m with 1 ampere (flowing perpendicular to the field).
