electromagnetism Flashcards
what is a magnetic field?
EMG
a magnetic field is a region of space where a force is exerted on magnetic materials
what are the features of a magnetic flux [/field] line diagram?
EMG
some of the features of a magnetic flux line diagram are:
⋅ magnetic fields can be represented by flux lines (aka field lines)
⋅ flux lines go from the north to south magnetic poles
⋅ the strength of the magnetic field is represented by how tightly packed the flux lines are - the closer together the lines, the stronger the magnetic field
⋅ each flux line always joins up the north and south poles in one continuous line
⋅ the flux lines around a bar magnet, or between a pair of magnets, have characteristic shapes (as shown below)
⋅ if flux lines are equally spaced and in the same direction, the field is uniform (i.e. same everywhere)
what happens when a current flows through a wire or in any other long straight conductor?
EMG
when a current flows through a wire or any other long straight conductor, a magnetic field is induced around the wire
what shape are the field lines around a current-carrying wire when a magnetic field is induced?
EMG
the field lines around a current-carrying wire when a magnetic field is induced are concentric circles centred on the wire
how do you work out the direction of the induced magnetic field around a current-carrying wire?
you can work out the direction of the induced magnetic field around a current-carrying wire using the right hand [corkscrew] rule, as shown below:
what do the field lines look like for a loop of wire or a solenoid (lots of loops)?
EMG
for a loop of wire, the field lines form a that field is donut-shaped - whilst the field lines for a solenoid (coil with lots of loops and with length) forms a field that is similar/the same to the field of a bar magnet
what will a wire carrying a current through an external magnetic field experience?
a current-carrying conductor in an external magnetic field will experience a force
what happens in detail when a current-carrying wire cuts through an external magnetic field?
EMG
⋅ if you put a current-carrying wire into an external magnetic field (eg, between two magnets), the field around the wire and the field from the magnets are added together
⋅ the shape of the resultant flux lines is a combination of the two fields
⋅ magnetic fields in the same direction repel (same repel) and magnetic fields in the opposite direction to each other cancel out (opposite cancel out)
⋅ in the example shown, the flux lines at the top travel in the same direction - so they repel - and the flux lines at the bottom are travelling in the opposite directions - so they cancel out
⋅ this results in the top magnetic fields repelling each other, resulting in a force downwards - and since the bottom magnetic fields are canceled out there is nothing to resist the force, so the overall force is downwards
⋅ however, if the current is parallel to the magnets’ flux lines, no force acts bc the fields are perpendicular, so they don’t affect each other
⋅ the direction of the force is always perpendicular to both the current and the magnetic field (the direction of the force is given by fleming’s left hand rule)
how can you remember how fleming’s left hand rule works?
EMG
you can also just remember FBI, where the thumb finger = direction of the force [of motion] (F), index finger = direction of the magnetic field (B), middle finger = direction of the current (I)
the size of the force on a current-carrying wire perpendicular to a uniform magnetic field is proportional to what?
EMG
the size of the force on a current-carrying wire perpendicular to a uniform magnetic field is proportional to:
⋅ current, I
⋅ the length of the wire cutting through the magnetic field at right angles, l (<- that is a lower case L)
⋅ the magnetic flux density, B
bc F = BIl
“F = bill”
what is the equation for the force exerted on a current-carrying wire perpendicular to the uniform magnetic field?
F = BIl
B = magnetic flux density [/magnetic field strength]
I = current flowing through the wire
l = length of wire cutting through the magnetic field at right angles
what is magnetic flux density (B) a measure of?
the magnetic flux density (B) is a measure of the magnetic field strength per unit area
what is magnetic flux density defined as and what are its units?
the magnetic flux density is “the force on one metre of wire carrying a current of one amp perpendicular to the magnetic field”
bc F = BIl so B = F/Il
the units for magnetic flux density are: Teslas, T
you may also see people call magnetic field called B-fields as a result
is magnetic flux density a vector quantity?
YES, magnetic flux density is a vector quantity with both direction and magnitude
what is 1 Tesla equal to?
1 Tesla = Wb/(m^2)
what is a helpful way to think about flux density?
EMG
to understand flux density, it helps to think of flux density as the number of flux lines [cutting through a/]per unit area
Φ = BA
∴ B = Φ/A
describe how you would use a digital balance to investigate flux density?
⋅ you can use set-up shown to investigate uniform magnetic field between poles of magnet + obtain value for flux density, B:
1) square hoop of rigid metal wire is positioned so that top of hoop (length l) passes through magnetic field, perpendicular to it. when current flows, this horizontal length of rigid wire in magnetic field will experience downwards force (fleming’s left-hand rule)
2) power supply should be connected to variable resistor so that you can alter current. connect crocodile clips + zero digital balance when there is no current through wire. then turn on power supply
3) note mass showing on digital balance + current. then use variable resistor to change current. repeat this until you have tested large range of currents, then conduct whole experiment twice more + calculate mean mass for each current reading
4) convert your mass readings into force using F = mg. plot data on graph of force F against current I. draw line of best fit
5) bc F = BIl, gradient of your graph is equal to B x l. measure gradient, then divide by length l to get value for B
⋅ (length l is length of wire cutting through magnetic field at right angles)
explain how a conducting rod becomes electromagnetically induced
EMG
1) if a conducting rod moves perpendicular to a magnetic field, electrons in the rod will experience a force, which causes them to accumulate at one end of the rod
2) this induces an electromotive force (emf) [/pd] across the ends of the rod exactly as connecting it to a battery would
⋅ if the rod is part of a complete circuit, then the induced current will flow through it too
3) this process of inducing an emf is called electromagnetic induction
what do changes in magnetic flux induce?
EMG
changes in magnetic flux (aka a changing magnetic flux) induce an electromotive force
describe when an emf is induced?
EMG
⋅ an emf is induced whenever there is relative motion between a conductor and the magnetic flux
⋅ (this can be the conductor moving and the magnetic field staying still, or it can be that the conductor is staying still and the magnetic field is moving)
⋅ an emf is induced in general whenever flux lines are cut
⋅ flux cutting always induces an emf but it will only induce a current if the circuit is complete
what can you think of magnetic flux as?
EMG
to help understand it, you can think of magnetic flux as the total number of flux lines per area
what is the equation that defines the total magnetic flux (Φ) passing through an area (A) perpendicular to the magnetic field (B)?
EMG
the equation that defines the total magnetic flux (Φ) passing through an area (A) perpendicular to the magnetic field (B) is:
Φ = BA
when you move a current-carrying coil in a magnetic field, what does the size of the induced emf depend on?
EMG
when you move a current-carrying coil in a magnetic field, the size of the induced emf depends on:
⋅ the magnetic flux passing through the coil (Φ)
⋅ the number of the turns on the coil (N)
⋅ how quickly the flux and the conductor move relative to each other (the faster you move the coil in the magnetic field, the greater the size of the emf induced)
what is the product of the magnetic flux (Φ) passing through the current-carrying coil and the number of turns (N) on the current-carrying coil?
EMG
the product of the magnetic flux (Φ) passing through the current-carrying coil and the number of turns (N) on the current-carrying coil is the flux linkage (NΦ)
what is the unit of flux linkage (NΦ)?
EMG
the unit of flux linkage is: Webers, Wb OR Weber turns
what is Faraday’s Law?
EMG
Faraday’s Law states that: “the induced emf is directly proportional to the rate of change of flux linkage”
EMG
how can you write faraday’s law as an equation?
EMG
the minus in the equation comes from lenz’s law
what does the gradient of a flux linkage (NΦ) against time graph show?
EMG
the gradient of the graph for flux linkage (NΦ) against time is the size of the emf
what does the area under an emf against time graph show?
EMG
the area under the graph of emf against time gives the change in flux linkage [Δ(NΦ)]
how does lenz’s law explain the resistance produced when you pull a conductor through a magnetic field?
how can you use lenz’s law to find the direction of the induced emf and the induced current in a conductor travelling at right angles to a magnetic field?
EMG
method:
1) lenz’s law states that the induced emf will produce a force that opposes the motion of the conductor - in other words, resistance
2) using fleming’s left-hand rule, point your thumb in direction of the force of resistance - which is in the opposite direction to the motion of the conductor
3) your second finger will now give you the direction of induced emf
4) if the conductor is connected as part of a circuit, a current will be induced in the same direction as the induced emf
how can you use this equipment to induce an emf in a coil by dropping a magnet through a coil?
EMG
1) you can investigate induced emf by dropping magnet through coil
2) emf is induced because conducting coil cuts flux lines of magnet
3) by connecting data logger or oscilloscope to coil + recording emf in coil at very small time intervals (eg, 0.002 s), you can plot graph of induced emf against time
can you explain this data obtained from investigating the emf induced by dropping a magnet through a coil?
EMG
1) the peak emf occurs when the change in flux linkage [Δ(NΦ)] is greatest, which is when the pole passes through the ends of the coil
2) the amplitude of the second peak is greater bc the speed of the magnet has increased (due to acceleration due to gravity)(so Δt has decreased, and so the rate of change of flux linkage is greater (ΔΦ/Δt)
3) the area under each peak is the same, bc the total change in NΦ (flux linkage) must be zero (bc there was no emf before the magnet was dropped, and there is no emf after)
how do you explain the differences in data for a short magnet with a wide coil and a long magnet with a narrow coil? (investigating emf induced when dropping magnet through a coil)
EMG
you’ll get different graphs if you change the magnet or the coil that you use:
⋅ if you use a wider coil, the magnitude of the induced emf will be lower than if you used a narrow coil bc the wider coil will cut fewer flux lines
⋅ if you use a long bar magnet, there will be a longer period between peaks bc there is only a change in NΦ when the poles of the magnet enter or leave the coil
⋅ the second peak will have an even greater magnitude and shorter duration bc the magnet will have been accelerating (due to gravity) for more time when the second pole passes through the coil, and so will be travelling faster (so Δt will be even smaller and d(NΦ)/dt will be even larger)
how do transformers work? (more detailed)
EMG
1) the alternating current flowing in the primary coil produces magnetic flux
2) the changing magnetic field passes through the iron core to the secondary coil, where it induces an alternating voltage (emf) of the same frequency as the input voltage
3) from faraday’s law and lenz’s laws, induced emfs in both the primary and the secondary coils can be calculated:
how can these equations be combined to give the equation for the ideal transformer?
EMG
the equations can be combined into this equation for an ideal transformer:
how do step-up and step-down transformers work?
EMG
⋅ as for an ideal transformer, power in = power out
⋅ power is current x voltage, so I1V1 = I2V2, so you can rearrange for the equation below:
⋅ knowing this, step-up transformers increase the output voltage by having more turns on the secondary coil than on the primary coil - step-down transformers decrease the voltage by having fewer turns on the secondary coil than the primary coil
what is the equation for permeance?
EMG
how can you investigate the relationship between the number of turns of the coils and the voltage across the coils?
EMG
1) set up equipment as shown. put two C-cores together + wrap wire around each to make coils. begin with 5 turns in the primary coil and 10 turns in the secondary coil (coil turn ratio of 1:2)
2) turn on a.c. supply to the primary coil. use low voltage - remember [step up] transformers increase voltage, so make sure you keep the voltage at a safe level. record voltage across each coil
3) keeping V1 the same so it’s a fair test, repeat the experiment with different ratios of turns (eg, try 1:1 + 2:1). divide N2 by N1 + V2 by V1. you should find that for each ratio of turns:
how to investigate the relationship between the number of turns of transformer coils, the voltage across the transformer’s coils, and the current of the transformer coils?
EMG
1) use the same equipment as when investigating only the relationship between the number of turns and the voltage across the coils, but add a variable resistor to the primary coil circuit and an ammeter to both circuits
2) turn on the power supply and record the current through each coil and the voltage across each coil
3) leaving the number of turns constant, adjust the variable resistor to change the input current. record the current and the voltage for each coil, then repeat this process for a range of input currents
4) you should find that for each current:
how do dynamos work?
EMG
1) dynamos, or generators, induce an electric current by rotating a coil in a magnetic field
2) the output voltage and the current change direction with every half rotation of the coil, producing an alternating current (a.c. or aka AC)
3) a split ring commutator is used to change this AC current to direct current (d.c. or aka DC)
4) this current is then carried to an external circuit using brushes
