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
What is the unit of flux linkage?
Wb (same as flux)
How do you increases the size of the induced emf?
Increase no. turns
Increase magnetic flux
Increases the speed the conductor and flux are moving relative to each other (decreases dt)
What does the gradient of a flux linkage - time graph represent?
induced emf
What does the area under an emf - time graph represent?
change in flux linkage
What is Lenz’s law?
The induced emf is always in such a direction as to oppose the change that caused it
How does the idea that an induced emf will oppose the change that caused it agree with the conservation of energy?
The energy used to pull a conductor through a magnetic field, against the resistance caused by magnetic attraction, is what produces the induced current
What can Lenz’s law be used to find?
Direction of an induced emf and current in a conductor travelling at right angles to the magnetic field
How can you find the direction of the induced emf using Flemming’s left hand rule and Lenz’s law?
Lenz’s law says that the induced emf will produce a force that opposes the motion of the conductor - so is resistance
Using Flemmings left hand rule means that you can point your thumb in the direction of resistance (opposite direction to motion), index finger will point to direction of induced emf and middle finger points to direction of current
Why is an emf induced when a magnet is dropped through a coil?
The conducting coil cuts the flux lines of the magnet
How do you plot a graph of induced emf against time when you drop a magnet through a coil?
By connecting a data logger to the coil and recording the emf in the coil at very small time intervals
When is peak emf of a magnet being dropped through a coil?
When the magnet passes each pole as this is when the change in flux linkage is greatest
Why is the amplitude of the second peak greater on an emf - time graph of a magnet being dropped through a coil?
Magnet is accelerating due to gravity, so speed is greater meaning Δt has decreased and so the rate of change of flux linkage is greater
Why is the area under each peak of an emf - time graph of a magnet being dropped through a coil the same?
The total change in flux linkage must be zero as there was no emf before or after the magnet is dropped
How do the emf - time graphs of a magnet being dropped through a coil change depending on the width of the coils?
The wider the coil the less emf is induced as fewer flux lines are cut
(narrower coil means greater emf)
How does the length of the magnet being dropped through a coil affect an emf - time graph?
A longer magnet means a longer period between the peaks as there is only a change in flux linkage when a pole enters or leaves the coil.
The second peak also has a greater magnitude the longer the magnet as its had more time to accelerate so is travelling faster
What do transformers do?
Use electromagnetic induction to change the size of the voltage for an alternating current
What is a transformer made of?
Two coils wrapped around a laminated iron core
How do transformers work?
An alternating current flowing through the primary coil produces magnetic flux
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
How do you calculate the induced emfs of both the primary and the secondary coil in a transformer?
primary coil ε = - N1 dϕ / dt
secondary coil ε = - N2 dϕ / dt
What is an ideal transformer?
100% efficient
power in = power out
What is the equation for the voltage and number of turns of an ideal transformer?
V1 / V2 = N1 / N2
What is the equation for voltage and current for an ideal transformer?
V1 I1 = V2 I2
What is the equation for current and number of turns for an ideal transformer?
I2 / N1 = I1 / N2
What are step up transformers?
Ones that increase the voltage by having more turns on the secondary coil than on the primary
What are step down transformers?
Ones that decrease the voltage by having more turns on the primary coil than on the secondary
Why aren’t transformers 100% efficient?
Eddy currents are induced by the changing magnetic flux in the core which produces heat
Heat is also generated by resistance in the coils
How do you minimise heat lost from transformers through resistance in the wires?
Use thick copper wire which has a low resistance
How do you reduce the effect of eddy currents in transformers?
Laminate the core with layers of insulation
What is permeance?
The amount of flux induced for a given number of current turns
What is always true of magnetic flux lines?
continuous and form a closed loop
What does a high permeance mean?
A large amount of flux is induced
What is the equation for permeance and what is it the same as?
permeance = µA/L (same as conductance)
µ = permeability of the material
What is permeance proportional and inversely proportional to?
proportional to area
inversely proportional to length
What are the ideal dimensions for a transformer when only thinking about permeance?
Short and fat
Why is iron used as a transformer core?
High permeability
What is the ideal dimensions of the wire used in a transformer?
High conductance so want short piece of wire
(need to get right no. turns with the shortest piece of wire)
What is the problem when trying to get the best overall transformer performance?
Want to get lots of turns using a short, fat piece of wire around a fat transformer which doesn’t really work
Why does an air gap in a transformer cause a lower amount of flux?
Air has a very low permeability compared to iron
Why do you need an alternating current in a transformer?
To create an alternating magnetic field
What do dynamos do?
Convert kinetic energy to electrical energy
What is another word for dynamos?
generators
How do generators (dynamos) induce an electric current?
Rotate a coil in a magnetic field, the output voltage and current changed direction with every half rotation of the coil which produces an ac.
A split ring commutator is used to change this ac to dc which is then carried to an external circuit using brushes
What is the motor effect?
Current carrying wire experiences a force when placed in a magnetic field
What is the energy store transfers in a generator?
Kinetic energy to electrical energy
(no power supply connected as it is the power supply)
What is the stator coil and whats the rotor coil?
Stator coil is the one that’s stationary and rotor coil is the one that rotates
When do you use the term flux linkage?
When referring to a coil with turns ONLY
Why are magnets acting as a rotor in a generator often curved?
Reduces air gap between stator and rotor
Increase efficiency as increases permeance
What happens when the air gap is too small between the stator and the rotor?
Get air resistance
In a generator what is induced and in what order?
A changing flux (or flux linkage when its a coil) induces an emf across the stator coil which then induces a current (if connected in a circuit)
What are the energy store transfers in a motor?
Electrical energy to kinetic energy
(must be a power supply)
What happens when you put a current though a coil?
PRODUCE a magnetic field
What shape is an emf-time graph for a generator?
sinusoidal
How does a motor keep the rotor rotating?
By changing the direction of the current in the stator it changes the poles causing the magnetic rotor to then repel the like charge its next to and be attracted to the other pole meaning it rotates
How can you make a motor more efficient?
Have more stator coils and more poles so in one 360° rotation there are more max and min emfs
If you replace a magnet rotor with a metal such as copper why does it still work?
Due to the alternating current of the stator the copper rotor is sitting in a changing magnetic field so you get an induced emf which induces a current in the copper.
The current in the copper means the copper produces its own magnetic field which allows it to then behave as a magnet
What can reduce flux linkage and what does it result in?
Air gaps
Energy losses
