16: Electromagnetism Flashcards
What is a magnetic field?
A region where a force is exerted on a magnetic materials
What are magnetic fields represented by?
Flux lines\ field lines
In which direction do field lines go? (Unless in the magnet/coil)
North to south poles
What is the strength of magnetic field represented by?
By how tightly packed the lines are, the closer together the lines, the stronger the field
Each flux line always joins up the [ ] and [ ] poles in one [ ] line
North
South
Continuous
What shows that a field is uniform?
The flux lines are equally spaced and in the same direction
Whenever you have a wire carrying electric current, you also have?
A magnetic field around it
When current flows in a wire or any other long straight conductor,
…..
A magnetic field is induced around the wire
Describe the field lines around a current carrying wire
They’re in concentric circles
How do you work out the direction of the magnetic field around a current carrying wire?
Right hand rule
What is the right hand rule?
1) Stick your right thumb in the direction of the CONVENTIONAL current
2) Your curled fingers point in the direction of the field
What happens to the field if you put a current carrying wire into an external magnetic field?
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
What do curved flux lines have a tendency to do?
To contract and straighten
What is the effect of flux lines contracting and straightening?
Causes an electromagnetic force that pushes the wire
What happens if the current is parallel to the flux lines?
No force acts because the fields are perpendicular, so they don’t affect each other
What is the direction of the force with respect to the current and magnetic field?
Perpendicular - Fleming’s left hand rule
What is Fleming’s left hand rule?
First finger = external uniform magnetic field
Second finger = direction of the CONVENTIONAL current
Thumb = direction of the force and therefore motion
What is the force on a current carrying wire at right angles to a uniform magnetic field proportional to?
F = BIL
Current
Length of the wire
Magnetic flux density
What is magnetic flux density a measure of?
Measure of the strength of the magnetic field
What is magnetic field density defined as?
The force on one meter of a wire carrying a current of one amp at right angles to the magnetic field
Is magnetic flux density a scalar or vector?
Vector. It has a magnitude and a direction
What happens with charge if a conducting rod moves perpendicular to a magnetic field? Describe the electromagnetic induction
The electrons in the rod experience a force, which causes the, to accumulate at one end of the rod
This induces an emf across the ends of the rod exactly as connecting a battery to it would. If the rod is part of a complete circuit , then an induced current will flow through it too
This process of inducing an emf is electromagnetic induction
When is an emf induced?
Whenever there is relative motion between a conductor and magnetic field
Whenever lines of flux are cut
When will flux cutting induce an emf? A current?
Flux cutting always induces an emf but will only induce a current if the circuit is complete
What does magnetic flux density measure?
Strength of the magnetic field per unit area
Area must be perpendicular to field
What is flux linkage?
The number of turns on the coil * the magnetic flux passing through the coil
State the relationship between speed of coil in field and emf
The faster you move a coil in a field, the greater the size of the emf induced
What is Faraday’s law?
The induced emf is directly proportional to the rate of change of flux linkage
On a graph of flux linkage against time, what does the gradient represent?
Gradient = size of the emf
On a graph of emf against time, what does the area underneath the graph represent?
Area = flux linkage change
What is Lenz’s law?
The induced emf is always in such a direction as to oppose the change that caused it
Describe the link between Lenz’s law and energy
Agrees with the conservation of energy
The energy used to pull a conductor through a magnetic field, against the resistance cause by magnetic attraction, is what produces the induced current
How can you use Lenz’s law to find the direction of an induced emf and current in a conductor travelling at right angles to a magnetic field?
The induced emf will produce a force that opposes the motion of the conductor, eg. resistance
Using Fleming’s left hand rule, point your thumb in the direction of the force of resistance, which is the opposite direction to the motion of the conductor
Your second finger will now give you the direction of the induced emf
If the conductor is connected as part of a circuit, a current will be induced in the same direction as the induced emf
Why is emf induced when you drop a magnet through a coil?
The conducting coil cuts the flux lines of the magnet
How can you investigate induced emf by dropping a magnet through a coil?
By connecting a data logger or oscilloscope to the coil and recording the emf in the coil at very small time intervals, you can plot a graph of induced emf against time
Describe the graph of induced emf against time for a magnet dropped through a coil
Peak emf occurs when the change in flux linkage is greatest, which is when each pole passes through the coil
The amplitude of the second peak is greater because the speed of the magnet has increased, so the change in time as decreased, and so the rate of change of flux is greater
The area under each peak is the same because the total change in flux linkage must be zero. There was no emf before the magnet was dropped, and there is no emf after
What happens to the induced emf if you use a wider coil?
Magnet dropped through coil experiment
The magnitude of the induce emf will be lower than if you used a narrower coil because the coil will cut fewer flux lines
What happens when you use a longer bar magnet?
Magnet dropped through coil experiment
There will be a longer period between the peaks because there is only a change in flux linkage when a pole enters or leaves the coil. The second peak will have an even greater magnitude and a shorter duration because the magnet will have been accelerating for more time when the second pole passes through the coil, and so will be travelling faster (so Δt will be even smaller and Δ flux linkage/Δt will be even larger)
What are transformers?
Devices that make use of electromagnetic induction to change the size of the voltage for an alternating current. They use the principle of flux linking in two coils of wire, wrapped around an iron core
Describe how transformers work
Alternating current flowing in 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
What is an ideal transformer?
One that is 100% efficient. You can assumed a transformer is ideal unless it is stated otherwise.
The power in each coil will also be the same
What are step up and step down transformers?
Step up transformers increase the voltage by having more turns on the secondary coil than the primary
Step down transformers reduce the voltage by having fewer turns on the secondary coil
Why, in real life, are transformers not ideal?
There will small losses of power from the transformer, mostly in the form of heat
Heat can be produced by eddy currents in the transformer’s iron core
Heat is also generated by resistance in the coils
What are eddy currents?
Currents induced by the changing magnetic flux in the core of a transformer
How can reduce the effect of eddy currents in a transformer?
Laminating the core with layers of insulation
How can you reduce the power loss due to heat generated by resistance, in transformers?
Thick copper wire is used, which has a low resistance
What is the equivalent of the power supply in an electrical circuit, in a magnetic circuit? Why?
magnetic flux lines are always continuous and form a closed loop, so the current carrying wire is equivalent to the power supply
Why is the emf of an electrical circuit, equivalent to in a magnetic circuit?
The number of current turns, NI
Why is the current of an electrical circuit, equivalent to in a magnetic circuit?
Magnetic flux
What is permeance? What is it equivalent to in an electrical circuit?
Conductance
It’s the amount of flux induced in the objects for a given number of current turns that surround it.
The higher the permeance of an object, the greater the amount of flux induced
What is the relationship between area, length and permeance/conductance?
Both permeance and conductance are inversely proportional to the length of the object, and proportional to the cross sectional area
How would you design an ideal transformer? Talk about the core and the coils. What issue comes up?
Make the permeance of the core as high as possible to get the magnetic flux induced in it
Ideally you want the core to be short and fat and made from a high permeability material like iron
However you also want the conductance of the copper coils to be as high as possible, to limit energy loss. So you want to make the right number of turns with the shortest piece of wire possible, eg. Use small radius coils. This doesn’t work when you have a fat core to wrap them around. So you have to get a balance in dimension to get the best overall performance
Unlike an electric circuit, a magnetic circuit will still work with an air or vacuum gap in it. What happens?
Transformers: if there is an air gap in an otherwise iron core, magnetic flux still ‘flows’ around the magnetic circuit. But because air has a very low permeability compared to iron, the total amount of flux in the magnetic circuit will be dramatically lower than without the air gap
What is the circuit symbol for an a.c supply?
A wiggly line ~ between two dots. Dots at end of vertical wires with wiggly line between with in gap in wire
What do dynamos do?
They convert kinetic energy into electrical energy
They induce an electric current by rotating a coil in a magnetic field
Describe how dynamos work
They induce an electric current by rotating a coil in a magnetic field
The output voltage and current change direction with every half rotation of the coil, producing an alternating current
A split ring commutator is used to change this a.c current into direct current
This current is carried to an external circuit using brushes
Which hand rule do you use for motors? For generators/dynamos?
For motors: left hand. Motor cars drive on the left
Generators: right
What is another word for a dynamo?
Generator
how do you achieve a constantly changing flux in a transformer’s primary coil
you connect an alternating potential difference to a transformer’s primary coil, this drives an alternating current, in the primary coil. and as it is the primary coil that produces the flux, you produce a constantly changing flux in the primary coil
what does a turns ratio less than one mean
its a step up transformer, more turns in secondary than in primary
what does a turns ratio more than one mean
its a step down transformer, more turns in primary than in secondary
what are eddy currents and how are they produced in an iron core transformer
circulating currents
iron has a high magnetic permeance but it’s also a good conductor, so the alternating flux in the primary coil induces current in the secondary coil but also within the core itself.
by lenz’s law, the flux then produced by eddy currents will oppose the flux induced in the primary coil reducing the efficiency of the transformer.
this wastes energy which is dissipated as heat within the core by the eddy currents’ heating effect
how do you reduce eddy currents
why in this plane rather than the other?
by making the iron core out of vertical laminations, stack of flat plates instead of a single solid lump, separated by electrically insulating layers.
eddy currents flow horizontally in the iron core so by making the core out of vertical laminations we further minimize eddy currents and their consequences.
graphically for flux linkage against time
what is the gradient
when is emf greatest
when is it 0
the gradient of flux linkage against time is emf
its greatest when flux linkage-time graph is steepest, as that is when flux linkage changes the most
emf is 0 at a minimum or maximum as that is when the gradient is 0
what is back emf in a motor
emf produced when a motor starts spinning which opposes the pd driving the motor due to lenz’s law, which limits the speed of the motor
how does a generator work
uses motion often produced by a turbine to produce the flux changes needed to induce an emf
equation for working out emf
emf = v * L * B
