Magnetic Flux Flashcards
(46 cards)
1
Q
What does magnetic flux density measure?
A
- Magnetic flux density measures the strength of a magnetic field B, or B field in teslas (T)
- It helps tot his of it as number of field lines per unit area
2
Q
What is the magnetic flux (phi)?
A
- Magnetic flux is density x cross sectional area perpendicular to field direction (measured in Webers)
- It is defined as magnetic flux density B (in teslas T) multiplied but the area of the surface A (in m^2) where the area A is perpendicular to the lines of flux
- Phi = BA
- The total magnetic flux phi, passing through an area A perpendicular to a magnetic field B is defined as phi=BA
3
Q
What is magnetic flux measured in and what is it?
A
- Magnetic flux is measured in webers (Wb) where 1 Wb equals Tm^2
- The weber (Wb) is the unit of magnetic flux equal to 1 Tesla metre^2 (1Tm^2)
4
Q
What happens when an area is not perpendicular to the lines of magnetic flux?
A
- The flux through area A is now the component
- Phi = BAcostheta
5
Q
What happens when an object passes through a magnetic field?
A
-It ‘cuts’ the magnetic flux lines
6
Q
Describer what happens when a wire of length l is moving downwards in a magnetic field with horizontal field lines?
A
- The wire cuts across the flux lines as it moves perpendicular to them
- The wire cuts through more flux lines each second if:
1. Length l is longer
2. The wire moves faster
3. The magnetic flux density is stronger
7
Q
What happens if a conductor moves perpendicular to field lines?
A
- It ‘cuts’ the flux lines
- But if the conductor moves parallel to field lines, they are not cut
8
Q
What is magnetic flux linkage?
A
- Magnetic flux linkage is the magnetic flux linked by a coil, calculated as magnetic flux (phi) x number of turn (N) of the coil (measured in Weber turns)
- Weber turns are the unit of magnetic flux linkage
- Magnetic flux linkage defined as NPhi where phi is the number of flux lines that pass through or links with each of the turns of a coil of N turns
9
Q
How does flux relate to flux linkage?
A
- Since flux is phi = BA for a single loop fo wire then flux linkage is NPhi = BAN
- If the coil of wire has N turns that are perpendicular to the lines of flux
- Flux linkage is measured in Weber turns
- Nphi = BANcostheta
10
Q
What does flux linkage depend on?
A
- The flux density
- The orientation of the coil and flux lines
- The coil’s cross-sectional area
- The number of turns on the coil
11
Q
Why is flux linkage important?
A
Because an emf is induced in a coil in which the flux linkage changes
12
Q
How are electromotive forces induced in conductors when they cut magnetic flux?
A
- If there is reactive motion between a conducting rod and a magnetic field, the electrons on the rod will experience a force which causes them to accumulate at one end of the rod
- This induces and electromotive force (emf) across the ends of the rod and this is called electromagnetic induction
13
Q
How can you induce an emf in a flat coil or solenoid?
A
- Moving the coil towards or away from the poles of the magnet
- Moving a magnet towards or away from the coil
- In either case the emf is caused by the magnetic field (or ‘magnetic flux’)that passes through the coil changing
- If the coil is part of a complete circuit an induced current will flow through it
14
Q
How can you demonstrate electromagnetic induction?
A
- You can demonstrate electromagnetic induction using a coil of wire connected to a micrometer
- The micrometer flicks one way when a bar magnet into the coil and the other way when the magnet is pulled out
- It is zero when the magnet is stationary inside the coil
- An emf is induced if thee is relative movement between the coil and a magnetic field (either the magnet or the coil moves) or the magnetic flux linkage changes (e.g. the strength of the electromagnet changes)
15
Q
What is the electromagnetic induction caused by?
A
A length of wire moving between two magnets, the wire is connected to the micrometer which flicks one way when he wire moves down and flicks in the opposite direction when the wire moves up
16
Q
Why is an emf induced in the wire?
A
- Because an electric charge moving perpendicular to a magnetic field experience a force, BQV and using FLHR you can see that electrons in a wire move towards one end of the wire when the wire moves perpendicular to the magnetic field
1. This leaves one end of negatively charged overall and the other end positively charged overall creating. potential difference across the wire
2. A current can flow if the wire is part of a complete circuit e.g when the wire is connected to a microammeter
17
Q
When you move a coil in a magnetic field what does the size of the emf generated depend on?
A
- The magnetic flux passing through the coil (phi) and the number of turns in the coil that cut the flux (N) and the product of these is the magnetic flux linkage
- For a coil with N turns, perpendicular to a filed with flux density B, the flux linkage is give by Nphi=BAN - The rate of change of flux linkage tells you how strong the electromotive force will be in volts
- A change in flux linkage of one weber per second will induce an electromotive force of 1 volt in a loop of wire
18
Q
What is Faraday’s Law?
A
The indued e.mf. is directly proportional to the rate of change of flux linkage
19
Q
How can you calculate the magnitude of the induced emf in a coil?
A
Emf = Delta(Nphi) / Deltat
-It is the magnitude of the emf, because you only know its size not the direction
20
Q
What does Faraday’s law become in a coil of wire?
A
Since a coil of wire has a fixed number of turns
Emf = N Deltaphi / Deltat
21
Q
How can Faraday’s law be represented?
A
- The size of the emf is shown by the gradient of the graph of flux linkage (Nphi) against time
- If the line is flat the gradient is zero and no emf is induced - The area under ht graph of the magnitude of emf against time give the flux linkage change
22
Q
What does Faraday’s law help to understand?
A
- Relative movement between a magnet and a coil changes the flux linkage in the coil, this generates an emf
- Rotating a coil in the plane perpendicular to the field changes the cross sectional area through which the flux passes, this changes the flux linkage and generates an emf
- Increasing the relative motion or the speed at which the coil roadies increases the rate of change of flux linkage, which increases the induced emf
- If there is no relative movement or rotation, the flux linkage does not change, so no emf is generated
23
Q
What is Lenz’s law?
A
- The direction of the induced emf causes effects that oppose the change producing it
- The induced emf is always in such a direction as to oppose the change that caused it
24
Q
What does the combination of Faraday’s law and Lenz’s law create?
A
Emf = -N Delat(phi)/Delta(t)
-The minus sign shows the direction fo the induced emf
25
Why is the minus sign in Lenz's law significant?
- The idea that an induced emf will oppose the change that caused it agrees with the principle of the conservation of energy
- As the energy used to pull a conductor through a magnetic filed, against the resistance caused by magnetic attraction is what produces the induced current
26
What can Lenz's law be used for?
-To find the direction of an induced emf and current in a conductor travelling at right angels of a magnetic field
27
How is Lenz's law the result of conservation of energy?
1. When the South Pole of a magnet is pushed into the coil, a current is induced in the wire, which become an electromagnet
2. If the South Pole of the electromagnet faces the moving magnet the poles repel and work must be done to keep pushing the magnet into the coil of the wire
- If you try this with a very strong magnet you may feel the force you are working against
28
What would happen if Lenz's law did not apply, and instead the North Pole of the coil faced the magnet's South Pole?
- The magnet would be attracted
1. This would make the magnet accelerate into the coil increasing the induced emf
2. This would start a process in which increasing the emf increased the acceleration, which increased the emf and so on
3. That would imply that energy can be created without doing any work and this of course cannot happen
29
How do you use Lenz's law?
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 FLHR point thumb in direction of the force of resistance (opposition direction to the motion of the conductor)
3. Point your first finger in the direction of the field and your second dinger will now give you the direction of the 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
30
What happens to a metal sheet moving into or out of a magnetic field?
- It can become very hot
| - This happens if very large currents called eddy currents are set up in the metal sheet
31
What are eddy currents?
- Eddy currents are circulating electric currents flowing in the plane of the metal
- They are caused by the change of flux linkage when the metal moves perpendicular to the field, and the currents in a direction to oppose the motion creating them
- Eddy currents can become very large because metals have low resistance
32
How are eddy currents used?
1. Eddy currents are used in induction cookers
2. Here, a high frequency alternating current in the cooker produces a rapidly changing magnetic field which induces a large alternating current in the base of a saucepan causing it to heat up
33
How can eddy currents cause magnetic breaking?
-A pendulum swinging between two magnets slows down quickly because eddy currents are set up in the metal when it enters and leaves the field
34
How do magnetic fields and eddy currents relate? How can this be avoided?
- Magnetic fields created by these eddy currents interact with the fixed magnetic field opposing the motion and stopping the pendulum
- Cutting slits in the pendulum prevents eddy currents forming and the pendulum continues to swing
35
How do guitar strings use changing magnetic fields and electromagnetic induction?
- Guitar strings are made of magnetised steel
1. When eh strong are plucked, they vibrate directly above pick ups which are fixed on the guitar's neck
2. The pick ups are bar magnets wrapped in up to 7000 coils of very fine wire
3. The vibrating strong causes vibrations in the magnetic filed surrounding the coil of the pick up
4. This change is converted into an emf and amplified
5. The distinctive electric guitar sounds come from deliberate distortion when the note is amplified
36
How do microphones use electromagnetic induction to change a sound wave into an electrical signal?
1. The microphone has a light weight coil suspended in a circular groove between the poles of a permanent magnet
2. The coil is attached to a diaphragm that vibrates when a sound wave reaches it
3. Since the coil and magnetic field are perpendicular an emf is induced in the vibrating coil, which depends on the frequency and amplitude of sound waves
4. The induced end is amplified and a loudspeaker changes there signals back to audible sound
37
How can an induced emf be caused by a conductor moving in a straight line?
- An induced emf can be caused by a conductor moving in a magnetic field
- E.G a straight wire may be dropped through a uniform magnetic field, or a plane may fly at a constant height and speed in the Earth's magnetic field
38
What is an example of an induced emf be caused by a conductor moving in a straight line?
1. A credit card includes information stored on a magnetic strip
2. The credit card reader has a small coil in it, and when the credit card is swiped through the reader an end is induced in the coil
- It is important to swipe the card quickly enough so that the induced emf is large enough to be interpreted
39
What happens when a conductor moves at a velocity v perpendicular to the flux lines?
-Faraday's law applies and an emf is generated
-For a conductor of length l, travelling in a flux density B the area swept out per second is length x velocity
-The induced emf equals the rate of change of lux linkage so
emf=B dA/dt and because the area swept out per second is lv this becomes emf=Blv
40
What is power?
The rate of doing work
41
What is an expression for electrical power when a wire cuts flux lines?
-As P=IV
-When power is generated by electromagnetic induction:
P=emfI
42
What is another expression for electrical power when a wire cuts flux lines?
-Work done is Fxd where d is the distance travelled in the direction of the force
-So the rate of doing work is F x v where F is force (N) and v is velocity in the direction of the force (ms-1)
-Each second the change of lux linkage for a wire moving through a magnetic field is BA or Blv where l is the length of the wire perpendicular to the field and substituting P=emfI
P=BlvI P=BIIv
-Since B I and l is force on a conductor in a filed, BIl x v is consistent with the rate of doing work or power generated F x v
43
How do you calculate an induced emf for a rotating coil?
- When a coil rotates in a magnetic field an ac voltage is induced in the coil
- To calculate the value of the induced emf at time t you can sue the following equation for a plane coil in a uniform magnetic field so long as the axis of rotation is at right angles to the field
44
What is an equation for an induced emf for a rotating coil?
emf=BANwsinwt
- A is cross sectional area of the coil
- w is the angular speed of the rotating coil
45
How do you work out the maximum induce emf?
-Since the maximum value of sinwt is 1, the maximum induced emf is
Emfmax = BAN
46
What is a piece of equipment that can be used to measure variation in magnetic flux?
Search coil
