3. Magnetic Fields and Electromagnetic induction Flashcards
B = magnetic flux density
Definition:
The strength of the magnetic field
What quantity is B
A vector
u⁰ définition
The permeability of free space
4(pi) x 10‐⁷
Units of B
Teslas (T)
What is needed to use the right hand rule?
The distance from the wire is perpendicular
What represents the field INTO the page?
Cross
What represents the field OUT of the page?
Circle
How can you increase the B of a solenoid?
The number of coils
Add an iron core
Increase current
B equation with long straight wire
B = u⁰ x I / 2(pi) x a
B equation with a solenoid
B = u⁰ x n x I
What does n represent?
The number of turns or coils per metre
Number of turns / Length
Force on a current carrying wire
F = BIL sinø
What are the conditions for sin ø
It is the angle between the magnetic field line
If F = BIL and mv²/r then what can you figure out
Centripetal force
Omega
Frequency
Time period
What rule is used for figuring out directions of current
Fleming left hand rule
What does the 1st finger represent
Magnetic field
What does the 2nd finger represent
Current
What does the thumb represent
Force
Force on moving charge equation
F = Bqv Sinø
What are the three types of particle accelerators
Linear accelerator
Cyclotron
Synchrotron
What current does a linear accelerator use
constant AC
Why do the tubes get longer in a linear accelerator
The particle needs to spend the same time period in each tube as the speed increases
The frequency of AC is constant
Where is the particle accelerated in a cyclotron or synchrotron
In between the gaps and Dees
What is the difference between a cyclotron and a synchrotron
Synchrotron = circular (2 Dees) = constant radius= increasing field
Cyclotron = spiral = constant field= path with increasing radius
Why do the protons move in a spiral
The electric field accelerates protons across the gaps
The force is towards the centre
As speed increases so does the radius due to mv²/r
The force is centripetal keeping it in circular motion
how can the acceleration of a proton increase
increase the potential difference
what direction do field lines travel in
north to south
what conditions must there be for f=bvq
the magnetic field must be perpendicular to the velocity
what are the situations where the magnetic field = 0`
when the angle is 0 to moving parallel to the field
when the particle is at rest / velocity = 0
if a particle entered a uniform magnetic field and the field then increased what would happen to the path of the particles?
they would become more curved and the radius of both particles would decrease
What is the hall effect
A semiconductor is placed within a magnetic field and a potential difference is created due to the forces on the moving charge carriers
how to increase the hall effect
use a semi conductor to provide a better voltage
which decreases the uncertainty
also a semiconductor has a smaller n in I=nave so will decrease Vhall
Why can devices use the hall effect to measure magnetic fields
Vhall is directly proportional to B for a constant current
What is the cutting flux
when the field lines of a magnetic field are ‘cut’ by a metal
what happens when a magnet falls through a metal pipe
the magnet slows down and so takes longer to travel through the tube
what is lenzs law
the direction of any current resulting from an induced emf changes to the oppposite direction to the change in flux linkage
what is faradays law
the emf induced is proportional to the rate of change in flux linkage
also proportional to the rate of flux cut
What equations are used for a velocity selector?
F = eq = bvq
Rearrange to solve for a variable
how do particles travel in a straight line in a velocity selector
the two forces have to be equal to each other
Why does increasing the pd between plates increase the velocity
E = Bv
E = v / d
bigger pd means bigger electric field and bigger E means bigger velocity
How to use a hall probe
Place the probe within the magnetic field at right angles
How to derive the hall voltage
F= Eq
F= Bvq
Set them equal
E = V/d
Substitute that in and rearrage
Vhall = Bvd
What is the relationship between B and Vhall
Directly proportional
How do you induce an EMF
change the area over time
change the flux density over time
What are the four factors that effect the instantaneous EMF
Flux density
Area of the coil
Angular velocity of the rotation
Position of the coil (eg angle it is in the field)
