Magnetic Fields COPY Flashcards
Which way do magnetic field lines go?
Away from a north pole of a magnet and towards a south pole.
What are magnetic fields produced by?
Permenant magnets and current carrying conductors
Draw the field lines around a magnet
What does each finger represent in Fleming’s left hand rule?
First Finger Field
seCond finger Current
THumb THrust
What happens if the current and field are in the same direction?
The current will get no force on it
What is the force on a conductor in a magnetic field at right angles to?
- The direction of the current
- The direction of the field
Describe the experiment to investigate F = BIl
What does the force on a conductor in a magnetic field depend on?
- The magnetic field strength / flux density
- The current flowing
- The length of the wire
What is the condition for F = BIl to work?
The current and the B field are at right angles.
Define Tesla
A magnetic field has a strength of 1 Tesla when 1 metre of wire put at right angles to the field with a current of one amp through it has a force of one newton on it.
What does a simple d.c. motor consist of?
A rectangular coil wound on an iron core between the ples of a strong U-shaped permenant magnet
Describe how a simple d.c. motor works
- The two ends of the coil are soldered onto the two halves of a split ring comutator
- When a current flows, forces act on each side in opposite directions of the coil making it rotate. The forces put a couple on the coil
- The turning effect is the biggest when the coil is horizontal, it decreases as the coil rotates
When is the turning effect biggest on a simple d.c. motor?
When the coil is horizontal, it decreases as the coil rotates
What would happen to the simple d.c. motor if the current remained in the same direction?
The turning effect would reverse in direction when the coil passed through the vertical position
How would you avoid the turning effect reversing in direction in a simple d.c. motor?
- When the coil reaches a verticle position the brushes touch the gap between the two halves of the commutator, cutting off the current
- The coild keeps rotating past the vertical positiondue to momentum
- The current then reverses so that the turning effect continues to act in the same direction as previously.
What are the conditions that need to be met in order for a charged particle in a B field to have a force exerted on it?
- The particle needs to be moving
- It must not be moving parallel to the field
What is the formula for the force on a charged particle moving with velocity v at right angles to a magnetic field of strength B?
F = Bqv
What is the direction of the current is the charged particle flowing is positive?
In the direction of the charged particles
What is the direction of the current is the charged particle flowing is negitive?
It is opposite to the direction of flow of the charged particles
Draw the path of a particle of charge +q as it enters a magnetic (direction is into the page). Draw the force that acts on the particle on the diagram.
Why does a positive particle undergo circular motion after it enters a magnetic field?
- The force on it acts as a centripetal force
- Speed is constant
- The force does no work on the particle so the K.E doesn’t change
What is the equation for the energy of an accelerated charged particle in a magnetic field?
qV = 1/2 mv^2
What are the assumptions made when using the formula qV = 1/2 mv^2?
The particle is accelerated from rest.
What is the radius of the motion of the charged particle as it undergos circular motion in a magnetic field?
r = mv / Bq
How does a cyclotron work?
- A strong uniform magnetic field is used to make charged particles travel on a spiral path inside two hollow semi-circular “dees”
- An alternating p.d is applied to the “dees” at a certain frequency
Why does the frequency have to be right in a cyclotron?
- The frequency of the ac p.d. is chosen so that whenever the charged particle gets to a gap the polarity of the ac changes so that the particle is attracted to the other “dee”, and accelerated to a higher velocity
What is the same, no matter the radius or the velocity, in a cyclotron?
The time taken to travel a half cycle
Derive a formula for the time taken for a charged particle of mass m, charge q moving at velocity v in a radius r in a magnetic field B to do half a cycle in a cyclotron.
What does the time period of a charged particle to complete a hald cycle in a magnetic field in a cyclotron not depend on?
USING T = pi m/Bq
It doesnt depend on the radius of the motion or the velocity of the particle.
Why does the time period get bigger when the particle travels near to the speed of light in a cyclotron?
- Its mass increases significantly due to its kinetic energy having extra mass
- The time period depends on the mass and so the particle would take longer and would be out no longer be in step with the ac supply
Why can a cyclotron not be used to get charged particles to very high velocities near to the speed of light?
- As the particles get near to the speed of light their mass increases significantly
- This is due to kinetic energy having extra mass
- The time period depends on the mass and so the particle would travel slowly
- The particle would no longer stay in step with the ac supply.
What happens in a velocity selector? Use this top derive the formula for mula fo a charged particle the will pass straight through the crossed fields
- A charged particle enters crossed fields (magnetic and electric field)
- The particle experiences a force of qV/d upwards due to the e field and a force of Bqv down due to the magnetic field
- For a velocity to pass through Fe=Fm
- Vq/d = Bqv
so v = V/Bd
Derive an equation for the velocity of the charged particle that will pass straight through the combined field.
What are crossed fields?
Where an e field and a B field are at right angles
Which way does a the B field act in a velocity selector?
Into the page
Derive a formula for the velocity of charged particles that can go through the slit in terms of v, B and E
BqV = Eq
Bv = E
v = E/B
How is the mass of a charged particle measured in a mass spectrometer?
- Ions with velocity E/B (or V/Bd) will pass through the slit
- A magnetic field is applied in the mass spec and causes them to move in a circle.
- since r = mv/Bq then m = Bqr/v
- Since v=E/B and E,B,q and r can be measured, the mass can be measured.
Derive a formula for the mass of a charged particle in mass spec using V, d, B, q & r
What is likely the cause of ions arriving at r at slightly different points?
The ions are different isotopes and so ahve slightly different masses..
This is due to r being proportional to m.
What is likely the cause of the radius measured being half the radius?
The ion will be doubly charged as r is proporional to 1/q
