11. Magnetic Fields Flashcards
What is a ‘force’ field?
a region where an object experiences a non-contact force
What is a magnetic field?
an area where a magnet or magnetically susceptible material experiences a force of attraction or repulsion
What are magnetic field lines referred to as?
flux lines
What do magnetic flux lines show?
the direction of the force on the north pole of a magnet
Where do the arrows of magnetic flux lines always point when drawn?
from North to South
What is the magnetic field strength called?
magnetic flux density
What is magnetic flux density?
- a measure of the density of the flux lines
- the force on one metre of wire carrying a current of one amp at right angles to the magnetic field
What is the symbol for magnetic flux density?
B
What are the units of magnetic flux density?
Teslas (T)
How are flux lines going into the page represented?
with an X
How are flux lines coming out of the page represented?
with in O
What happens when a current flows through a wire?
a circular magnetic field is induced around it
What does the thumb show in Fleming’s right hand rule?
the thumb points in the direction of the conventional current
What do the fingers show in Fleming’s right hand rule?
fingers curl in the direction of the magnetic field
What happens when a current carrying wire is placed in a magnetic field?
it will experience a force
What does the thumb show in Fleming’s left hand rule?
the direction of the force
What does the index finger show in Fleming’s left hand rule?
the direction of the magnetic field
What does the middle finger show in Fleming’s left hand rule?
the direction of the conventional current
What four factors could affect the force on a current carrying wire?
- magnetic flux density
- size of current
- length of wire in magnetic field
- angle between the wire and field
When is the magnetic flux density equal to 1 Tesla?
when a 1 metre wire, carrying 1 amp perpendicular to the magnetic field experiences 1 Newton of force
What is the full equation used to calculate the force on a current carrying wire?
F = BILsin(θ)
What are the two conditions where a charged particle will not experience a force in a magnetic field?
- if it is moving parallel to the field lines (so Fleming’s left hand rule doesn’t apply)
- if it is stationary (v = 0 therefore F = BQv = 0)
What path do charged particles follow in a magnetic field?
A circular path
Why does a charged particle follow a circular path in a magnetic field?
- Fleming’s left hand rule states that the force experienced by a particle is always at right angles to the direction it is travelling in
- so this force will change the direction the particle is travelling in
- which changes the direction of the force
How can the radius of the circular motion of a charged particle in a magnetic field be derived?
Centripetal force (using linear velocity) = magnetic force
How can the time period and frequency of one rotation of a charged particle in a magnetic field be derived?
Centripetal force (using angular velocity) = magnetic force
What is a cyclotron?
A compact particle accelerator used often in hospitals to produce radioactive tracers and high energy radiation beams
What do cyclotrons consist of?
Two “Dees” (the area where there is a magnetic field) with a gap in the middle (where an alternating electric field is present)
How does a cyclotron work?
- A charged particle is inserted in the middle of the cyclotron and it experiences a force in the electric field, accelerating it towards the negative plate
- It follows a circular path once it enters the magnetic field
- The particle with re-enter the electric field
- The potential difference of the plates must be flipped to accelerate the particle to a higher linear speed
- The particle changes direction as it enters the other Dee
- The potential difference must be flipped again as the particle re-enters the electric field
- This process repeats until the particle reaches the desired speed
- It will then exit the cyclotron
Why can the magnetic field not increase the linear speed of the particle?
Because there is no force in the direction of travel, it only changes the direction
Why must the potential difference of the plates be flipped?
To accelerate the particle to a higher speed, otherwise the particle would get decelerated
Why does the radius of the particle’s circular path increase?
The radius of the circular path will be larger as the particle will have a greater linear speed
