Magnetic Fields A2 Flashcards
What is the direction of magnetic fields
From north to south
What is the motor effect
When a wire carrying a current is perpendicular to a magnetic field, this creates a force to act on the wire
What is the equation for the motor effect
F=BIL
What is the magnetic flux density
The magnetic field strength with units of B, in teslas
What is magnetic flux
BA, where B is flux density and A is area. Flux has the units of Wb, in webers
What is the equation of the force acting on a moving charge on a magnetic field
F=BQV
What occurs when charged particles are travelling perpendicular to the magnetic field
They travel in circles due to the force from the magnetic field acting as centripetal force, towards the centre of the created circle (this changes the direction of the electron, not speed)
What is inside an electric motor
A magnetic field, a coil and a commutator
How does the electric motor work
When the current starts to flow through the coil, the perpendicular magnetic field creates a force clockwise. When the coil is vertical, the commutator causes the circuit to stop flow so that the coil can continue rotating around. Then the current is switched by the commutator so the force is still clockwise.
What is a cyclotron
A device to produce high-energy beams of particles for radiation therapy
What is in a cyclotron
Two D-shaped electrodes and an AC supply connected to both
How do cyclotrons work
The protons inserted travel in circular motion around the first D and when they meet the gap between the electrodes, they are attracted to the negative D. This causes its velocity to increase. An AC supply is required to constantly switch the polarity of current in each electrode every half-turn of protons
What is the dynamo effect
Using the right-hand rule for when a moving conductor in a perpendicular magnetic field produces a current in the conductor
What occurs when you induce a current using the dynamo effect
Using the motor effect, a force is produced in the opposite direction of the original motion. This is Lenz’s Law
What is Lenz’s Law
The direction of the induced current is always such as to oppose the change that causes the current
What occurs when a magnet falls through a magnetic field of a coil of wire
A current is induced in the wire, and the top of the wire becomes a certain pole, as to oppose the magnet. This will slow the magnet down
How to find the flow of current when the falling magnet creates a magnetic field in the coil of wire
If the top of the coil is N, this is anti-clockwise. If the top is S, this is clockwise
What is induced emf
Voltage produced from the dynamo rule
What is Faraday’s Law
The negative of the rate of change of flux (change in flux / change in time)
How to calculate the emf of a wire cutting through a magnetice field
BLs/t , where s is change in height of wire or BLv
What is flux linkage
BAN
What is emf induced in a coil rotating uniformly in a magnetic field
BANωsin(ωt)
What happens when a motor turns
This produces the dynamo effect due to Lenz’s Law, creating back emf
What is the back emf when it is spinning fast
Emf is larger and current is lower
What is the back emf when it is spinning slow
Emf is lower and current is higher
What is the direction of current relative to an electron
Current is in the opposite direction to the motion of electrons
