Magnetic fields Flashcards
Magnetic field lines direction
Magnetic point in the direction in which a free North pole would move (from North to South)
Electromagnetism
When a wire carries a current, a magnetic field is created around the wire
Magnetic field lines are concentric circles centred on the wire and perpendicular to it
Right hand grip rule
Thumb points in direction of conventional current and direction of field is given by direction of fingers curling around the wire
Motor effect
Force is generated when current carrying wire is in a magnetic field due to interaction of magnetic fields
Fleming’s left hand rule
Thumb represents force
Index fingers represents field
Middle finger represents current
This shows force experienced by current carrying wire in a magnetic field
Equation for force acting on current carrying wire in magnetic field
F=BILsinϴ
where ϴ is angle between magnetic field and current
Investigating force on current carrying wire
Place magnet on a balance and zero
Use current carrying wire and read change
Multiply by g to find force
Motion of individual charged particles moving in a magnetic fields
Charged particles moving in a field will experience a force perpendicular to its velocity so it moves with circular motion
Derivation of F=Bev
F=BIL = BI(vt)
It = Q
F=BQt
for 1 electron, F=Bev
Radius of charged particle in magnetic field
F=mv^2/r
F=Bev
mv^2/r =Bev
r=mv/BQ
Velocity selector
Uniform electric field is applied perpendicular to uniform magnetic field so that they act in opposite directions
For a particle not to be deflected, EQ=BQv
v=E/B
Used to select velocities
Mass spectrometer
Atoms are ionised so each have the same charge
Uses velocity selector so that all ions have the same velocity
They enter a uniform magnetic field so r=mv/BQ
r is proportional to m so radius of circle can be used to find the mass of the ion and the and therefore find the relative abundance of different types of atoms
Electromagnetic induction
When a magnet moves through a current-carrying coil, an emf is induced across the ends of the coil
Reversing the direction of movement reverses emf so oscillating magnet produces alternating current
Generator effect
Coil is made to rotate through electric field and motion of wire generates an emf
Work done to move magnet is transferred to epe when electrons experience a force which causes them to move
Magnetic flux
Component of magnetic flux density perpendicular to area x area
BAcosϴ
Where ϴ is the angle between the field lines and the normal to the area
Magnetic flux linkage
turns on coil x magnetic flux
BANcosϴ
Faraday’s law
The magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linkage
Lenz’s law
The direction of induced emf or current is always such as to oppose the change creating it
This must be true so that conservation of energy is not violated
If forces didn’t oppose force, it would accelerate change so energy would be created out of nowhere
Alternating current generator
Rectangular coil of cross-sectional area A and N turns of coil rotate in a uniform magnetic field
Angle changes so magnetic flux changes, flux = BAcosϴ so graph of magnetic flux linkage against time is cos graph
Emf = gradient so forms a curve
alternating current is generated
How do transformers work
Alternating current is supplied to primary coil
This produces a varying magnetic flux in the laminated soft iron core
Magnetic flux changes, meaning emf is produced across ends of secondary coil
How can transformers be made more efficient
Transformers can be made efficient by using low-resistance wiring
Laminated core insulates it and prevents eddy currents in the core itself
Soft iron is easy to magnetise and demagnetise
