Magnetic fields Flashcards
How does a magnetic field form
When current passes through a long straight conductor
How can you work out the direction of the field produced by a current carrying wire
The field line are concentric rings around the wire and use FRH rule to work out direction with thumb as current
Why is a force produced when a current carrying wire is placed near a permanent magnet
The magnetic field of the current carrying wire interacts with the magnetic field of the permanent magnet
Define 1 Tesla
The magnetic flux density if a force of 1 N is produced when 1 A of current passes through a 1 m long wire
What is the formula for the force produced when a current carrying wire is placed in a magnetic field
F = BILsinθ
where B is the flux density of the external magnetic field
I is the current in the conductor
L is the length of the conductor within the external field
θ is the angle between the conductor and the external flux lines
How can you find the direction of the force produced when a current carrying wire is placed in a magnetic field
- Flemming’s left hand rule
- index finger is direction of external flux lines
- middle finger is direction of current
- thumb is direction of force
What is the formula for the force produced when a moving charged particle is placed in a magnetic field
F = BQv
where B is flux density of external fied
Q is charge of particle
v is velocity of particle
When would you use F = BQv and not F = BIL
If it is an individual particle and not a sustained current
How can you calculate the DIRECTION of force produced when a moving charged particle is placed in a magnetic field
- Still use Flemings left hand rule
- But now:
middle finger is direction of velocity of particle
Why does the moving charged particle move with circular motion when the force is produced
Force is perpendicular to velocity
Derive a formula to find the radius of path for the moving charged particle
BQv = mv^2 /r
r = mv / BQ
What is a cyclotron
- A type of particle accelerator made up of 2 electrodes with an alternating voltage between them and a magnetic field which acts perpendicular
- Is an application of the circular motion of moving charges
How does a cyclotron work
- Charged particle starts in the centre of the 2 electrodes and accelerated towards 1 of them due to pd
- Force due to perpendicular B field causes circular motion in 1 of the electrodes
- As it passes out of electrode back in to the centre, pd alternates causing it to be accelerated towards other electrode
- Now that it is faster, the radius of circle in the other electrode is bigger
- Process repeats until particle is fast enough to leave cyclotron
Define flux
Measures magnetic field in a given area
(think of it as amount of field lines in the area)
Define flux density
The flux per unit area
Define flux linkage
total flux in all of the coils
flux x number of coils
What is the formula for flux and flux linkage
Φ = BAcosθ (for flux)
NΦ = BANcosθ (for flux linkage)
where B is the flux density of the magnetic field
A is the area being measured across
θ is the angle between the field lines and the normal to the coil
N is the number of coils
What is electromagnetic induction
The process by which an EMF is induced when you move a conducting rod relative to a magnetic field
How can a current be induced during electromagnetic induction
If the circuit is complete, the EMF induced will cause a current
What is Faraday’s law
The induced EMF is equal to the rate of change of flux linkage that the coil experiences
i.e. EMF = - N ΔΦ / Δt
(negative because Lenz’s law)
What is Lenz’s law
The induced EMF opposes the change in flux linkage that caused it to conserve energy
e.g. if there was an increase in flux linkage upwards the induced emf would act downwards
Derive another formula for the induced EMF
- EMF = - N ΔΦ / Δt
- s = vΔt so area = lvΔt
- ΔΦ = BA = BlvΔt
- EMF = BlvΔt/Δt = Blv
where B is the flux density
l is the length of conductor
v is the velocity of conductor being moved through the changing flux
How can you find the emf induced when you rotate a coil in a magnetic field instead of moving it in a straight line
NΦ = BANcos(wt) as w = Δθ/ Δt
EMF is d/dx
= BANwsin(wt)
so flux linkage against time is a function of cos and emf against time is a function of sin
What is the difference between the oscilloscope graph of current/time for AC and DC
AC is a sin curve
DC is in the form y=k
For a oscilloscope with no time base
AC is a vertical line at x = 0
DC is a point at (0,k)
What is the difference between peak voltage (or current) and peak to peak voltage (or current)
Peak is the distance from equilibrium to the peak
Peak to Peak is the distance from trough to peak
What is the relationship between the peak voltage (or current) and the root mean square voltage (or current)
RMS = peak / sqrt2
How does a transformer work
- Alternating PD is applied across primary coil causing an AC current
- This causes an alternating magnetic field in the primary coil as current carrying wire produces a magnetic field
- The alternating magnetic field transfers across the iron core to the secondary coil
- Secondary coil is experiencing changing B so EM induction causes EMF to be induced, causing a current
When would you use a step up or step down transformer
Step up to increase the voltage and decrease current so used in transferring electricity efficiently in power cables with less heat loss
Step down to decrease voltage and increase current so used for safety in households
What is the formula for power lost in a transformer which isn’t 100% efficient
P = I^2 R
What is the formula for efficiency of a transformer
Efficiency=IsVs / IpVp x100
What are the 3 ways energy can be lost in a transformer causing less efficiency
- Eddy currents
- Resistance in coils causing energy to be dissipated as heat (as lost P = I^2 R)
How do eddy currents form
When iron core (a conductor) experiences alternating magnetic field, causing EM induction and currents to form
How do Eddy currents reduce effiency
- They oppose the alternating magnetic field in the primary coil due to Lenz’s law, reducing net field strength
- They cause energy to be dissipated as heat
How can you reduce the effect of Eddy currents
Laminate the core with layers of insulation so eddy currents can’t form
How can you reduce the inefficiency caused by resistance in the coils
Use a thicker wire which has a lower resistance
