3.7.5 Magnetic Fields Flashcards
What is a magnetic field?
An area around a magnet (or current carrying conductor) that exerts a non-contact force on other magnets or magnetic materials
What is the null point?
A neutral point where the magnetic fields cancel out so there is no field
What are the magnetic fields around a wire?
When current flows in a wire, a magnetic field is induced around the wire.
The field lines are concentric circles centred on the wire.
How do you find the direction of a magnetic field around a current-carrying wire?
Right-Hand Rule
Make a thumbs up:
Thumb = direction of current through the wire
Curl of fingers = direction of the field
What is a solenoid?
Lots of coils
What is the magnetic field for a coil?
Doughnut shaped
What is the field like for a solenoid?
Like a bar magnet
What does the force on a wire in a magnetic field depend on?
Current
Magnetic field strength
Length of wire
Angle wire is within field
What is magnetic field strength also known as?
Flux density
What is the definition of magnetic field strength/ flux density ?
The force per unit length per current, on a current carrying conductor at right angles to the magnetic field lines
B=F/IL
What is the unit(s) for magnetic field strength/ flux density?
Nm^-1A^-1
Or
Tesla (T)
What is the equation for magnetic force?
Force = magnetic field strength * current * length of wire * number of turns
F = BILn
What is force between a wire and magnetic field caused by?
Force is caused by interactions between a permanent magnetic field and the electromagnetic field (of the wire)
What causes a turning effect in a motor?
Each side of the coil experiences equal and opposite forces.
What is a pair of forces called?
Couple
What is the turning effect also called?
Torque
What is the equation for torque?
Torque = F * d
(d = perpendicular distance between forces)
What happens when the coil is parallel to the field?
Maximum force
F=BILn
Torque = BIAn
(A = area of coil)
What happens when the coil is at an angle to the field?
Reduced force
Torque = F d (d = wcos(x))
Torque = BIAncos(x)
( cos(x) gives the proportion of the maximum force:
Cos(0)=1 =full force =parallel coil
Cos(90)=0 =no turning effect =perpendicular coil )
How does an Electron Deflection Tube work?
Electric field produced by applying a constant pd between metal deflecting plates.
Electron beam enters the field at right angles (to field line) - is deflected towards positive plates.
Force on each electron = constant (in magnitude + direction) - because field is uniform, so beam curved in a parabolic path.
Fluorescent screen enables path of beam to be observed and measured.
What are safety considerations when using an electron deflection tube?
Because high voltages are used, thick cables are needed due to the increase in temperature.
What is Flemming’s LHR?
The force for a charge moving through a uniform magnetic field at a speed perpendicular to the direction of the field.
F = B Q v
What is the work done for a charged particle in circular orbits?
On an equipotential, no work is done.
Stays on circular path, so direction changes, but radius = constant.
Speed=constant
KE=constant.
What is the force for a charged particle in circular orbits?
(if velocity takes on a circular path)
= perpendicular to velocity
Towards centre of the curvature.
What is the acceleration for a charged particle in circular orbits?
Centripetal acceleration
Particle follows circular path if it stays inside uniform field at all times.
How does a galvanometer/ sensitive ammeter work?
A magnet and wire/ coil is connected to the meter.
When magnet is moved near wire (vice versa) a small current passes through the meter.
- this is because an emf is induced in wire = electromagnetic induction.
When does electromagnetic induction occur?
Whenever a wire cuts across the lines of a magnetic field.
How to increase the induced emf?
Move wire/coil faster
Use stronger field/magnet
Increases amount of coils of wire
How does moving wire/coil at angle affect induced emf?
Reduces induced emf because cuts through less field lines.
What are 2 other ways of generating electricity/ an induced emf?
Using an electric motor in reverse. (coil spinning in an electric field)
Using a dynamo. (magnet spinning in/near coil)
When is emf induced?
When there is relative motion between a coil and a magnet (field).
What is Flemming’s RHR?
The reverse of LHR (works in opposite direction)
When is the LHR used?
Motor effect
(we drive on the left hand side)
[2 magnetic fields are needed to interact to create a force]
When is RHR used?
Electromagnetic Induction
What is the 100% efficient way in which energy relates to electromagnetic induction?
Current transfers energy, which is equal to the work done in moving the coil or magnet.
What is the non-100% efficient way in which energy relates to electromagnetic induction?
Movement has to work against the reaction force between magnetic fields (permanent + electromagnet)
What is a cyclotron?
A type of particle accelerator used to accelerate charged particles (e.g protons, alpha particles) to high speeds.
What are the 5 main components of cyclotron?
Ion source
Dees
Mag. field
Vacuum chamber
Outer port
How do the dees work in a cyclotron?
These are 2 D-shaped electrodes placed opposite each other in the mag. field connected to an alternating voltage, accelerating the particle each time.
What is the use of magnetic field in cyclotron?
Field keeps particles travelling in circular path because they experience a centripetal force.
What is the use of electric field in cyclotron?
Field accelerates particles (occurs between Dees), which increases KE.
= radius of circular path increases, so takes longer to complete orbit.
What are some uses of cyclotron?
Medical - production of radioisotopes for medical imaging and radiotherapy.
Nuclear - accelerate and collide target nuclei to study nuclear reaction and fusion.
Industry and Space - semi-conductor manufacturing, radiation sources for sterilisation.
What are some advantages to cyclotron?
Efficient
Compact design
Continuous operation
What are some disadvantages to cyclotron?
Size limitations
Require large heavy electromagnets = expensive.
What is Lenz’s Law?
The direction of an induced e.m.f. is always opposite to the change that causes it.
(Mag. field from induced current opposes movement of mag. field making it)
This is because energy must be conserved.
What is Faraday’s Law?
Moving conductor has induced emf within it (RHR predicts direction )
Wire experiences a force which opposes the motion (LHR predicts force direction)
What is magnetic flux?
The number of magnetic field lines
[BA]
=magnetic flux density * area
What is magnetic flux density?
Number of flux lines per unit area.
[B]
What is magnetic flux linkage?
Number of flux lines passing thru the area, perpendicular to the field.
[NBA]
What is the induced emf also?
Rate of change of flux linkage
What happens to a moving conductor in a mag. field?
Emf induced in conductor (if conductor cuts across mag. field lines)
Conductor moves - change in mag. flux due to change in area of conductor exposed to mag. field lines.
induced emf = BLv
What happens to a fixed coil in a changing mag. field?
Mag. field of solenoid passes thru small coil - if current in solenoid changes, emf induced in small coil.
- because mag. field thru coil changes - flux linkage changes = induced emf.
magnitude of induced emf = (ANΔB)/Δt
What happens to a rectangular coil moving into a uniform magnetic field?
change of flux linkage per sec = BNLv
-once coil is completely in field, flux linkage thru it doesn’t change, so induced emf = 0
What happens to flux linkage for generator?
As coil spins within uniform mag. field, flux linkage changes:
Flux linkage = BANcosθ = BANcos(2πft)
What is induced emf in coil rotating uniformly in a mag. field?
ε = BANωsin ωt
What is peak induced emf, ϵ0?
(equation)
= 2NBLv
How can peak emf be increased?
By increasing:
- speed
- number of turns
- length of coil
- strength of mag. field
How do power station generators (alternators) work?
3 coils 120º apart [120º out of phase with each other] - stationary so called ‘stators’.
Electromagnet spins in middle = the rotor.
Produced 3 alternating induced emfs [3 phases]
-1/3 of each of users receive 1 of the phases
How is back emf made?
Voltage creates movement [motor effect]
Coil moving in mag. field so emf induced [induction]
Voltage and emf oppose each other [Lenz.s Law]
So new induced emf = back emf.
What is ‘resultant voltage’?
(equation)
V - ε = IR
What happens to current and induced emf at different speeds?
Low speeds = low induced emf = high current
High speeds = high induced emf = low current
What is the power supplied using back emf?
Power [IV] = mechanical power [Iε] + power dissipated in circuit [I^2 R]
What is an alternating current [A.C.] or alternating voltage?
One that changes direction with time.
What does rms mean?
The root of the mean of the square.
Why do we use rms when using A.C?
To negate the effects of the negative values in AC supply.
Rms is numerically equal to the D.C, which will transfer the same power as A.C.
How do we calculate peak value and rms value?
rms = peak / √2
How do we calculate power of an A.C supply?
Pmax = Imax * Vmax
How do we calculate dissipated power of an A.C supply?
P = (Irms)^2 * R
P = (Vrms)^2 / R
How do we calculate mean power of an A.C supply?
P = Irms * Vrms
P = 1/2 * (Imax)^2 * R
What is a transformer?
A device that changes the peak value of an alternating p.d. (using electromagnetic induction)
What is the structure of a transformer?
2 coils (primary and secondary) wrapped around a laminated iron core.
How does a transformer work?
An A.C. flowing in primary coil creates an alternating magnetic field.
This is then passed through iron core, inducting an alternating emf in the secondary coil.
