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)
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)
