Magnetism Flashcards
What are the rules for drawing magnetic field lines?
The lines should never cross
The spacing of the lines represents the strength of the field
An arrow should be drawn to show the direction of force that a North Pole would feel
Describe the magnetic field pattern around a bar magnet
Fields lines are closer at the poles and flow from north to south
Describe the field pattern for magnets repelling and attracting
See diagram
Describe the field lines around a straight conductor
Circular rings
Describe the right hand curl rule
Magnetic fields around a straight conductor are concentric circles.
Curl you right hand into a fist and point your thumb in the direction of the current.
Your fingers will curl in the direction of the magnetic field
Describe a solenoid
A wire wrapped into a coil
Describe the magnetic field produced by a solenoid
Inside the coil its a strong and uniform field
Outside the field is similar to a bar magnet
How can you turn a solenoid into an electromagnet?
Adding an iron nail
Describe the motor effect
A current carrying wire produces a magnetic field field.
If this wire is placed near a permanent magnet, then the two fields will either attract or repel each other, exerting a force on each other.
Describe three ways to increase the size of the force experienced with the motor effect
- larger current
- Stronger magnets
- Longer length of wire in the magnetic field
Describe two ways the direction of the force in the motor effect can be reversed?
- Reversing the direction of the current
- Reversing the poles of the magnets
Describe Fleming’s left hand rule
Thumb, first finger and second finger are placed at right angles to each other.
ThuMb - Motion
First Finger - direction of magnetic field (North to south)
seCond finger - direction of current (positive to negative)
What are the units of magnetic flux density?
Tesla (T)
Define magnetic flux density
The force on one meter of wire carrying a current of one amp at right angles to the field
(B = F/IL)
Define the tesla
1 tesla = 1 NA-1m-1
Describe how the motor effect causes a motor to rotate
Current flows into the coil and creates a magentic field.
This magnetic field interacts with the permanent magnetic field creating a force.
The current flows out the coil. As the current has changed directions the force chages direction.
As each side of the motor has a force in opposite directions it rotates.
Describe a charged particles motion when it travel through a magnetic field thats at a right angle to its motion?
Flemings LHR shows that a particles velocity and force are always perpendicular (90°) to each other.
This means that a charged particles will always move in a circular path.
The particle will have a constant speed in a magnetic field but will always be accelerating towards the centre of the circle.
Why is the frequency of rotation independent of velocity?
If a particles speed increases it will follow a circular path with a larger radius so the time period and frequency remain constant
What is a velocity selector?
A velocity selector uses both electric and magnetic fields to select particles at a particular velocity.
It contains two parallel plates connected to a power supply with a uniform magnetic field applied perpendicular to the magnetic field.
Particles with a range of speeds enter and are deflected in opposite ways by the electric and magnetic fields.
Only particles with a specific speed will these deflections cancel so they travel in a straight line and emerge without being deflected.
Describe the structure of a cyclotron
A cyclotron is made up of two hollow semicircular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes.
An alternating p.d. is applied between the electrodes.
Describe how a cyclotron works
- Charged particles enter the accelerator with a small velocity at the centre of one of the dees
- A magnetic field is applied at right angles to the dees, which accelerates particles into a semi circular path until they reach the gap
- An alternating voltage is applied between the dees to produce an electric field to accelerate the particles
- The frequency of the electric field must reverse polarity as particles reach the gap each time.
- The particles gain energy so travel faster and move in a larger semi circular path meaning they take the same time to travel each semi circle.
What is the difference between flux density and flux?
Magnetic flux density, B is a measure of the strength of a magnetic field. (number of field lines per unit area)
The magnetic flux, Φ passing through an area, perpendicular to the magnetic field is define as (the total number of field lines): Φ = BA
Describe electromagnetic induction
If there is relative motion between a conductor and a magnetic field, the free electrons will experience a force (F = BQv), which causes them to move to one end of the conductor.
This induces an electromotive force (e.m.f.) across the ends of the conductor.
If the coil is part of a complete circuit then an induced current will flow through it.
What is required for magnetic induction?
- A conductor
- Movement
- A magnetic field
- A complete circuit
How can you calcualte the induced e.m.f. in a straight conductor moving through a magnetic field?
e.m.f. = Blv
State Faradays law
The induced e.m.f. is directly proportional to the rate of the change of flux linkage.
Magnitude of the induced e.m.f. = rate of change of flux linkage
State Lenz’s law
the direction of the induced e.m.f. or current is always such as to oppose the change producing it
Sketch a graph of flux linkage against time for a coil rotating in a magnetic field.
Maximum flux linkage when the coil is perpendicular to the magnetic field
Zero flux linkage when the coil is parallel to the magnetic field
see diagram
Sketch a graph of induced emf against time for a coil rotating in a magnetic field.
Zero emf when the coil is perpendicualr to the magnetic field.
Maximum emf when the coil is paralle to the magnetic field
How can emf induced be found from a graph of flux linkage against time?
Gradient
If a coil that was rotating in a magentic field had its speed doubled how would
a) flux linkage against time graph change
b) emf against time graph change
a) flux linkage against time graph -
flux linkage stays the same
frequency doubles
b) emf against time graph -
emf doubles
frequency doubles
Why would a pendulum bob that swings through a magnetic field be heavily damped?
- Pendulum bob cuts through magnetic field which induces a voltage across metal bob
- Currents circulate in bob (eddy currents)
- Current produced a magnetic field
- Current will always produce a magnetic field that apposes the motion that created it `This is known as Lenz’s law
- Magnetic fields (from current and permanent magnets) interact and dampen motion
How could you compare a 2V dc battery and a 2V ac supply?
To compare them properly you need to average out the ac voltage. We can find the root mean square (rms) voltage
Describe the structure of a step up transformer
An iron core with an insulated primary coil and a secondary primary coil. The secondary coil must have more turns.
Describe the structure of a step down transformer
An iron core with an insulated primary coil and a secondary primary coil. The secondary coil must have less turns.
Describe the role of a step up transformer
Increases the potential difference and decreases the current
Describe the role of a step down transformer
Deceases the potential difference and increase the current
Explain how a transformer works?
- a.c. flows in primary coil this creates an alternating magnetic field in primary coil
- Magnetic field from primary coil cuts through secondary coil via the iron core
- This induces a voltage and current in the secondary coil
- Current in secondary coil is also a.c.
Do transformers work with ac or dc? Why
only ac
Need a changing magnetic field
Explain why transformers are not 100 % efficient.
- The iron core is cut by magnetic flux, this induces an emf and allows eddy currents to flow. Current creates heat energy (P = I2R) and therefore energy is lost.
- The eddy currents produced their own magnetic field that opposes the magnetic field that created it (Lenz’s law). This can be reduced by laminating the core.
- Heat is also produced by the resistance in the coils. To reduce this use low resistance wires with a large diameter.
- Energy is needed to magnetize and demagnetize the core, to minimize this use a magnetically soft material (Iron) for the core
- Not all the flux from the primary coil will pass through the secondary coil. To reduce this wrap the coils on top of each other on the same part of the core
