Magnetic Fields Flashcards
Describe and explain what happens when a current carrying wire is entered into an external magnetic field
Which hand rule can be used to describe the components involved?
A current carrying wire is entered into an external magnetic field
Field of wire and magnetic field are added to produce a resultant force
Magnetic field lines are perpendicular to the direction of current
LHF
At what angle does a current carrying wire experience the greatest resultant force?
90 degrees
Define magnetic flux density
Unit? (2 possibilities)
Is it a scalar or vector quantity?
The force per unit length per unit current on a current carrying at right angles to the magnetic field lines
T or Nm-1A-1
Vector
What does F represent in the equation F=BIL
The maximum force that a current carrying wire experiences in an external magnetic field - when it is at 90 degrees to the field
Describe right hand rule
When are these two components switched?
Fingers represent direction of magnetic field
Thumb is direction of current of current carrying wire
When describing a wire coil
Describe left hand rule
What else can the thumb represent? When does the thumb represent this component?
First finger field
Second finger current
Thumb is movement of a current carrying through an external mag field
Direction of resistive force which is opposite to motion due to external force
When emf is induced, when lenz law is applicable, when external force is acting
What is the direction of current relative to an electron moving though a magnetic field?
Current is in opposite direction to direction of electron
What happens if an electron leaves a magnetic field?
What happens if it doesn’t? Why?
It travels in a straight line
Circular motion will occur because the direction of force is at right angles to the direction of the electrons movement. (Centripetal force)
Describe and explain the cyclotron
4 points
Magnetic field forces particle into and out of electrode via a semi circular path
Alternating p.d accelerates particle into opposite electrode
V increases so r increases and the direction of p.d switches, causing process to continue
Particle leaves cyclotron as a beam
Define total magnetic flux
Unit?
Total magnetic flux density in a given area of an object
Webers, Wb
Define flux linkage
Unit?
The size of emf induced when a wire coil is moved through a magnetic field or vice versa
Wb
What is Faraday’s law?
Induced emf is directly proportional to the rate of change of flux linkage
What is the gradient of a Flux linkage against time graph?
Emf
What is the area of an induced emf against time graph?
Flux linkage
What is Lenz’s law?
The induced emf is always in such a direction as to oppose the change that caused it
Describe how the opposing force to movement is generated, when a wire coil is moved through a magnetic field
Object moves right with velocity due to an external force
It cuts field lines, generating eddy currents
Current produces an opposing field which causes an opposing field to movement
When lenz’s law applies, what does the thumb represent in the LHF rule?
I’m which direction is the motion of the conductor?
Direction of force of resistance
Opposite
How does flux linkage and emf vary when a coil rotates uniformly in a magnetic field?
Sinusoidally
What is the phase difference between flux linkage and emf when there is a rotating wire coil?
They are 90 degrees out of phase
When and at what angles is flux linkage greatest?
When and at what angles is emf greatest?
At 0 and 180 degrees - coil is perp
At 90 and 270 degrees - coil is parallel
How does a generator work?
What is another name for a generator?
Converts kinetic energy to electrical energy by inducing an alternating current from a rotating coil in a magnetic field
Dynamo
What happens if the number of turns in a transformer doubles?
The voltage doubles
What are the reasons for energy loss in a transformer? What are their solutions?
(3)
Heating of coils - using low resistance windings
Generated eddy currents heat core - use laminated core
Constantly changing magnetic field - use a soft iron core because it magnetizes and demagnetizes easily
What is the relative number of coils on a step up transformer?
No of coils on secondary coil is greater then number of coils on primary coil
Why do we step up voltage?
To decrease current so that heat loss is minimized
How does a transformer work?
5 points
Alternating current flows through primary coil
This causes the core to magnetise and demagnetise continuously in opposite directions
Rapidly changing magnetic flux is produced across core
Rapidly changing magnetic flux passes through secondary coil
This induces an alternating voltage of the same frequency but a different voltage (assuming no of coils is different)
How does current vary with an AC power supply
Sinusodially
What does “same load “ mean?
Under the same resistance
What is the relation between the Irms value of an AC and the I value of a DC?
Irms has the equivalent heating affect as direct current with the same value
How can you use an oscilloscope as a voltmeter to find:
- Vrms
- Frequency
Connect power supply to INPUT of the oscilloscope
Switch time base setting OFF and adjust y gain until a complete vertical line is seen
Measure length of the line and convert it to peak to peak voltage WITH CALIBRATION
(For D.C, measure just the peak voltage)
Divide by root 2 to get Vrms
Switch time base setting ON, adjust it until a minimum of 1 cycle is seen on screen
Measure length of one cycle and convert it to time period using time base setting
F = 1/T
Does an electron undergo circular motion when perpendicular to an electric field?
No. Just parabolic motion
What happens to the motion of an electron due north is inserted into a magnetic field due south
It will continue moving north with constant velocity
What happens when a wire with alternating current passes through a magnetic field? Why?
It will vibrate vertically because the force is perpendicular to Magnetic flux density and current
Force acts in opposite direction when current reverses
Continual reversal means process is repeated
