Magnetic fields Flashcards
What is the difference between current and ‘conventional current’?
Current is the direction in which the electrons actually flow (from negative to positive) whereas conventional current is the flow from positive to negative (we use conventional current)
What is the motor effect?
When a current-carrying wire is placed within a magnetic field it will experience a force
(the direction of this force depends on the direction of the current (I) and the direction of the field (B) which can be figured out using Flemming’s left hand rule)
When is the force felt by a current carrying wire in a magnetic field the strongest and weakest?
When the current-carrying wire is perpendicular to the direction of the field the force is at a maximum and when they are parallel the force is zero
Define one Tesla (T)
The magnetic flux density that would cause a 1 meter length of wire, carrying a current of 1A to experience a force of 1N (It is one Newton per amp per meter)
What is magnetic flux density?
The force acting on a one meter of wire carrying a current of one amp at right angles to the magnetic field
What are the two situations when a charged particle will experience no magnetic force?
1) When it is stationary (v=0)
2) When the velocity of the particle is parallel to the magnetic field
Why does a charged particle follow a circular path in a magnetic field?
The magnetic force acts perpendicular to the velocity, providing a constant centripetal force that changes the direction of motion of the particle but not the speed
What factors affect the radius of a particle’s path in a magnetic field?
The radius increases with mass and velocity, and decreases with magnetic field strength and charge
What is a cyclotron and how does it work?
A cyclotron is a particle accelerator that consists of two hollow semicircular electrodes called dees, that are placed in a perpendicular uniform magnetic field. As a charged particle starts at the centre it is accelerated across the gap between the dees by an alternating electric field and once it is inside a dee, the magnetic field forces the particle to move in a semicircular path and after half a circle it exits the dee and is accelerated into the other dee by the electric field (the electric field is now in the other direction). This process repeats as the electric field causes the particles speed and radius to increase creating a spiral and the particle will then be able to leave the cyclotron at a high speed
What is magnetic flux?
A measure of the number of field lines passing through a given area (it is the product of magnetic flux density and the area through which the field lines pass, perpendicular to the field)
What is magnetic flux linkage?
The total magnetic flux passing through all of the coils in a wire (it is the magnetic flux through one coil multiplied by the number of coils)
Define one Weber
The amount of magnetic flux that passes through an area of 1 square meter when the magnetic flux density is 1 Tesla.
Give two simple ways of inducing a potential difference
1) By pushing a magnet into a coil of wire
2) By moving a wire through a horseshoe magnet
What is Faraday’s law?
The induced e.m.f is directly proportional to the rate of change of flux linkage (and the magnitude of an induced e.m.f is equal to the rate of change of flux linkage)
Sketch the graph of how flux linkage changes during a full rotation of a coil and draw the corresponding graph for the induced e.m.f
Look in booklet for answer
What is Lenz’s law?
The direction of the induced e.m.f. is always in the opposite direction to the change that caused it
What is the peak voltage?
The maximum voltage reached by an alternating current supply during one cycle (It is measured from equilibrium up to the peak of a wave)
What is the peak-to-peak voltage?
The total voltage of an alternating current suppy from the maximum positive to the maximum negative peak (it is measured from one peak to another (vertically))
What is the rms voltage?
The root mean square voltage is the voltage of the direct current (DC) that would produce the same power in a resistor as an alternating current (AC) produces over one cycle
Draw both an AC and a DC supply with their time bases switched on and then off
Look in booklet for answers
Describe how to take an accurate measurement of the peak voltage from an oscilloscope
Adjust the time base so that one wave fits onto the screen (helping make the wave clearly visible), then adjust the volts per division so that the wave nearly fills the screen vertically, this helps to maximise accuracy because you can now count the number of vertically divisions (boxes) from the equilibrium position to the peak of the wave, then to find you peak voltage you multiply the number of boxes by the volts per division (scale)
Describe how make an accurate calculation of the frequency from an oscilloscope
Adjust the time base so that multiple full wave cycles appear on the screen (e.g. 5 or 10) then count the number of horizontal divisions (boxes) across all of the wave cycles, then to find out the time period for all of these waves you multiply the number of boxes by the time base (scale), that will give you their total time period, so to find the time period of one wave you divide it by how many full waves there are, then the frequency is 1 over that
Describe the basic structure of a transformer
A primary coil (connected to an A.C. supply) and a secondary coil are wrapped around an Iron core
How does a transformer work?
An AC current flows through the primary coil, creating a changing magnetic field in the iron core. The Iron core then directs this changing magnetic field to the secondary coil, which induces an electromotive force (EMF) in the secondary coil, as explained by Faraday’s Law of Induction.
