Magnetic Fields Flashcards
In the Formula F=BIL, what does each letter stand for
F=Force (N)
B=Flux Density (Tesla)
I=Current (A)
L=Length (M)
Define Magnetic Flux Density
The force per unit current per unit length on a wire placed at 90 degrees to the direction of the magnetic field
In Flemming Left Hand Rule, what does each finger represent
Thumb= Force
First= Direction of Magnetic Field
Second=Conventional Current
When would you use Flemmings Left Hand Rule
Motors. If a wire is carrying an electric current, electrons are moving along the wire. If the wire is placed in a magnetic field, the force exerted by the field on the moving electrons can move the wire.
Define the Tesla
A unit of magnetic flux density, 1T is the magnetic flux density when 1m of wire carrying 1A of current at right angles to a magnetic field experiences a force of 1N
In the formula F=BQv what does each letter stand for
F=Force
B= Magnetic Flux Density
Q=Charge of Particle
V=Speed
Name the 3 particle accelerators that use magnetic fields
Cyclotron, Line Arc and Synchrotron
How does a Cyclotron Work
- They use circular deflection between 2 hollow dees
- An alternating electric field is maintained across the gap between the 2 dees by an alternating applied pd
- The direction of the electric field between the dees needs to be in the direction that the charged particle is moving in order to accelerate it.
- When it returns to the dee it will be travelling in the opposite direction and therefore the electric field needs to be in the opposite direction. This is achieved by reversing the polarity of the applied pd
- A constant uniform magnetic field acts throughout the whole of the accelerator and provides the centripetal force
What is the unit for Magnetic Flux and Magnetic Flux Density
Magnetic Flux= Weber (Wb)
Magnetic Flux Density= Tesla (T)
Difference between magnetic flux and magnetic flux density
The amount of magnetic flux a magnet displays is a direct result of the material that makes up the magnet. The magnetic flux density is a product of the magnetic flux and the area that this flux is present within.
Define Magnetic Flux Linkage
The product of the magnetic flux passing through a conducting coil and the number of turns of wire on the coil
State Faraday’s Law
The induced emf is directly proportional to the rate of change of flux linkage
State Lenz’s Law
The direction of an induced current opposes the change of magnetic flux that produces it
What is the magnitude of the induced EMF equal to
The rate of change of flux linkage
What is the frequency and rms voltage of mains electricity
Frequency=50Hz and V=230
Define root mean squared
equivalent DC voltage that produces the same heating effect
Whats the difference between Peak voltage and peak-to-peak voltage
Peak=Distance from equilibrium to Peak
Peak-to-Peak=Distance between peak and trough
If the line on an oscilloscope is horizontal what are the 3 potential reasons
- It is a direct voltage
- The voltage gain is zero
- No signal
If the line on an oscilloscope is vertical what is the potential reason
The time base is very small
On an oscilloscope what does the horizontal and vertical height represent
Vertical Height= the input voltage
Width=Time base
What does a transformer do
They change the size of the voltage for an alternating current
How does a transformer work
- An alternating voltage is applied to the primary coil, causing an alternating current to flow
- The current creates an alternating magnetic field
- This creates a changing magnetic flux in the secondary coil
- The changing magnetic flux induces a change in the rate of change of flux linkage
- This results in an emf being produced
What are eddy currents
They are looping currents induced by the changing magnetic flux in the core, they create a magnetic field that acts against the field that induced them
What are the 3 causes of inefficiencies in transformers
- Power is wasted due to heating effect of the current in the coils due to a resistance
- Eddy currents which dissipate energy due to heating, they also produce magnetic flux which acts to reduce the overall flux in the core
- Energy lost due to work done repeatedly magnetising and demagnetising the core
How do you reduce the power wasted due to resistance in the wires in transformers
Use thick copper wires as they have less resistance
How do you reduce the energy lost due to eddy currents in transformers
Laminate the core with layers of insulation
How do you reduce the energy lost due to work done magnetising and demagnetising the core in a transformer
Use a soft iron core as it is easy to change the magnetic polarity of it
Why do the National Grid use transformers ( why do they transmit at high voltages)
They use they to decrease and increase voltages, because transmission over long distances is more efficient at high voltages, because higher voltages mean lower currents, which means less heating. However it has to be stepped down for recreational consumption
