Electricity And Magnetism

Question 1

What are the laws of electromagnetic induction

Answer 1

Faraday’s laws
•when a magnetic field linked to a conductor changes an EMF is induced in the conductor
•The size of the EMF is proportional to the rate of change of the field
Lenz’s law
•The EMF is in a direction that opposes the changes causing it

Question 2

What are the equations involving power

Answer 2

P=VI
Power loss= I2R

V= volts
I= current(A)
R= ohms

Question 3

What is the equation involving volts

Answer 3

V=IR

V= volts
I= amps
R= ohms

Question 4

Why is electricity better transmitted at high voltage low current form

Answer 4

Due to the fact that power loss is I2R so you want to reduce the current to reduce the power loss but as P=VI you need to increase the voltage to increase power

Question 5

How does capacitance differ as distance between placates increase

Answer 5

The capacitance is proportional to 1/d so double the distance half’s capacitance as there is reduced attractive forces

Question 6

Energy transformations in the filament

Answer 6

• Electricity
• Heat
• Kinetic energy of electrons

Question 7

Energy transformations in the target

Answer 7

• kinetic energy of electrons
• heat (99%)
• X-rays (1%)

Question 8

Heat transfer in fluids

Answer 8

• Convection
• heat causes molecules to vibrate taking up more space so the region becomes less dense
• this causes hot fluid to rise transferring heat and cooling as it does

Question 9

What is the rest mass of an electron in MeV

Answer 9

0.51 MeV

Question 10

What is diamagnetism?

Answer 10

• Typically weaker magnet
• There are an equal amount of electrons spinning in opposite directions and there is no set arrangement so they cancel
• If there is an electron field applied a minute magnet effect occurs caked diamagnetism die to a disturbance in orbits
• Typically diamagnetic material is water
• These will make a magnet weaker if put into a magnetic field so MRI sequences need to be adjusted

Question 11

What is paramagnetism?

Answer 11

• Atoms have built in magnetism due to dipoles creating a small bar magnet
• The small magnets mean the dipoles line up (copper is an example)
• They aren’t magnetic when the current is switched off
• Some are used as a contrast agent in MRI- gadolinium (Gd) is an example
• If an external magnetic field is applied the magnets become aligned and this increases the strength
• During a scan metal could heat inside you though and maybe move

Question 12

What is Ferromagnetism?

Answer 12

• The magnets or dipoles don’t act as independently as in paramagnetic materials and are already arranged in small areas
• The smaller groups within a region interact so their magnetic areas line up
• When they are aligned they are a magnet and are magnetically saturated

Question 13

Ferromagnetic material examples

Answer 13

•Iron, nickel and cobalt and and also stalloy which is a steel alloy

Question 14

What does the X-Ray tube voltage do?

Answer 14

•It affects both the type and number of photons and determines the photon energy

Question 14

What does the HTT (step up transformer do)

Answer 14

• Produces the operating voltage

* Increases the potential difference across the tube

Question 15

What is magnetic Flux density?

Answer 15

• The number of lines of magnetic force passing through the unit area at right angles to the line of force
• It’s how strong the magnetic force is and in clinical MRI it’s 1-3T
• the SI unit is Tesla (T)

Question 16

What is hysteresis?

Answer 16

• The resistance to changes in magnetism in all ferromagnetic materials is know as hysteresis
• This happens as removing magnetising force still leaves the material with some residual magnetism and a reverse magnetising force called a coercive force must be applied to destroy this alignment
• Hysteresis results in power loss

Question 17

Two uses of a dialectic

Answer 17

• To prevent contact between the two plates (Left hand plate can store charge)
• They increase the capacitance of the capacitor because it displays polar properties so less work is done to charge the capacitor

Question 18

Three things that impact capacitance

Answer 18

• Distance between the plates: increased distance means a decreased capacitance
• Area of the plates: increased area means an increased capacitance
• Dielectric constant: a high dialectic constant means an increased capacitance