Transformers

Question 1

what do transformers do

Answer 1

• they step up or step down pds
• by having a coil with a current and magnetic field
• induce an emf and therefore current within the other coil

Question 2

what is the name of the coil producing the initial magnetic field

Answer 2

the primary coil

Question 3

what is the name of the coil that is having an emf induced within it due to the magnetic field of the primary coil

Answer 3

the secondary coil

Question 4

what needs to be in the setup for the primary coil in order for this induction to work (other than a cell) and why

Answer 4

• a switch
• because there needs to be a change in the magnetic field (the second coil is feeling) in order for an emf to be induced
• as faradays law states

Question 5

if the switch is simply held down, producing a constant magnetic field, what would eventually happen to the current in the secondary coil

Answer 5

• there will be no change in flux linkage overtime
• so the induced emf would drop to 0
• meaning theres no current or pd present anymore

Question 6

if a voltmeter was connected to the secondary coil, what would it show from the moment the switch was held down to about a couple of seconds later

Answer 6

• it would initially show a spike in pd across the coil

- then would promptly die back down to 0

Question 7

what would be seen on the voltmeter if the switch was opened again

Answer 7

• there would be another spike in the pd across the coil
• but it would be equal and OPPOSITE to the first spike
• either by a needle kicking the opposite direction or by showing a -ve reading
• then once again promptly returning back to 0

Question 8

why does the voltmeter behave like this when the switch is opened again

Answer 8

• when opened, the primary coil of wire stops producing a magnetic field
• this results in there being a change in the flux linkage
• so an induced emf would be temporarily produced in the secondary coil
• but in the opposite direction

Question 9

why does the induced emf within the secondary coil alternate at the same rate as the ac current supplied to the primary coil

Answer 9

• the magnetic field produced by the p coil would alternate at the same rate as the ac current
• the alternating magnetic field would induce the emf that would alternate at the same rate as said magnetic field
• aka the induced emf would alternate at the same rate as the ac current supplied

Question 10

recalling the equation for flux linkage, what would the strength of the coils’ magnetic fields in this transformer setup depend on, given B and A are equal in both

Answer 10

• flux linkage = NBA

- so the number of turns the coils have would determine the strength

Question 11

if the rate of change of flux linkage determines the emf present within the coils, and the rate at which the current in both wires alternates is equal, what equation can you derive for the relationships between both coils number of turns and the pd across their wires

Answer 11

• the ratio of the coils’ number of turns would equal the ratio of the emf / pd across them
• Vp / Vs = Np / Ns

Question 12

what would that equation be in words

Answer 12

the pd across the primary coil over the pd across the secondary coil = the number of turns of the primary coil over the number of turns of the secondary coil

Question 13

now that you know this equation, what would the setup of a step up transformer be like

Answer 13

• the secondary coil would have more turns than the primary coil
• which would increase the output voltage compared to the input voltage

Question 14

what would a step down transformer be like

Answer 14

• the secondary coil would have less turns than the primary coil
• which would decrease the output voltage compared to the input voltage

Question 15

where are the magnetic fields these two coils of wire produced (describe in the sense of it being a picture)

Answer 15

• the magnetic fields of both would create a circle
• that would circle around the inside of the iron core
• usually clockwise

Question 16

why are the magnetic field lines like so

Answer 16

• the coils of wire essentially make a solenoid

- and the magnetic field lines of inside a solenoid would be straight