6.8 AC and transformers

Question 1

What is alternating current?

Answer 1

Current that periodically changes direction

Question 2

What is direct current?

Answer 2

Current flowing only one way.

Question 3

What else can we have that is alternating?

Answer 3

voltage/pd

Question 4

What is an alternator? it uses what

Answer 4

generator of alternating current, uses electromagnetic induction

Question 5

how to alternators work?

Answer 5

as an electrical conductor is turned in a magnetic field an emf and this current is induced. Since the direction of rotation is switched the direction of emf/current in reversed. This is repeated

Question 6

What energy store is energy transferred to and from in alternators?

Answer 6

Ek to EPE

Question 7

What are transformers?

Answer 7

devices that use electromagnetic induction to change size of pd/voltage for alternating current supplies

Question 8

What does a transformer consist of?

Answer 8

An iron core, primary coil and secondary coil

Question 9

What are the 2 types of transformers?

Answer 9

Step Up and Step Down

Question 10

What is a step up transformer?

Answer 10

Where pd is increased due to the secondary coil having more coil turns than primary

Question 11

What is a step down transformer?

Answer 11

Where pd is increased due to the secondary coil having less coil turns than primary

Question 12

How does a transformer work?

Answer 12

• The primary coil is supplied with a ac power supply and so creates its own magnetic field
• the change is magnetic flux linkage relative to secondary coil means emf is induced in it (faradays law). Since AC is the supply this happens constantly as magnetic flux linkage keeps changing
• The emf created is alternating since it would backtrack to oppose change of magnetic flux linkage. Its magnitude is determined upon the turns ratio.

Question 13

How do we determine the pd created in the secondary coil?

Answer 13

We use the ratios using the formula Vs/Vp = Ns/Np = Ip/Is
where v = voltage/pd/emf
n = turns
i = current

Question 14

Why is the current represented as Ip/Is in this equation instead of the other way round?

Answer 14

As it will do the opposite to voltage/pd (increase or decrease), since conservation of energy means power must stay the same. So IpVp = IsVs

Question 15

Why do transformers have an iron core? What is its structure?

Answer 15

A bunch of iron layers insulated from each other

It reduces the loss of energy transferred between coils

Question 16

Why are the layers of iron in the iron core insulated from each other?

Answer 16

Avoid heating effect and therefore energy loss as thermal energy

Question 17

Why would a DC not work for a transformer?

Answer 17

As teh magnetic flux linkage wouldn’t change so that no emf would be induced in the secondary coil (faradays law)

Question 18

What needs to decrease to increase voltage in secondary coil?

Answer 18

The current

Question 19

What needs to decrease to increase current in secondary coil?

Answer 19

voltage

Question 20

Why?

Answer 20

As power = IV must stay equal throughout to obey conservation of energy

Question 21

What is alternating current produced by ?

Answer 21

Alternating voltage

Question 22

What is the waveform (pd/t or i/t) for current and pd?

Answer 22

Sine graph

Question 23

What is the waveform for power of an ac supply?

Answer 23

Postive peaks only

Question 24

What happens to the the magnitude of power supply of an ac/av?

Answer 24

It won’t be the same as peak voltage/current or voltage/current of DC supply but lower as most of the time the pd/I will be below the peak.

Question 25

What do we need to do to compare ac/av to dc?

Answer 25

we have to use root mean square (r.m.s)

Question 26

What is root mean square?

Answer 26

voltage or current divided by (square root)2

Question 27

What can the root mean square be used for?

Answer 27

current

pd/voltage

Question 28

What is the r.m.s power of an ac supply?

Answer 28

Vr.m.s x Ir.m.s

Question 29

Using the waveform how can we work out the frequency?

Answer 29

Using the distance between peaks and f = 1/t

Question 30

How do we work out the r.m.s?

Answer 30

peak value divided by (square root)2

Question 31

How can we work out peak value from r.m.s?

Answer 31

r.m.s x (square root)2

Question 32

Is alternting current or emf constant?

Answer 32

No, varies from V0 to 0 each half a cycle

Question 33

How to work out power at a time?

Answer 33

p = V^2 / r = V0^2cos^2(omega x time)/R = V^2/2R

hence average power is half the peak power

Question 34

Equations for voltage and current for sine graphs?

Answer 34

V = V0 sin 2πft 
I = I0 sin 2πft 
f = frequency 
t = time  
2πf is essentially angular velocity