6.20

· recall and use the relationship (for 100% efficiency):

input power = output power

Vp Ip = Vs Is

6.19 and 6.20 Plenary answers

a. Vs/Vp=ns/np

Vs/2=80/20

Vs=8V

b. Vs/Vp=ns/np

Vs/10=20/100

Vs=2V

c. Vs/Vp=ns/np

Vs/240=20/400

Vs=12V

d. Vs/Vp=Ns/Np

Vs/4=2000/50

Vs=160V

2. Vs/Vp=ns/np

3/24=ns/480

ns=60turns

a. Iron (Why? Iron is a magnetically soft material - it can be magnetised and lose its magnetism easily. This is necessary in a transformer as the magnetic field needs to change repeatedly)
b. electrical energy in the primary coil => magnetic energy in the core =>electrical energy in the secondary coil
c. dc current produces a steady magnetic field in the core. To induce a current in the secondary coils there must be a changing magnetic field in the core.

a. Step down (because the voltage decreases)
b. Vs/Vp=ns/np

12/240=ns/1000

ns=50turns

c. P = V x I

24 = 12 x I

I = 2A

d. Power in secondary = 24W

e. Power in primary = 24W

f. P = V x I

24 = 240 x I

I = 0.1A

g. The current would be greater/double

Why?

Efficiency = Useful Pout/Total Pinx100

50 = 24/Total Pin x 100

Total Pin = 48W

P = V x I

48 = 240 x I

I = 0.2A

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