A.C. Currents

Question 1

What is the equation for alternating current I

Answer 1

I = I.sinωt

Question 2

What is the equation for alternating V

Answer 2

V = V.sinωt

Question 3

What is the room mean square value of an A.C.

Answer 3

It is defined as the value of the steady direct current what would dissipate heat at the same rate as the a.c. in a given resistor

Question 4

How to determine rms value of an ac

Answer 4

1. square the ac and plot the I^2 - t graph
2. Determine the mean of I^2, <i></i>
3. Determine the square root of <i></i></i>

Question 5

What is the rms current of a sinusoidal graph

Answer 5

I = I./2^1/2

Question 6

What is the average power for a sinusoidal ac

Answer 6

<p> = 1/2 P.</p>

Question 7

What are the features of an ideal transformer?

Answer 7

• resistances of the primary and secondary coils are 0
• The is no flux leakage so both coils have the same flux through them
• it is 100% efficient: input power at primary coil = output power at the secondary coil

Question 8

What is the principles of operation of an ideal transformer

Answer 8

1. When an alternating emf Vp is applied to the primary coil, an ac flows in primary coil
2. This ac sets up a changing magnetic field and hence changing magnetic flux in the primary coil which is linked by iron core
3. By Faraday’s Law, this changing magnetic flux linkage induces an emf in the 2 coils
4. Since there is no flux leakage, flux per turn in both coils are equal. Hence rate of flux change per turn must be same in both coils.
   
   • Ns/Np = Vs/Vp
   • Ns/Np is called the turns ratio of the transformer
5. When load is connected across secondary coil, a current Is flows. Since no power loss,
   
   • VsIs = VpIp
6. Ns/Np = Vs/Vp = Ip/Is

Question 9

What causes energy loss in a practical transformer

Answer 9

• Resistance in coils leads to power loss known as Joule Heating. The loss of power can be reduced by using thick wire
• Eddy currents in iron core due to the changing magnetic flux inducing currents in the core itself. This causes the core to get hot, loss of energy. Can reduce eddy currents by making the core out of many insulated strips.
• Flux leakage between primary and secondary coils causes the electrical power to not be completely transferred to the secondary output. This can be reduced by improving the design of the core such as ensuring that there is no air gaps btw laminated core
• Hysteresis: Repeated magnetisation and demagnetisation of iron core causes energy loss in the form of heat. Can reduce by using a core material with low hysteresis loss.

Question 10

How to do rectification

Answer 10

Use diode