Part A-Power lines carry electricity at about 120,000 volts. If a power line wer

Question

Part A-Power lines carry electricity at about 120,000 volts. If a power line were stepped down by a transformer to 120 volts, how many turns would there be on the secondary coil if there were 1000 turns on the primary? Show calculations.

Part B-If the secondary coil is connected to a resistor with a resistance of 10 ohms, what is the current in the secondary coil? Show calculations.

Part C-What is the power in the secondary coil? Show calculations.

Part D-What is the current drawn by the primary coil? show calculations.

Part E-Why is it useful for the power lines to transfer electricity with a current of the magnitude of the answer in part D?

Explanation / Answer

Use V for volts, N for turns, subscript p for primary and subscript s for secondary. Rewrite the formula as below:
Vs/Vp = Ns/Np
Hence
Ns = Np (Vs/Vp) --------> (1)

Given:
Vp = 120,000 volts
Vs = 120 volts
Np = 1000 turns

Substituting above quantities in (1)

Ns = 1000 (120/120000)
= 1

Current in secondary Is = Vs/Rs = 120/10 = 12 ampere
c. what is the power in the secondary coil?
Power in Watts Ps = Vs x Is = 120 volts x 12 ampere = 1440 watts

Power in primary coil = Power in secondary coil
Ip x Vp = Is x Vs = 1440
Ip = 1440/120000 = 0.012 ampere (which is comparatively low amperage)

Power loss = (I^2) R.
Thus, power loss increases in square proportion of current I for a given resistance R. By reducing I to low level such as 0.012, the power companies avoid power loss during power delivery from power station which is far away fro home or place of power consumption.

b. if the secondary coil in part a is connected to a resistor with a resistance of 10 ohm's, what is the current in the secondary coil?

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