Tag: physics

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Multiple choice power transmission household electricity household circuits electricity and magnetism physics

An electric fan marked 60 watt consumes 3 units for the duration:

  1. 50 hours

  2. 150 hours

  3. 10 hours

  4. 15 hours
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Answer is A.

The total energy consumed is 3 units = 3 kWh = 3000 Wh.
The total enrgy dissipated is given as Power * Time.
Therefore, time = Energy dissipated / Power.
Time taken = 3000 Wh / 60 W = 50 hours.
Hence, an electric fan marked 60 watt consumes 3 units for the duration of 50 hours.

Multiple choice power transmission household electricity household circuits electricity and magnetism physics

A battery is charged at a potential of 15V for 8 hours when the current flowing is 10A. The battery on discharge supplies a current of 5A for 15 hours. The mean terminal voltage during discharges is 14 V. The "Watt hour" efficiency of the battery is :-

  1. 80%

  2. 90%

  3. 87.5%

  4. 82.5%
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

% Watt hour efficiency = $\frac{E _{out}} {E _{in}} \times 100$
= $\frac{\left ( 14 \right )\left ( 5 \right )\left ( 15 \right )} {\left ( 15 \right )\left ( 10 \right )\left ( 8 \right )} \times 100 = 87.5$%

Multiple choice power transmission household electricity household circuits electricity and magnetism physics

Power transmission is carried out at ........................ and  ..........................?

  1. high voltage, high current

  2. low voltage, high current

  3. high voltage, low current

  4. low voltage, low current
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

One of the key concerns in the transmission of electricity is the power loss in transmission lines, dissipated as heat due to the resistance of the conductors.
The power loss ${P} _{loss} = { I }^{ 2 }R={ (\dfrac { P }{ V } ) }^{ 2 }R(\because p=VI)$
Since $P $and $R $ are fixed conditions, less power will be lost if high voltages V are used.
The transmission line then uses a transformer to step down the voltage.
For a given amount of power, a higher voltage reduces the current and thus the resistive losses occurs in the conductor. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the$ {I}^{2}{R}$ losses by a factor of 100, provided the same sized conductors are used in both cases. 

Multiple choice power transmission household electricity household circuits electricity and magnetism physics

A town situated $20km$ away from a power house at $440V$, requires $600KW$ of electric power at $220V$. The resistance of transmission line carrying power is $0.4\Omega$ per km. The town gets power from the line through a $3000V-220V$ step down transformer at a substation in the town. Which of the following is /are correct

  1. The loss in the form of heat is $640KW$

  2. The loss in the form of heat is $1240KW$

  3. Plant should supply $1340kW$

  4. Plant should supply $640kW$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Total length of wire = $20 \times 2 = 40km$


Resistance of wire $R = 40 \times 0.4 = 16\Omega $ ,


${I _{\max }} = \dfrac{P}{V} = \dfrac{{600kW}}{{3000V}} = 200A$

Power loss in the form of heat $ = {I^2} _{rms} \times R = {\left( {200} \right)^2} \times 16 = 640kW$