Tag: electric power

Questions Related to electric power

A neon lamp is connected to a voltage a.c. source. The voltage is gradually increased from zero volt. It is observed that the neon flashes at $50 V$. The a.c, source is now replaced by a variable dc source and the experiment is repeated. The neon bulb will flash at ?

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  1. $50V$

  2. $70V$

  3. $100V$

  4. $35V$

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Correct Option: B
Explanation:

Answer is B.

The RMS (Root mean square) value tells us what equivalent DC voltage we would need to get the same power, for the neon bulb to glow.
In this case, the neon bulb glows at 50 V ac voltage. Therefore, for the bulb glow with dc voltage, $50\times \sqrt { 2 }  =50\times 1.414=70V$ dc voltage should be applied.
Hence, the neon bulb will flash at 70 V dc.

Two dissimilar bulbs are connected in series, which bulb will be brighter ?

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  1. low resistance bulb

  2. more resistance bulb

  3. high current pass bulb.

  4. low current pass bulb

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Correct Option: B
Explanation:

Components connected in series are connected along a single path, so the same current flows through all of the components. The current through each of the components is the same, and the voltage across the circuit is the sum of the voltages across each component.
When two bulbs are connected in series the first bulb will receive more current. As the bulb has its own resistance, less current will flow to the next bulb.
Hence the first bulb will glow more.

The wire having a green plastic covering is a

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  1. live wire.

  2. neutral wire.

  3. earth wire.

  4. none of these.

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Correct Option: C
Explanation:

Black, Red and Blue are used for hot wires and White is used as the neutral wire in a 120/208 V circuit. Brown, Orange and Yellow are used as hot wires and gray is used as the neutral wire in a 277/480 V. For grounding, regardless of the voltage, Green is used.The wire having a green plastic covering is a earth wire.

An electrician not aware of the colour coding of resistors connected two resistors A and B in series to a 6V battery of internal resistance $3\Omega$ and an ammeter. The ammeter connected in the circuit was not working and hence he disconnected the ammeter from the circuit. The sequence of the colour bands on resistor. A is yellow, violet and brown while that on resistor B is red, violet and black respectively. By using the colour coding of resistors, help the electrician to determine the current flowing through the circuit. 

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  1. $12$ mA

  2. $24$ mA

  3. $48$ mA

  4. $32$ mA

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Correct Option: A
Explanation:

Given: resistors A and B connected in series to a $6V$ battery of $3\Omega$ internal resistance. The sequence of the colour bands on resistor. A is yellow, violet and brown while that on resistor B is red, violet and black respectively. 

To find the current flowing through the circuit.
Solution:
According to the question the A and B are 3-band resistors, so the thirdcode will be a multiplier.
And using the standard resistor color code table the value of:
yellow is 4, violet is 7, brown is $\times 10^1$, red is 2, black is $\times 10^0$.
The corresponding value of A and B are:
A= $47\times 10^1=470\Omega$, B= $27\times 10^0=27\Omega$
Now A and B are in series are also the internal resistance will also be in series. So the effective resistance of the circuit will be $R _{eq}=470+27+3=500\Omega$.
We know according to Ohm's Law,
$V=IR\implies I=\dfrac VR\\implies I=\dfrac 6{500}=0.012A$
Or the current flowing through the circuit = $12mA$

One unit of electric energy is equal to :

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  1. One kilowatt hour

  2. one watt hour

  3. 100 Joules

  4. none of these

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Correct Option: A
Explanation:

1 unit of electric energy is equal to


$1unit=1kWh$

$1kWh=1kW\times 1h$

$=1000W\times 3600s$

$\implies 1kWh=3.6\times 10^6J$

Answer-(A)

The wire having a red plastic covering is a

physics electrical circuits domestic electric circuits and safety precautions domestic and commercial circuit electric power

  1. live wire.

  2. neutral wire.

  3. earth wire.

  4. none of these.

Show answer
Correct Option: A
Explanation:

The wire has a red plastic covering is a live wire. Red wire signifies the phase in an electric circuit. It is the live wire that cannot be connected to another red or bulb wire Red is used in some types of the switch leg.

The electricity tariff in a town is $Rs.3.00$ per unit. Calculate the cost of running an $80 W$ fan for $10 h$ a day for the month of June.

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  1. $Rs. 12.00.$

  2. $Rs. 52.00.$

  3. $Rs. 72.00.$

  4. $Rs. 92.00.$

Show answer
Correct Option: C
Explanation:

Total time for which the fan is used $= (10 hours) \times (30 days) = 300$ hours.
Energy consumed $= (80 W) \times (300$ hours$)$
$= 24,000 Wh = 24 kWh.$
But $1 kWh =1$ unit.
Therefore, $24 kWh = 24$ units
The cost of this power $= 24$ units $\times Rs. 3= Rs. 72.00.$

The wire having a black plastic covering is a

physics electrical circuits domestic electric circuits and safety precautions domestic and commercial circuit electric power

  1. live wire.

  2. neutral wire.

  3. earth wire.

  4. none of these.

Show answer
Correct Option: B
Explanation:

Black, Red and Blue are used for hot wires and White is used as the neutral wire in a 120/208 V circuit. Brown, Orange and Yellow are used as hot wires and gray is used as the neutral wire in a 277/480 V. For grounding, regardless of the voltage, Green is used.The wire having a black plastic covering is a neutral wire.

The colour code of a resistor is brown, black and brown. Then the value of resistance is _____

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  1. $10$ $\Omega$

  2. $100\ m \Omega$

  3. $0.1\ k$$\Omega$

  4. $1000$ $\Omega$

Show answer
Correct Option: C
Explanation:

First two colours says about significant digits of resistance and the third colour gives multiplier.


Brown colour gives digit $1$, brown colour gives digit $0$ and for multiplier brown colour gives $10^1$.

Hence, resistance  $=R=10\times 10^1=100\Omega$

$\implies R=0.1k\Omega$

Answer-(C)

An electric oven of 2 kW power rating is operated in a domestic electric circuit that has a current rating of 5A. If the supply voltage is 220V, what result do you expect?

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  1. circuit will be breaked

  2. fuse will blow

  3. both

  4. none

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Correct Option: C
Explanation:

Given,
Power of the oven $(P) = 2kw = 2 \times 103 W$
Voltage supplied $(V) = 220V$
Current $(I) = ? A$
Power $= V \times I$
$\Rightarrow I = P/V = 2 \times 103/220 = 9.09 A$
Since the domestic electric circuit has a current rating of 5A, the flow of 9.09 A by the Oven exceeds the safe limit. Here, fuse will blow and break the circuit.