Tag: physics

Questions Related to physics

One unit of electric energy is equal to :

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

  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.

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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.

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

  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.

What is the function of distribution box in domestic electrical circuit

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

  1. It distributes voltage across appliances as per need

  2. It provides isolation between set of appliances and improves reliability

  3. It distributes Losses among all parts of circuit equally

  4. It allows user to manually distribute power as per his need

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Correct Option: B
Explanation:

Distribution box in domestic electrical circuit provides isolation between appliances and allow them to work paralelly. It also ensures failure in one part do not affect the functioning of the other.

Mark the incorrect statement.

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  1. The metallic body is connected to the earth wire

  2. The earth wire, has insulation of red colour

  3. Both

  4. None

Show answer
Correct Option: B
Explanation:

One of the wires in this supply, usually with red insulation cover, is called live wire (or positive). Another wire, with black insulation, is called neutral wire (or negative). In our country, the potential difference between the two is $220   V$. At the metre-board in the house, these wires pass into an electricity meter through a main fuse. Through the main switch they are connected to the line wires in the house. These wires supply electricity to separate circuits within the house. Often, two separate circuits are used, one of $15   A$ current rating for appliances with higher power ratings such as geysers, air coolers, etc. The other circuit is of 5 A current rating for bulbs, fans, etc. The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house. This is used as a safety measure, especially for those appliances that have a metallic body, for example, electric press, toaster, table fan, refrigerator, etc. The metallic body is connected to the earth wire, which provides a low-resistance conducting path for the current. 

Determine the power in 2 hours in an electric motor which takes $5  A$ from $220  V$.

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

  1. $2200 W$

  2. $1100 W$

  3. $500 W$

  4. $1000 W$

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Correct Option: B
Explanation:

$P = VI$
  $= 220 \times 5 = 1100  W$.

A transformer is used to light $100\ W - 110\ V$ lamp from $220\ V$ mains. If the main current is $0.5\ A$, the efficiency of the transformer is

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

  1. $90$%

  2. $95$%

  3. $96$%

  4. $99$%

Show answer
Correct Option: A
Explanation:

Voltage in primary coil     $V _p = 220$ V

Current in primary coil    $I _p = 0.5$ A
Thus input power     $P _{in} = V _pI _p$
Output power  (in secondary coil )      $P _{out} = 100$ W
$\therefore$  Efficiency of transformer     $\eta = \dfrac{P _{out}}{P _{in}}\times 100$
$\implies   $        $\eta = \dfrac{100}{(220)(0.5)}\times 100 = 90$ %.