Tag: electric power

Questions Related to electric power

Units of power and voltage are represented by

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  1. W, V

  2. A, V

  3. W, $\Omega$

  4. V, $\Omega$

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Correct Option: A
Explanation:

In our every day lives we use electrical energy to do work for us. This electrical energy is provided either by our power company through the outlets in our wall, or we can buy stored electrical energy in the form of batteries. Different electrical appliances use electrical energy at different rates. Power is the rate at which work is done. The power rating of an appliance tells us the rate at which it uses electrical energy. For example, a 1500 watt hair dryer uses 1500 joules of electrical energy per second.
In any electrical appliance their is a simple relationship between the power output of a component, the potential difference across that component, and the current running through it. This is because charge times potential difference is energy, so (charge per unit time) times (potential difference) gives (energy per unit time).
Thus, we have $P=V I.$
Substituting $V=I R$ from ohm's law, the power relationship can be rewritten as $P={ I }^{ 2 }R$. From this formula, the resistance can be calculated.

hence correct option is A

Fill the blank.
Voltmeter is always connected in . . . .with the device.

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  1. series

  2. parallel

  3. series and parallel both.

  4. the cell.

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Correct Option: B
Explanation:

A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. A voltmeter is connected in parallel with a device to measure its voltage.
Hence, the statement is false as the voltmeter is connected in parallel to the device in a circuit.

The melting point of filament of a bulb is 

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  1. very low.

  2. very high.

  3. normal.

  4. none of these.

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Correct Option: B
Explanation:

The electric bulb is the glass enclosure around the filament that often contains a vacuum or is filled with a low-pressure noble gas to prevent the filament from burning out due to evaporation at high temperature. The coiled filaments of incandescent lamps are made up of tungsten, a high resistance material that is drawn into a wire .It has both a high melting point (3382 degrees Celsius) and a low pressure which keep it from melting or evaporating too quickly. Supplying electricity through this coiled tungsten wire generates the light. Due to the resistance, it is heated until the wire becomes white-hot. The emission of light by heating the filament wire to white-hot level is known as incandescence. Here, the electrical energy is converted to heat energy by the resistance of the wire.A fused electric bulb has its filament cut, because at higher temperature for a longer period of time, the tungsten filament melts and the flow of electric current stops and the bulb loses its original utility. 
Hence, the statement is true.

The wire having a red 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: A
Explanation:

According to old convention, live wire was coded by red colour. Hence live wires were covered with red plastic.

A live wire is ______ in colour.

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  1. red

  2. blue

  3. green

  4. yellow

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Correct Option: A
Explanation:

A live wire is red on color .The wire had a red plastic covering.

The heater element in an electric iron is made of

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  1. Copper

  2. Tungsten

  3. Nichrome

  4. Iron

  5. Platinum

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Correct Option: C

Which of the following is not a type of wire in power supply.

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  1. Phase Wire

  2. Neutral wire

  3. Zero wire

  4. Earth wire

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Correct Option: C
Explanation:

In power supply system, there are three wires - Phase(live) wire, neutral wire and earth wire. hence there is no any wire named Zero wire . so correct option is option C.

The wire having a black 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: B
Explanation:

The wire having a black plastic covering is a neutral wire whereas that having a red covering is a live wire.

A certain network consists of two ideal and indentical voltage sources in series and a large number of ideal resistor. The power consumed in one of the resistor is $4W$ when either of the two sources is active and other is replaced by a short circuit. The power consumed by same resistor when both sources are simultaneously active would be: 

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  1. $0$ or $16W$

  2. $4W$ or $8W$

  3. $0$ or $8W$

  4. $8W$ or $16W$

Show answer
Correct Option: A
Explanation:

$ P=VI$

$ I= \cfrac{nE}{R+r}$ (r=0)
In this case 
$ I= \cfrac{nE}{NR} ; E _1 = E _2$
$ P=VI$
In first case 
$ 4 =E \times \cfrac{E}{NR} = \cfrac{E^2}{NR}$
In second case 
$ x =2E \times \cfrac{2E}{NR} = \cfrac{4E^2}{NR}$
$ \cfrac{x}{4} = \cfrac{4E^2}{NR} \times \cfrac{NR}{E^2}$
$ x= 0 \quad or \quad 16W$

State whether true or false.
The earth wire is generally black in colour.

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  1. True

  2. False

Show answer
Correct Option: B
Explanation:

The earth wire is generally black in colour. This statement is false because the earth wire is green in colour.