Tag: domestic electric circuits and safety precautions

Questions Related to domestic electric circuits and safety precautions

An electric bulb is rated 250 W, 230 V. The energy consumed in one hour is :

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

  1. $9 \times 10^5$ J

  2. $15 \times 10^5$ J

  3. $25 \times 10^5$ J

  4. $23 \times 10^5$ J

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Correct Option: A
Explanation:

Energy ($power\times time$) is measured in Joules and by including time (t) in the power formulae, the energy dissipated by a component or circuit can be calculated.


Energy dissipated Q= Pt.

In the question, the time taken is given as 1 hour = 60 minutes, that is $60\times 60=3600\quad seconds$ and the power is 250 W.

Hence, the energy consumed by the bulb is $250\quad W\times 3600\quad s=9.00,000\quad J.\quad That\quad is,\quad 9\times { 10 }^{ 5 }\quad Joules$. 

A geyser is rated 1500 W, 250 V. This geyser is connected to 250 V mains. The current drawn will be:

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

  1. $6 A$

  2. $5 A$

  3. $40 A$

  4. $10 A$

Show answer
Correct Option: A
Explanation:

Power, $P=VI$
For a $1500\ W$ geyser rated for $250\ V$, the current through is given as $I=\dfrac { P }{ V } =\dfrac { 1500 }{ 250 } =6A$
Hence, the current flowing through the heater is $6\ A$.

Which is or are the correct for filament of bulb ?

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

  1. Low melting point

  2. High melting point

  3. Made of tungsten

  4. Made of nichrome

Show answer
Correct Option: B,C
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 false.

Three heaters each rated 250 W, 100 V are connected in parallel to a 100 V supply. The total current taken from the supply is :

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

  1. $2.5 A$

  2. $5 A$

  3. $7.5 A$

  4. $25 A$

Show answer
Correct Option: C
Explanation:

Power is given as: $P=VI$


For a $60\ W$ lamp rated for $ 250\ V$, the current through is given as: $I=\dfrac { P }{ V } =\dfrac { 250 }{ 100 } =2.5\ A$

Hence, the current flowing through the heater is $2.5\ A$.

In a parallel circuit, the voltage across each of the components is the same, and the total current is the sum of the currents through each component.
Therefore, when three such heaters are connected in parallel, then the current obtained from the supply is given as $2.5+2.5+2.5 = 7.5\ A$

Household electrical appliances are joined using .......... combination of resistors.

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

  1. Parallel

  2. Alternating

  3. Continuous

  4. Series

Show answer
Correct Option: A
Explanation:

Household electrical appliances are always joined in parallel.

The current through a 60 W lamp rated for 250 V is 

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

  1. 0.24 A

  2. 4.2 A

  3. 0.5 A

  4. 6 A

Show answer
Correct Option: A
Explanation:
The electric power in watts associated with a complete electric circuit or a circuit component represents the rate at which energy is converted from the electrical energy of the moving charges to some other form, e.g., heat, mechanical energy, or energy stored in electric fields or magnetic fields. The power is given by the product of applied voltage and the electric current. That is, $P=VI$. 

For a 60 W lamp rated for 250 V, the current through is given as $I=\dfrac { P }{ V } =\dfrac { 60 }{ 250 } =0.24A$.

Hence, the current flowing through the lamp is 0.24 A.

Substituting I=V/R in the above formula, we get, $P=\dfrac { { V }^{ 2 } }{ R } $.

Given that the voltage is 250 V and the power is 60 W, the resistance of the 

bulb is calculated as follows.

$R=\dfrac { { V }^{ 2 } }{ P } =\dfrac { { 250 }^{ 2 } }{ 60 } =1041.67\Omega$.

When the voltage drops to 200 V, the power is calculated as follows.

$P=\dfrac { { V }^{ 2 } }{ R } =\dfrac { { 200 }^{ 2 } }{ 1041.67 } =\quad 38.40W$

Hence, the power of the bulb is reduced to 38.40 W.

We should not use many electrical appliances simultaneously. True/False?

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

  1. True

  2. False

Show answer
Correct Option: A
Explanation:

When many appliances are switched on simultaneously, a lot of current flows through the main circuit and the current may exceed the permissible amount. This causes overloading and may cause a fire. Thus many electrical appliances should not be used simultaneously.

An electric heater consists of a nichrome coil and runs under $220 V$, consuming $1 kW$ power. Part of its coil burned out and it was reconnected after cutting off the burnt portion, The power it will consume now is:

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

  1. More than $1 kW$

  2. Less that $1 kW$, but not zero

  3. $1 kW$

  4. $0kW$

Show answer
Correct Option: A
Explanation:

Answer is A.

The electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made.The resistance of a wire can be expressed as $R=\rho \frac { L }{ A } $, 
where,
$\rho $ - Resistivity  - the factor in the resistance which takes into account the nature of the material is the resistivity
L - Length of the conductor
A - Area of cross section of the conductor.
From this relation, we observe that the length is directly proportional to the resistance and the area of cross section is inversely proportional to the resistance.
In this case, the length of the nichrome coil is reduced due to the burn and so the resistance will also be reduced proportionally. 
When the resistance is decreased, more current flows through the coil and apparently, more power is consumed by the heater as power consumed P = VI.
Hence, the power it will consume now is more than 1 kW.

An air conditioner is rated $240\;V,\;1.5\;kW$. The air conditioner is switched on for $8$ hours each day. How much electrical energy is consumed in $30$ days ?

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

  1. $360\;kW\;h$

  2. $8.64\;kW\;h$

  3. $120\;kW\,h$

  4. $240\;kW\;h$

Show answer
Correct Option: A
Explanation:

Answer is A.

If a certain amount of power is dissipated for a given time, then energy is dissipated. Energy (powertime) is measured in Joules and by including time (t) in the power formulae, the energy dissipated by a component or circuit can be calculated.
Energy dissipated = Pt .
In this case, the power dissipated is 1.5 kW and it runs for 8 hours a day.
So, in a day, the energy dissipated is 1.5 * 8 = 12 Kwh.
For 30 days, the power dissipated is 12 * 30 = 360 Kwh.
Hence, the electrical energy is consumed by the air conditioner in 30 days is 360 Kwh.

A washing machine rated $300W$ is operated for one hour/day. If the cost of a unit is Rs $3.00$ then the cost of the energy to operate a washing machine for the month of March is

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

  1. $Rs 25.60$

  2. $Rs 27.50$

  3. $Rs 27.90$

  4. $Rs 26.90$

Show answer
Correct Option: C
Explanation:

In this case, the washing machine that is rated $ 300\ W$ is operated for $1$ hr/day. The electric bill has to be calculated for the month of March. That is, the total number of hours the washing machine is operated in this month is $31$ hours as this month has $31$ days.
The total energy that is used by the washing machine in a day is given by the formula $Q=Pt$.
Here, the power is $300\ W$ and the time used is $1\ hour$.
So, $Q=300W\times 1h=300Wh= 0.3\ kWh$
Therefore, the total number of $kWh$ for a month is $0.3kWh\times 31=9.3\ kWh $
The cost of $1\ kWh$ is given as $Rs. 3$.

Hence, the total cost for a month is $=9.3kWh\times 3=Rs. 27.90$