Tag: domestic electric circuits and safety precautions

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

What is the function of distribution box in domestic electrical circuit

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

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

Mark the incorrect statement.

  1. The metallic body is connected to the earth wire

  2. The earth wire, has insulation of red colour

  3. Both

  4. None
Reveal answer Fill a bubble to check yourself
B Correct answer
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. 

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

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

  1. $90$%
  2. $95$%
  3. $96$%
  4. $99$%
Reveal answer Fill a bubble to check yourself
A Correct answer
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   $        $\eta = \dfrac{100}{(220)(0.5)}\times 100 = 90$ %.

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

Why is parallel arrangement used in domestic wiring?

  1. For getting same potential difference across each electrical appliance.

  2. For getting different potential difference across each electrical appliance.

  3. For getting high potential difference across each electrical appliance.

  4. For getting low potential difference across each electrical appliance.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The usage of parallel arrangement in domestic circuits has many advantages.

  • Each circuit will have the same potential difference which is equal to  the potential difference of the supply line. As a result, each electrical appliance will work under constant voltage.
  • When two or more appliances are used at the same time, each appliance will be able to draw the current as needed. The appliances having low resistance will draw higher current and vice versa.
  • When distribution circuits are in parallel, then each circuit operates separately. So, if one of the distribution circuits gets overloaded, only the fuse in that circuit will be blown off. The other distribution circuits will remain unaffected.

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

The most important safety method used for protecting home appliances from short circuiting or overloading is

  1. earthing.

  2. use of fuse.

  3. use of stabilizers.

  4. use of electric meter.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Answer is B

Fuses are safety devices that are to be built into our electrical system. If there were no fuses and we operated too many appliances on a single circuit, the cable carrying the power for that circuit would get extremely hot, short circuit, and possibly start a fire. To prevent electrical overloads, fuses are designed to trip or blow, stopping the flow of current to the overloaded cable.

Fuse is a piece of wire of a material with a very low melting point. When a high current flow through the circuit due to overloading or short circuit, the wires gets heated and melts. As a result, the circuit is broken and current stops flowing.

The fuse must always be connected to the mains and it must be of correct value. For example, a 15-ampere fuse should trip when the current through it exceeds 15 amperes. A 20-ampere fuse should blow when the current through it exceeds 20 amps.

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

The resistance of hot turngsten filament is about $10$ times the cold resistance. What will  be the resistance of $100W$ and $200V$ lamp when not in use

  1. $40\Omega $
  2. $20\Omega $
  3. $400\Omega $
  4. $200\Omega $
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Given,

$P= 100W$
$V= 200V$
${ R } _{hot}= 10 \cdot { R } _{ cold }$
We know,
$ P= I \cdot V$
$ \rightarrow P= \cfrac{ V }{ R } \cdot { V }= { V }^{ 2 } \cdot R$
$ \therefore R= \cfrac{{ V }^{ 2 }}{P}$
where, 
$ P= $ Power
$ I = $ Current
$ R = $ Resistance
$ V =$ Voltage
$ { R } _{ hot }= $ Hot Resistance
${ R } _{ cold }=$ Cold Resistance
So, $ { R } _{ hot}= \cfrac {200 \cdot 200}{ 100}= 400 \Omega$
${ R } _{ cold }= \cfrac{{ R } _{ hot }}{ 10 }= \cfrac{ 400 }{ 10 }= 40 \Omega $
$\Rightarrow { R } _{ cold }= 40 \Omega$
$ 40 \Omega $ will be the resistance of $100W$ and $200V$ lamp when not in use.

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

The armature current in D.C motor is maximum when the motor has -


  1. picked up maximum speed

  2. just started

  3. intermediate speed

  4. just been switched off
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

In a DC motor, the back emf is proportional to the speed of the motor (E_b = k * omega). The armature current is given by I_a = (V - E_b) / R_a. When the motor has just started, its speed is zero, so the back emf is zero, resulting in the maximum possible armature current.

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

The line that draws power supply to your house from Street has

  1. $220\sqrt{2}$ average voltage.

  2. 220 V average voltage
  3. voltage and current Out of phase by $\pi/2$
  4. voltage and current possibly differing in phase $\phi$ such that $| \phi| < \dfrac{\pi}{2}.$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

As the line has some resistance $(R \, \neq \, 0)$. Also, line that draws power supply has some inductance and capacitance, hence voltage and current differ in phase $\phi$ such that $|\phi| \, < \, \dfrac{\pi}{2}$.

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

Which arrangement is used for domestic circuits ?

  1. Series

  2. Parallel

  3. Series and parallel both

  4. None
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

For Domestic circuits parallel arrangement is always used because:

  1. In parallel circuits, if one electrical appliance stop working due to some effect then all other appliances keep working normally.
  2. In parallel circuit, each electrical appliance has own switch due to which it can be turn off or on independently, without effecting  other appliances.
  3. In parallel circuits, each electrical appliance gets same voltage as that of the power supply line.
  4. In the parallel connection of electrical appliances, the overall resistance of the house hold circuit is reduced due to which the current from the power supply is high.