Tag: physics

Questions Related to physics

A light bulb has the resistance of tungsten resistance which convert about 10% electrical energy into visible light.  If energy other than visible is waste energy. How many kilo-joules does a light bulb wasted in one hour?

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. 36 kJ

  2. 90 kJ

  3. 3240 kJ

  4. 360 kJ

  5. 32,400 kJ

Show answer
Correct Option: C
Explanation:
$90$ % of electrical energy is wasted in one hour.
Using        $1kWh  = 3600$ $kJ$
$\therefore$ Energy wasted       $E _{waste} = 0.9 \times 3600  =3240$  $kJ$

An electric kettle is rated as 2.5 kW, 250 V, Find the cost of running the kettle for two hours at 60 paisa per unit.

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. Rs. 6

  2. Rs. 3

  3. Rs. 9

  4. Rs. 1

Show answer
Correct Option: B
Explanation:

Total unit consumed in two hours $E=2.5\times 2=5\,unit$

Total cost is, $Cost=5\times 0.6=3 $

A table lamp of power 60 W consumed 9 (commerical) units of electricity in the month of April. For how many hours per day, on an average, was the lamp in use?

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. 1 h

  2. 3 h

  3. 5 h

  4. 6 h

Show answer
Correct Option: C
Explanation:

1 unit is $3.6\times 10^6J$

let us suppose it has been used for y hours a day
energy consumed in y hours =$60\times y\times 60\times 60$ , y in hours, so converting it into seconds

now it has been used for whole month so,
$60\times y\times 60\times 60\times  30=9\times 3.6\times 10^6$

$y=5 hours$

A $100W, 200V$ bulb is connected to a $160V$ supply. The actual power consumption would be

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. $185W$

  2. $100W$

  3. $54W$

  4. $64W$

Show answer
Correct Option: D
Explanation:

Resistance of the bulb $R = \dfrac{V^2}{P}$

where $P = 100 W$ and $V = 200$ V.
$\therefore$ $R = \dfrac{(200)^2}{100} = 400\Omega$
Actual power consumption $P' = \dfrac{V _1^2}{R}$
where $V _1 = 160$ V
$\therefore$ $P' = \dfrac{(160)^2}{400} = 64 W$

$4$ bulbs rated $100$W each, operate for $6$ hours per day. What is the cost of the energy consumed in $30$ days at the rate of Rs. $5$/kWh?

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. Rs. $360$

  2. Rs. $90$

  3. Rs. $120$

  4. Rs. $400$

Show answer
Correct Option: A
Explanation:

$\displaystyle E= Power(in kW)\times time=\frac{4\times 100\times 6}{1000}$kWh$=2.4$kWh
Consumed in $30$ days$=30\times 2.4=72$
Total cost $=72\times 5=360$Rs.

Why is the switch for any electrical appliance always fitted on to the live wire ? 

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. No current flows in the neutral wire.

  2. There will be a short circuit if the switch is in the earth lead.

  3. The device can never be switched off if the switch is in the neutral lead.

  4. The device can only be isolated if the switch is in the live lead

Show answer
Correct Option: C
Explanation:

Why is the switch for any electrical appliance always fitted on the live wire.

The answer will be$\rightarrow$ The device can never be switched off if switch is in the neutral load.
Solution:- The live wire is always at higher potential where as neutral wire is always at zero potential.
The switch must be placed in live wire to maintain the body of appliance at zero potential in the switch off position and if the switch is kept in neutral wire, then the body of the appliance to keep the body at  infinite potential in the switch on position.

In an electrical circuit three incandescent bulbs A, B and C of rating 40 W, 60 W and 100 W respectively are connected in parallel to an electric source. Which of the following is likely to happen regarding their brightness?

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. Brightness of all the bulbs will be the same

  2. Brightness of bulb A will be the maximum

  3. Brightness of bulb B will be more than that of A

  4. Brightness of bulb C will be less than that of B

Show answer
Correct Option: C
Explanation:

Answer is C.

As the three bulbs are connected in parallel, the voltage applied across them would be the same, lets say its V.
Now, due to the fact that the power dissipated by the three bulbs is different the current flowing through them would be different and thus they will have different brightness.
Now, the brightness of a bulb is directly proportional to the power it uses or the amount of current it draws. 
Thus, in this case the brightness of bulb B (60 W) will be more than that of the bulb A (40 W) but less than that of the bulb C (100 W). That is, C has the maximum brightness and the bulb has has the least brightness. Bulb B in between.
Hence, the correct statement is option C.

A table lamp of power 60 W consumed 9 (commercial) units of electricity in the month of April. For how many hours per day, on an average, was the lamp in use?

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. $1\ h$

  2. $3\ h$

  3. $5\ h$

  4. $6\ h$

Show answer
Correct Option: C
Explanation:

$9\ units = 9kWh$
For the month of April (30 days), we get an average of $ \cfrac{9000}{30} =300 Wh$ energy per day.
For an appliance of $60\ W$, this clearly corresponds to $ t = \cfrac{E}{P} = \cfrac{300}{60} = 5\ h $

An air conditioner is rated $240\ V, 1.5\ kW$. The air conditioner is switched on $8\ hrs$ each day. What is electrical energy consumed in $30\ days$?

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. $2.88\ kWh$

  2. $360\ kWh$

  3. $120\ kWh$

  4. $240\ kWh$

Show answer
Correct Option: B
Explanation:

Energy consumed is given by:

$E= Pt$
    $= 1.5 KW \times  30 \times 8 hrs= 360 KWh $

Two electric bulbs rated ${P} _{1}$ and  watt at $V$ volt are connected in series across $V$ volt mains, then their total power consumption $P$ is

physics effects of electric current thermal effect of electric current heating effect of electric current electric current and its effects

  1. $\left( { P } _{ 1 }+{ P } _{ 2 } \right) $

  2. $\sqrt { { P } _{ 1 }{ P } _{ 2 } } $

  3. $\dfrac {P _{1}+P _{2}}{2} $

  4. $\dfrac {P _{1}P _{2}}{P _{1}+P _{2}} $

Show answer
Correct Option: C
Explanation:

Resistance of Ist bulb $={ R } _{ 1 }=\cfrac { { V }^{ 2 } }{ { P } _{ 1 } } $
Resistance of IInd bulb $={ R } _{ 2 }=\cfrac { { V }^{ 2 } }{ { P } _{ 2 } } $
when both bulbs are connected in series
$\quad { R } _{ eq }={ V }^{ 2 }\left[ \cfrac { 1 }{ { P } _{ 1 } } +\cfrac { 1 }{ { P } _{ 2 } }  \right] =\cfrac { { V }^{ 2 }\left( { P } _{ 1 }+{ P } _{ 2 } \right)  }{ { P } _{ 1 }{ P } _{ 2 } } $
Hence, power consumed $P=\cfrac { { V }^{ 2 } }{ R } =\cfrac { { V }^{ 2 } }{ { V }^{ 2 }\left( \cfrac { { P } _{ 1 }+{ P } _{ 2 } }{ { P } _{ 1 }{ P } _{ 2 } }  \right)  } =\cfrac { { P } _{ 1 }{ P } _{ 2 } }{ { P } _{ 1 }+{ P } _{ 2 } } \quad $