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

Which of the following is true for electrical energy?

  1. $E = P \times t$

  2. $E = \dfrac{P}{t}$

  3. $E = \dfrac{{V}^{2}}{Rt}$

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Electrical energy consumed by an electric appliance is equal to product of its power rating and time for which it is used $E = P \times t$.

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

When we pay for our electricity bill, we are paying for the:

  1. charge used.

  2. current used.

  3. power used.

  4. energy used.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Electricity bill is measured in units where 1 unit is equal to kWh which is the unit of energy. Hence, when we pay for our electricity bill, we pay for the amount of electrical energy consumed by us.

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

Four bulbs, each of rating (100 W, 220 V) and connected in parallel across a voltage supply of 220 V, are operated for five hours daily. If all the bulbs are replaced by LEDs of rating (8 W, 220 V), how many units of electrical energy will be saved every month (30 days)? 

  1. 55.2 units

  2. 60 units

  3. 4.8 units

  4. 32 units
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For bulbs :

$Energy\,\,=\,power\times time$

Total hours in 30 days $5\times 30=150hours$

So, $ energy=0.1\times 150=15J $

 $  $Power of 4 bulbs

 $ =4\times 15=60kW $

 $  $For LED:

Total hours in 30 days $=150hours$

$E=0.008\times 150=1.2J$

So, power of 4 LEDs $=1.2\times 4=4.8hours$

Saved energy 

  $ =\,\,60-4.8 $

 $ =55.2units $


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

How much energy in kilowatt hour is consumed in operating ten 50 watt bulbs for 10 hours per day in a month (30 days)

  1. 1500

  2. 5000

  3. 15

  4. 150
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Energy consumed $=10\times 50\times 10\times 30\times 3600 J$
$[1 Kwh=3600\times 1000J]$
$=\frac {10\times 50\times 10\times 30\times 3600}{3600\times 1000}kWh=150$


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

A geyser of $2.5 kw$ is used for $8$ hours daily. Calculate the monthly consumption ( 30 days) of electrical energy units. Also calculate the cost of electricity units consumed in a month if rate per unit is $3.50$

  1. $Rs. 2100.00$

  2. $Rs. 155.00$

  3. $Rs. 150.00$

  4. $Rs. 30.00$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Daily usage of electricity   $E _1 = 2.5 \ kw\times 8 = 20 \ kwh = 20 \ units$

Energy used in one month   $E = 30E _1 = 30\times 20 = 600\ units$
Cost    $ = Rs. 600\times 3.5 = Rs 2100.00$

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

If voltage across a bulb rated $220V-100W$ drops by $2.5$% of its rated value, the percentage of the rated value by which the power would decrease is

  1. $5$%

  2. $10$%

  3. $20$%

  4. $2.5$%
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Power P is proportional to V^2. If V decreases by 2.5%, V_new = 0.975 V_old. P_new is proportional to (0.975)^2 = 0.9506. The decrease is approximately 5%.

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

A padcular ohmmeter uses a battery to provide a potential difference across an unknown resistance  whose value S to be measured. The meter measures the resulting current through this resistor and is calibrated to read out corresponding value of resistance. Suppose that this ohmmeter is used to measure he resistance of a typical incandescent tungsten-filament light bulb. The value of the resistance of the light bulb will be

  1. less then when the bulb will be in use in a 120 volt circuit

  2. more then when the bulb will be in use in a 120 volt circuit

  3. the same as then when the bulb will be in use in a 120 volt circuit

  4. more information when needed to determine whether it's A,B and C
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

An ohmmeter measures resistance at low voltage/current. A tungsten bulb's resistance increases significantly with temperature. When in use, the filament is hot, so its operating resistance is much higher than its cold resistance measured by an ohmmeter.

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

A $500\ W$ heating unit is designed to operate on a $115\ V$ line. If line voltage drops to $110\ V$ line, the percentage drop in heat output will be:

  1. $7.6\ \%$

  2. $8.5\ \%$

  3. $8.1\ \%$

  4. $10.2\ \%$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given:
$H _1 = 500\ W$
$V _1 = 115\ V$
$V _2 = 110\ V$

From Joule's Law of heating,
$H _1 = \cfrac{(V _1)^2}{R}$ and $H _2 = \cfrac{(V _2)^2}{R}$
$\Rightarrow R = \cfrac{(V _1)^2}{H _1} = \cfrac{(V _2)^2}{H _2}$
$\Rightarrow H _2 = \cfrac{V _2^2}{V _1^2} H _1$
$\therefore H _2 = \cfrac{(110)^2}{(115)^2} (500)$
$\therefore H _2 = 457.46 W$

The percentage drop in heat output will be:
$\cfrac{H _2-H _1}{H _1} \times 100 = \cfrac{500 - 457.46}{500} \times 100 = \cfrac{42.54}{500} \times 100 = 8.5\ \%$