Q42 / 57 · KVPY 2019 (SA) · Physics

A student in a town in India, where the price per unit (1 unit =1kWh ) of electricity is ₹ 5.00, purchases a 1kVA UPS (uninterrupted power supply) battery. A day before the exam, 10 friends arrive to the student's home with their laptops and all connect their laptops to the UPS. Assume that each laptop has a constant power requirement of 90 W. Consider the following statements.
I. All the 10 laptops can be powered by the UPS, if connected directly.
II. All the 10 laptops can be powered, if connected using an extension box with a 3 A fuse.
III. If all the 10 friends use the laptop for 5 h, then the cost of the consumed electricity is about ₹ 22.50 .
Select the correct option with the true statements.

  • A
      I only
     
  • B
    I and II only
     
  • Correct answer
    C
    I and III only
     
  • D
    II and III only .

Solution

Power requirement for 1 laptop, P1=90 W
So, power r quirement for 10 laptops,
P=10×P1=10×90-900 W=0.9 kW
In 5 h, electrical energy used by all laptops,

E=P×t=0.9×5

=4.5kWh

cost of electrical energy used is
cost =E× Unit cost

=4.5×5
=₹ 22.50

So, statement III is correct.
For laptop charger, input voltage is 220 V
So, current when all 10 laptops are connected through an extension,

I=PV=900220≈4.1 A

As, line current exceeds current rating of fuse, therefore 3 A fuse cannot be used. So, statement II is incorrect.

Hence, the correct answer is option: 'C'.

More Current Electricity Questions