Power

Q.

State and define the S.I. unit of power.

Q.

Three bulbs 40W, 60W and 100W are connected to 220V mains. Which bulb will glow brightly if they are connected in series?

Q.

A boy takes 3 minutes to lift a 20-liter water bucket from a 20 m deep well, while his father does it in 2 minutes. Compare

(i) the work and

(ii) power developed by them.

Take density of water=${10}^{3}kg{m}^{-3}andg=9.8Nk{g}^{-1}$.

Q.

A boy weighing 350 N climbs up the 30 steps, each 20 cm high in 1 minute. Calculate:(i) the work is done, and (ii) the power spent.

Q.

A pump is used to lift 500 kg of water from a depth of 80 m in 10 s.Calculate:

(a) the work done by the pump,

(b) the power at which the pump works, and

(c) the power rating of the pump if its efficiency is 40%.

Q.

A man raises a box of mass 50 kg to a height of 2 m in 20 s, while another man raises the same box to the same height in 50 s.

(a) Compare:(i) the work done, and (ii) the power developed by them.

(b) Calculate:(i) the work done, and (ii) the power developed by each man.Take g=10 N $k{g}^{-1}$.

Q.

One megawatt is equal to how many units of electricity?

Q. Out of two bulbs 50W-220V and 100W-220V, which one will glow brighter:- a) when connected in series b) when connected in parallel .

Q.

An ox can apply a maximum force of 1000 N.It is taking part in a cart race and is able to pull the cart at a constant speed of 30 m $s^{-1}$ while making its best effort.Calculate the power developed by the ox.

Q.

Rajan exerts a force of 150 N in pulling a cart at a constant speed of 10 m $s^{-1}$.Calculate the power exerted.

Q.

Differentiate between watt and watt hour.

Q.

An electrical appliance of 500 W is used for 5 hours per day. Energy consumed in a day will be:

1. 25 k W h

2. 75 k W h

3. 2.5 k W h

4. 150 k W h

Q.

State two factors on which power spent by a source depends.Explain your answer with examples.

Q.

Three equal resistors connected in series across a source of emf together dissipate 5watt of power. If the same resistors are connected in parallel across the same source of emf, the power dissipated will be

Q.

A heater coil is rated 100w, 200v. It is cut into two identical parts. Both parts are connected together in parallel to the same source of 200v. Calulate the energy liberated per second in the new combination.

Q.

What are the limitations of the energy that can be obtained from the oceans?

Q.

A water pump raises 50 litre of water through a height of 25 m in 5 s. Calculate the power of the pump required.(Take g=10 N $k{g}^{-1}$ and density of water=1000 kg $m^{-3}$.

Q.

Prove that $1HP=746W$

Q.

A machine raises a load of 750 N through a height of 16 m in 5 s.Calculate:

(i) the energy spent by the machine,

(ii) the power of the machine if it is 100% efficient.

Q.

From the state of rest, a car accelerates while the engine supplied constant power p. The velocity of the car varies with time as

1. $v=\frac{pt}{m}$

2. $v=\frac{2pt}{m}$

3. $v=\sqrt{\frac{2pt}{m}}$

4. $v=\sqrt{\frac{pt}{m}}$

Q.

Define the term power. State its S.I. unit.

Q. A boy of mass 40 kg climbs up a flight of 30 steps each of $20cm$ high in 2 minutes and a girl of mass 30 kg does the same in 1.5 minutes. Compare the power developed by them.(Take $g=10m{s}^{-2}$)

1. 1:1
2. 4:3
3. 2:3
4. 1:4

Q.

What are some examples of work?

Q.

A tube-well pumps out 2400 kg of water per minute. If water is coming out with a velocity of 3 ms^–1, what is power of the pump? How much work is done, if the pump runs for 10 hours?

Q.

Name the physical quantity which is measured in
(i) kW, (ii) kWh and (iii) Wh.

Q. Which will glow brighter, 100w bulb or 200 w bulb if they are joined in series and the supplied voltage is 5 v?

Q. A thin lens has a focal length $-25cm$. Calculate the power of lens in dioptre.

1. -4

Q.

(a) Name the physical quantity measured in terms of horse power.

(b) How is horse power related to the S.I.unit of power?

Q.

The electric meter in a house records the consumption of :

(a) charge

(b) current

(c) energy

(d) power.

Q. A convex lens of focal length $30cm$ is placed in contact with a concave lens of focal length $20cm$. What is the power of the combination of lens?

1. $-0.0167D$
2. $-10D$
3. $-1.67D$
4. $+1.67D$