Power dlp

2. 1. What is required to do work ?
2. Have you measured any work done?
3. Have you seen the energy transformation?
4. How to classify the range of electricity?
5. Electricity is a form of ______ ?

3. Definition of power
Power is a measure of how fast or slow a work is done. Power
is defined as the rate of work done or energy delivered.
https://www.youtube.com/watch?v=12h5T1xPmoI

4. Average power
The average power (Pav) is defined as the ratio of the total
work done to the total time taken.

5. Instantaneous power
The instantaneous power (Pinst) is defined as the power delivered
at an instant (as time interval approaches zero)

6. Unit of power
Power is a scalar quantity. Its dimension is [ML2 T–3]. The SI unit
of power is watt (W), named after the inventor of the steam engine
James Watt. One watt is defined as the power when one joule of
work is done in one second, (1 W = 1 J s–1).
The higher units are kilowatt(kW), megawatt(MW), and Gigawatt(GW)

7. For motors, engines and some automobiles an old unit of power
still commercially in use which is called as the horse-power (hp).
We have a conversion for horse-power (hp) into watt (W) which is

8. All electrical goods come with a definite power rating in watt printed on
them. A 100 watt bulb consumes 100 joule of electrical energy in one
second. The energy measured in joule in terms of power in watt and time
in second is written as, 1 J =1 W s.

9. When electrical appliances are put in use for long hours, they consume
a large amount of energy. Measuring the electrical energy in a small
unit watt. second (W s) leads to handling large numerical values.
Hence, electrical energy is measured in the unit called kilowatt hour
(kWh).

10. Relation between power and velocity
The work done by a force F for a displacement dr is
Left hand side of the equation can be written as

11. Since, velocity is v. Right hand side of the equation can
be written as
After substitution we get,

12. This relation is true for any arbitrary value of dt. This
implies that the term within the bracket must be equal to
zero, i.e.,

13. Assignment
A vehicle of mass 1250 kg is driven with an acceleration 0.2
ms-2 along a straight level road against an external resistive
force 500 N. Calculate the power delivered by the vehicle’s
engine if the velocity of the vehicle is 30 ms -1 .