1.Give some examples of positive, negative, and zero work.
2.Write 5 energy transformations you see in your daily life.
3.What are the factors on which kinetic energy depends?
4.Distinguish between work and power.
5.A bulb consumes 3000J of energy when lighted for the 30s. Calculate its power.
6.An electric heater is rated 1500W. How much energy does it use in 10 hrs?
7.A certain household has consumed 250KWh of energy in a month. How much energy is this in joules?
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Answers
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(1)
→ Positive work: when force and displacement are in the same direction, the work performed on an object is said to be positive work.
Example: When a body moves on the horizontal surface, force and displacement act in the forward path. The work is done in this case known as Positive work.
→ Negative work: Negative work is performed if the displacement is opposite to the direction of the Force applied.
Example: Work was done the gravity on a rocket going perpendicular upwards.
→ Zero work: When force and displacement are perpendicular to each other, or when force or displacement is zero.
Example: When we hold an object and walk, the force acts in a downward direction whereas displacement acts in the forward direction.
(2)
→ The Sun transforms nuclear energy into heat and light energy.
→ Our bodies convert chemical energy in our food into mechanical energy for us to move.
→ An electric fan transforms electrical energy into kinetic energy.
→ when electricity moves from a wall plug, through a charger, to a battery.
→ like in a hydroelectric dam that transforms the kinetic energy of water into electrical energy.
(3) The amount of translational kinetic energy (from here on, the phrase kinetic energy will refer to translational kinetic energy) that an object has depends upon two variables: the mass (m) of the object and the speed (v) of the object.
(4) Work is defined as the process of energy transfer to the motion of an object through the application of force. This is usually represented as the product of force and displacement. The SI unit of work is Joule.
Power is defined as the amount of energy transferred in unit time. The SI unit of power is the watt. One watt is equal to one joule per second. Power is a scalar quantity.
(5)
→ Energy consumed by the bulb is 3000 J
→ Time taken = 30s
→ Power of bulb = P = E/t
= 3000J/30s
= 100 W
(6) Energy consumed by an electric heater can be obtained with the help of the expression,
→ P= W/T
Where, Power rating of the heater,
→ P= 1500 W
→ = 1.5 kW
Time for which the heater has operated,
→ T= 10 h
Work done = Energy consumed by the heater
Therefore, energy consumed
= Power - Time = 1.5 — 10
= 15 kWh
Hence, the energy consumed by the heater in 10 h is 15 kWh.
(7) Energy consumed by certain households in a month
→ E = 250 units
→ = 250kWh
∴ E = 250 × 3.6 × 10^6
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