Physics · Work, Energy And Power · NEET
kWh is a unit of ENERGY, not power. Power (kilowatt) multiplied by time (hour) gives energy. So kWh = power x time = energy. NCERT states this clearly: 'kWh is a unit of energy and not of power.' 1 kWh = 10^3 W x 3600 s = 3.6 x 10^6 J. Your electricity bill charges you for energy used (kWh).
1 hp = 746 W. This is the value NCERT uses (some books round it to 745.7 W or ~0.75 kW). So a 100 hp car engine has an output power of about 100 x 746 = 74,600 W = 74.6 kW. Horsepower is still used for cars and motorbikes; the SI unit is the watt.
Power is the rate of doing work: P = Work / time. Work is measured in joules and time in seconds, so power = joule/second = watt. That is why 1 W = 1 J/s. A machine doing 1 J of work every second delivers 1 watt of power.
A 100 W bulb uses 100 joules of electrical energy every second. If it stays on for 10 hours, the energy used = 100 W x 10 h = 1000 Wh = 1 kWh = 3.6 x 10^6 J. The 100 W is the power rating; the 1 kWh is the energy consumed, which is what you pay for.
Multiply kilowatts by 1000 (to get watts) and hours by 3600 (to get seconds), then multiply: 1 kWh = 1000 W x 3600 s = 3.6 x 10^6 J = 3.6 MJ. This exact conversion (1 kWh = 3.6 x 10^6 J) is a common one-mark NEET fact.
The power of a crane, which lifts a mass of 1000 kg to a height of 20 m in 10 s, is (g = 9.8 m/s^2)
An electric lift with a maximum load of 2000 kg (lift + passengers) is moving up with a constant speed of 1.5 m/s. The frictional force opposing the motion is 3000 N. The minimum power delivered by the motor to the lift (in watts) is (g = 10 m/s^2)
Water falls from a height of 60 m at the rate of 15 kg/s to operate a turbine. The losses due to frictional force are 10% of the input energy. How much power is generated by the turbine? (g = 10 m/s^2)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The watt (W), named after James Watt. 1 watt = 1 joule per second (1 W = 1 J/s). Its dimensional formula is [M L^2 T^-3].
1 hp = 746 W (as given in NCERT). Horsepower is still used to rate car and motorbike engines, but the SI unit is the watt.
Energy. 1 kWh = 3.6 x 10^6 J. Power (kW) times time (h) equals energy (kWh). Electricity bills charge energy in kWh.
1 kWh = 1000 W x 3600 s = 3.6 x 10^6 J = 3.6 MJ.
Because power is the rate of doing work: P = work/time = joules/seconds. So the unit is J/s, which is defined as the watt.
1 hp = 746 W = 0.746 kW, so roughly 1 hp is about three-quarters of a kilowatt. Conversely, 1 kW is about 1.34 hp.
