darrynm

When we buy petrol (or gas or electricity) we are buying ENERGY, not POWER.

One litre of petrol contains about 35MJ (Mega or million Joules) of chemical ENERGY.

Used in an engine it delivers about 8.8MJ (25%) of mechanical ENERGY.

The POWER (in horsepower or kilowatts) delivered by an engine depends on how fast (i.e. over how many seconds) the energy is used.

One Joule of ENERGY delivered every second for one hour (60 x 60 is 3,600sec) totals 3,600J of ENERGY

One Joule of ENERGY delivered in one second is defined as POWER of one Watt.

Hence POWER of one Watt running for one hour delivers 3,600J of ENERGY.

POWER of one kiloWatt running for one hour delivers one kiloWatthour of ENERGY which is

1,000 x 3,600J is 3,600,000J is 3.6MJ (Mega Joules) of ENERGY.

.

POWER of one kW (thousand Watts) running for one hour uses ENERGY of one kWh (kilowatthour).

This is different from one KW/hr (kilowatt per hour).

A radiator delivering 3kW of heat POWER running for one hour uses

3kW x one hour is 3kWh of heat ENERGY.

Because one kWh of energy = 3.6 MJ (Mega Joules) of energy

3kWh is 3 x 3.6 = 10.8 MJ.

A 3kW radiator running for 10 hours per day uses 3 x 10 is 30kWh per day.

30kWh is 30 x 3.6 is 108 MJ per day.

10kW running for 10hrs uses 10kW x 10h is 100kWh.

100kWh is 100 x 3.6 is 360 MJ is 360,000,000Joules.

from ghostgum (above):

1gram of water requires 4.18 Joules to raise its temperature by 1deg C.

1000g is one kilogram is one Litre of water.

One Litre of water requires 1000 x 4.18 is 4,180 Joules to raise its temperature by 1deg C.

When the temperature of one Litre of water falls by 20degC (e.g. from 60 to 40degC) it supplies

20 x 4,180 Joules.

If the temperature of water falls by 20deg C, to supply 360MJ requires

360,000,000J / (20 x 4,180)J is 4,306Litres of water.

Posted Thursday 23 May 2013 @ 9:20:37 pm from IP

#