Tuesday, February 16, 2010

More On Energy

More on Energy

This info may be a little more dry, but its useful for building on later energy concepts.

The first law of thermodynamics implies that energy is always conserved. This discovery is extraordinarily important. It indicated that however much energy you had in the beginning of a process the amount in the end although possibly in variant forms will always equal the beginning energy – never more, never less. As an example to point back to early posts on energy. When the calories or energy available in butters popcorn are converted into other forms of energy the amount before equals the amount after. (The old saying is the 1st rule of thermodynamics states you cant win. The 2nd rule which we will consider at a later time dictates you cant even break even) The trick is that the new form of energies may be varied and harder to quantify. With the popcorn, if you metabolize it the energy would be converted into various forms, from heat to the motion of your muscles. This is obvious when we heat up from vigorous exercise; part of the calories our body is converting to energy to exercise is being turned into heat. So the energy in always equal the energy out.

That said it’s important to understand a little about the measurements of energy and further the measurements of power. We dealt a bit in previous blogs with the Calorie, and the measurement of a Calorie comes form the simplest from of determining how much energy something contains, that is how it converts to heat. And as discussed previously the Calorie was originally just that, a measurement of how much the energy from a substance heated a specific amount of water. (One Calorie is defined as the amount of energy it takes to increase one kilogram of water by one degree Celsius) Over time this system of measurement was replaced or improved upon with other methods. Today we hear much more often of the Joule. There are approximately 4200 joules in a Calorie. (The Joule is equal to the work done by one newton moving an object through one meter. Further one newton is the amount of force required to accelerate one kilogram at a rate of one meter per second per second.)

Another form of energy that we are familiar with from our electric bill is the kilowatt-hour or kWh. A kWh is equal to 1000 watts of energy use for an hour. 1 kWh is approximately 1000 Calories.

Fun chart

Energy Unit


Calorie (food calorie or kilocalorie)

Heat 1kg of water 1 C


1/4182 Calories. Lift 1kg by 10cm

Kilowatt-hour kWh

861 Calories – 3600000 Joules

BTU – British Thermal Unit

(fun since its referenced in The Matrix)


So what is power?

Power is the rate something happens divided by time – or the rate of energy transfer. A familiar example of a “rate” is miles per hour. One would travel x-miles in 1-hour. So when defining the power of popcorn, if 1 gram of popcorn releases 7 Calories in 2 minutes you have the total that popcorn has the power of 210 Calories per hour.(those numbers are not necessarily the true power of popcorn just an expression to help understand how power is expressed)

Typically power is expressed in units other then Calories. The common ones are the watt, and often one hears of the horsepower. A watt is 1 joule per second. Horsepower was originally based on how much work a horse could do in a second which would prove useful for comparison to the steam engines of the time. Later James Watt came to a more technical definition of a horse power at .18 Calories per second. The story goes he made this measurement by finding that a pony could lift an average 220 lbf 100ft per minuet over an extended period of time. Watt then extrapolated that a horse was 50% more powerful then a pony and came to power for 1hp @ 33,000 ft-lb/min – which can be simplified to .18 Calories. All the exact numbers are not necessary to know. A simple conversion that may be useful, especially when comparing battery power from things such as the Tesla Roadster, is that 1hp is approximately equal to 1kW.

1 Horsepower 1 kilowatt

More conversions and useful info:


Equal Too

Real life example

1 watt

One joule per second


1 horsepower

Aprx. 1 kilowatt

(746 watts)

Man running fast up hill or stairs

1 megawatt (MW)

1 million watts

Electric power for small town

1 gigawatt 1GW

1 billion watts

Average power plant

San Onofre has two reactors generating aprx. 1.17GW each

2 terawatts

2x10^12 watts

Average power use entire world

Next week: Power Power Power