# What is Watt-Hour?

By Xah Lee. Date:

My physics is weak. Was reading up on what exactly a watt-hour is… Here's the summary from Wikipedia.

In physics and other sciences, Energy is a scalar physical quantity that is a property of objects and systems which is conserved by nature. Several different forms, such as kinetic, potential, thermal, electromagnetic, chemical, nuclear, and mass have been defined to explain all known natural phenomena.

In physics, force is that which tends to cause a body to accelerate. It may be experienced as a lift, a push, or a pull.

In physics, mechanical work is the amount of energy transferred by a force.

In physics, power is the rate at which work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time.

Now, about the units. The newton is the amount of force required to accelerate a body with a mass of one kilogram at a rate of one meter per second squared.

1 N is the force of Earth's gravity on a apple with a mass of about 102 g.

The force of Earth's gravity on a human being with a mass of 70 kg is approximately 686 N.

The scalar product of force and distance (N×m)— a force of 1 N carried out over a distance of 1 m— is one joule, the basic unit of energy in the SI.

One joule is the work done, or energy expended, by a force of one newton moving an object one meter along.

The watt is a unit of power, equal to one joule of energy per second. A human climbing a flight of stairs is doing work at the rate of about 200 watts. An automobile engine produces mechanical energy at a rate of 25,000 watts while cruising. A typical household incandescent light bulb uses electrical energy at a rate of 40 to 100 watts.

The Watt-hour is a unit of energy. One watt-hour is the amount of (usually electrical) energy expended by a one-watt load (e.g., light bulb) drawing power for one hour. 1 watt-hour = 3600 joules.

In a rough summary, things have energy. Energy exerted is a force. (Force is measured in newton) The ratio of force over time is work. (work is measured in joule) The ratio of work over time is power. (Power is measured in watt)

What exactly is energy is complicated, but i think it ultimately traces to mass by E=mc^2. Effectively, joule is a unit for energy.

Watt-hour is just measure of energy, just like joules. It is a unit used by US American for layperson because they wouldn't understand WTF is a joule.

## Reading Notes on Liter

Some Wikipedia reading on: Litre (a measuring unit for volume).

In Taiwan, where i grew up, often we use “cc” (abbrev for cubic center meter) for beverages, which is equivalent to ml (millie-liter). So, i wanted to know why did Taiwan predominantly use cc and not ml, the differences and social background of these units. (Of course, in USA, they use their stupid Cup, “fl oz” (fluid ounce) stuff, although thru government stipulation, beverage now sold must include measure in liter.)

Here's what i learned of personal interest:

• Liter is a unit of volume.
• One liter is equivalent to 1000 cm².
• One liter of water is 1 kilogram. (this is by definition originally. Not now because density of water changes a bit by temperature and pressure.)
• “cc” is used in Taiwan and some other countries because it was the centimeter-gram-second (CGS) system. The CGS system and meter-kilogram-second (MKS) system are precursors of the SI system. The basic idea of these systems is to reduce all sorts of measuring of different things to the bare core.
• There are many SI prefix to choose from. SI recommends the choice of prefix is that, their exponent be divisible by 3. That is, 10^3, 10^6, 10^-3, 10^-6 etc. This recommendation is especially adapted in scientific usages. For example, 0.03 meter or 30 millimeter are preferred over 3 center meter.
2007-11

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