Page 353 - ABCTE Study Guide_Neat

Who is doing more work, a weight lifter holding 500 pounds perfectly motionless over his
head for ten hours or a woman flicking a mosquito off her arm?

The weight lifter is certainly exerting himself more, but the woman flicking the mosquito is actually doing
more work. This doesn't make much sense until you realize that work has a specific meaning in science
that is different from its everyday meaning. In science, work is defined as the force times the distance over
which the force is applied. The weight lifter is exerting 500 pounds of force, but the distance is zero.
Hence his total work is also zero, despite his extreme fatigue. The woman is exerting a very small force,
but the mosquito moves as a result of this force. Hence her work is slightly greater than zero.

Simple machines such as levers and pulleys don't reduce the amount of work required to move
something. They reduce the amount of force required, but the force must be applied over a larger
distance. For example, if you use a lever to lift a large rock, the force you need to apply on the lever is
less than the weight of the rock. However, you must move the lever arm a greater distance than the rock
moves. Hence as required by the conservation of energy, the total work required to move the rock is the
same. Just the force is less.

Heat

Even though we use temperature to measure how hot something is, heat and temperature are not the
same thing. Heat is a form of energy that is related to both the temperature and the amount of material
present.

For example, a fluorescent light bulb does not feel hot to touch, but the temperature of the gas inside is
extremely hot (thousands of degrees). The temperature is high because the atoms are moving very fast.
The total heat energy is, however, still low because the gas is very thin and there are very few atoms at
this high temperature.

The three most commonly used temperature scales are Fahrenheit, Celsius (also called centigrade),
and Kelvin. The Fahrenheit scale is most common in everyday life in the United States, while many other
countries use the Celsius scale. The Celsius and Kelvin scales are used in scientific work.

The Kelvin scale is based on absolute zero. Zero kelvins is the temperature where the random molecular
motions are at the lowest amount. The other scales are based on the boiling and freezing points of water.

How is Heat Transferred?

Normally, heat energy will flow from hot to cold. There are three ways to transfer heat
energy: conduction,convection, and radiation. Conduction requires direct contact. The fast moving
molecules in the hot object collide with the molecules in the cold object, thereby increasing the speed of
the latter. The temperature of the hot object decreases while the temperature of the cold object increases.
When you burn your finger on a hot stove, the heat energy is transferred by conduction.

When you feel the warm sun on a nice day, heat energy is being transferred from the sun to you by
radiation. Light, infrared, and other forms of electromagnetic radiation transfer the heat energy.

A radiator with no fan heats a room by convection currents. The air just above the radiator rises as it
warms up and then moves to the other side of the room and drops as it cools. The resulting convection
currents transfer heat energy to the other side of the room. Convection currents in the earth's mantle
cause plate-tectonic motions. Most ocean currents and atmospheric wind patterns result from convection
currents.

Light and Waves