CONVERSIONS OF ENERGIES

Often the form of energy most readily available is not the most useful form. Coal can be burned to provide heat, but converting the chemical energy stored in coal into electrical energy requires a series of intermediate steps. In each step of the conversion process, some energy is wasted.

ELECTRICAL ENERGY TO MECHANICAL ENERGY OF MOTION

The model electric train illustrates the conversion of electrical energy into mechanical energy of motion. Electrical energy, which operates the train, is generated at a power plant. The electric motor in the train’s engine transforms that electrical energy into mechanical energy of motion. Since electrical energy is delivered to the train at a nearly constant rate, the train slows down as it climbs an incline because energy is required to lift the train up as well as to move it forward. As the train climbs the incline, some of its mechanical energy of motion is converted into gravitational potential energy.

MECHANICAL ENERGY OF MOTION TO ELECTRICAL ENERGY

Although we cannot easily store large amounts of electrical energy, electricity can be continuously transferred to us from a generating plant. In class, you will use hand-cranked generators to convert the mechanical energy of motion of your arm into electrical energy. In future periods we will learn more about the power plant generators that provide electricity for our homes and businesses.

A spark generator converts mechanical energy of motion into the electrical energy of a spark. When the spark generator’s spring is compressed, mechanical energy is stored as potential strain energy. When the stored strain energy is released, the spring exerts force on the surfaces of a certain type of crystal, which then becomes electrically charged. If enough charge accumulates on the surfaces, a spark will jump between oppositely charged parts of the crystal.

THERMAL ENERGY TO RADIANT ENERGY

Campers sometimes use gas lanterns as a light source. The lantern’s gas flame has a high temperature but gives off very little visible light. To convert its thermal energy into visible radiant energy, the lantern has a “mantle” of knitted fabric soaked with heavy metal oxides. The mantle glows brightly when heated in the gas flame, but does not burn. In class we use a Bunsen burner flame rather than the usual lantern propane or butane flame to illustrate this energy conversion.

ELECTRICAL ENERGY TO THERMAL ENERGY

The conversion of electrical energy into thermal energy is the most common energy conversion in our household appliances. Many appliances, such as toasters, hair dryers, and electric heaters, convert electrical energy into useful thermal energy.

Appliances designed for purposes other than providing heat waste energy by converting some of their electrical energy into undesirable thermal energy, giving the warmth you feel when you touch an operating television or computer.

THERMAL ENERGY TO MECHANICAL ENERGY OF MOTION

Appliances, such as toasters and hair dryers, contain thermostats to regulate their temperature. Some thermostats illustrate the conversion of thermal energy into mechanical energy of motion. In class you will see a demonstration of how thermostats with bimetallic strips work. A bimetallic strip consists of two layers of different metals, which react differently when they are heated. The metal in one layer of the strip expands more when heated than does the metal in the other layer, causing the strip to bend. When a bimetallic strip is heated to a critical temperature, the strip bends until it opens a switch, which interrupts the flow of electric current and turns off the appliance.

ELECTRICAL ENERGY TO RADIANT ENERGY

In class you will see an electroluminescent nightlight, which converts electrical energy to visible light, a form of radiant energy. The nightlight is dim because it operates with a very small amount of energy – only 0.02 watts of electrical energy compared to 100 watts for an average light bulb.

RADIANT ENERGY TO ELECTRICAL ENERGY

Solar cells convert radiant energy from the sun into electrical energy. In class, we will use solar cells to light a flashlight bulb, run a toy car, and operate a small motor.

MECHANICAL ENERGY OF MOTION TO THERMAL ENERGY

Mechanical energy of motion is converted into thermal energy when surfaces rub together, producing friction between the objects. We will illustrate thermal energy from friction by using an electric drill to rotate a metal tube filled with water. Two blocks of wood squeezed against the tube produce the friction. When we pour a small amount of water into the metal tube and cork it with a rubber stopper, the mechanical energy of motion converted into heat by friction turns the water into steam. The steam blows the cork out of the tube, thus converting thermal energy back into mechanical energy of motion of the stopper.

MAGNETIC ENERGY TO GRAVITATIONAL ENERGY

If two magnets are placed on top of one another with their like poles facing, the magnets repel each other. We can feel this force when we do work to push the top, floating magnet closer to the bottom magnet. When we release the floating magnet, some magnetic energy is converted into mechanical energy of motion and gravitational potential energy as the magnet moves upward and floats.

CHEMICAL POTENTIAL ENERGY TO ELECTRICAL ENERGY

Perhaps the most common energy storage device is the battery. Batteries store chemical potential energy, which is converted into electrical energy when the battery operates.

CHEMICAL POTENTIAL ENERGY TO THERMAL ENERGY

When substances burn, their stored chemical energy is converted into thermal energy and into visible and invisible radiant energy. Burning natural gas, coal, and petroleum products is one of our most economically important energy conversion processes. We will explore the economic and environmental consequences of this energy conversion.

CHEMICAL POTENTIAL ENERGY TO RADIANT ENERGY

Unlike light bulbs, which give off invisible infrared radiation as well as visible light, some chemical reactions produce visible light without giving off discernable amounts of infrared radiation. Such chemiluminescent reactions occur when a firefly gives off light.

RADIANT ENERGY TO CHEMICAL POTENTIAL ENERGY

Plants convert radiant energy from the sun into stored chemical energy through the process of photosynthesis. Nuclear energy produced in the Sun’s core radiates away from the Sun in all directions. Some of this radiant energy strikes the Earth and is absorbed by plants. Plant cells use radiant energy to produce chemical energy in the form of carbohydrates. When we consume plants, our bodies convert their stored chemical energy into energy stored in our bodies.