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Now let’s turn our attention to energy. First, we quickly review some ideas about electricity, and then we’ll move on to the different types of energy and discuss some important concepts about heat and light.
You have two bar magnets. The magnets are laid end to end in such a way that the two ends are attracted to each other. What will happen if you turn one magnet around but not the other?
The correct answer is C. All magnets have both a north and a south pole. Magnets that are either north poles only or south poles only do not exist. If the ends of the magnets in contact with each other attracted, they were opposite poles. Turning one of the magnets around brings like poles into contact, so they will repel each other.
In order for an electric current to flow, there must be a complete circuit. For a battery to light a bulb, the wires must connect from the battery to the bulb and then from the bulb to the other terminal of the battery.
This type of circuit is called a series circuit. If two bulbs are connected in series, the wire goes from the battery to one bulb, then to the other bulb, and finally back to the battery.
The other type of circuit is a parallel circuit. If the two bulbs are connected in a parallel circuit, the wire has a junction with a wire going to each bulb. After the bulbs, the wires then join again at another junction. The current is split between the two bulbs in this type of circuit.
One of the most fundamental laws of science is conservation of energy:
Energy can change into a different form, but it can be neither created nor destroyed.
It can change forms, but the total amount of energy in a closed system must remain constant.
Mechanical energy can take the form of either potential energy or kinetic energy. Potential energy is the energy that exists due to an object’s position. For example, a weight held above the ground has more gravitational potential energy than the same weight lying on the floor. Kinetic energy is the energy that an object has because it is moving. An object with either more mass or more speed will have a greater kinetic energy.
When the temperature of an object increases, the random motions of the individual atoms and molecules increase; so heat energy is just kinetic energy that is randomized at the molecular level. Energy can also be stored in either electric or magnetic fields, hence light and other forms of electromagnetic radiation are also forms of energy. The chemical energy that is released in a chemical reaction is the potential energy stored by the electrical forces within the atoms and molecules involved in the reaction.
Energy can interchange among any of these forms. Einstein’s famous equation E=mc2 means that energy can also take the form of mass. Mass and energy are interchangeable; the equation gives a formula for finding the amount of energy equivalent to a certain amount of mass. In nuclear reactions, energy is released because some of the mass in the atomic nuclei undergoing the reaction is converted into energy.
A couple paragraphs up, you were holding a weight above the ground. If you let go of the weight and allow it to fall to the floor, which of the following statements best describes the energy conversions from the time the weight is overhead until just before it hits the floor?
The correct answer is A. When the weight falls, its gravitational potential energy is converted to kinetic energy, and the weight moves at a higher speed. When the weight strikes the floor (or possibly your foot), the kinetic energy goes into deforming or breaking the weight, floor, or possibly your foot, the sound made, and so forth.
Who is doing more work, a weightlifter holding 500 pounds perfectly motionless over his head for ten hours or a woman flicking a mosquito off her arm?
The weightlifter 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 weightlifter 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. It’s only the force that is less.
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.
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.
A good start to this discussion is to ask: what creates a rainbow? White light, which is a mixture of all the different colors of light, is broken into its component colors by small water droplets in the atmosphere. The droplets are essentially acting as a prism, which will also divide light into its component colors.
Visible light is a form of electromagnetic radiation, or electromagnetic waves. Other forms of electromagnetic radiation include:
All of these forms of electromagnetic radiation are oscillating waves in the electric and magnetic fields. These fields oscillate from a negative value, to zero, to a positive value, and then back through zero to negative again and so on. Different colors of light and different forms of electromagnetic waves have different values of wavelength and frequency.
We often think of light and other electromagnetic radiation as having wavelike properties. Light also displays particle-like properties. Light comes in discrete packets called photons, which can be thought of as particles of light. When a photon of the correct amount of energy strikes an electron in an atom, the electron will absorb the energy and jump to a higher energy level. When the electron jumps back down to a lower energy level, it will emit a photon with the appropriate amount of energy.
These absorbed and emitted photons are the basis for spectroscopy as a way of finding the chemical composition of something. The absorbed and emitted photons cause spectral lines when the light passes through a prism and is broken into its component colors. Because each type of atom has its own unique set of energy levels, each type of atom has its own unique set of spectral lines. Scientists use these spectral lines to identify the chemical composition.
Unlike most other types of waves, light and other electromagnetic waves propagate in a vacuum. When a light wave encounters some medium, several things can happen. Depending on the type of medium, the possibilities include:
Refracted basically means bent. When light strikes a transparent medium, it can pass through the medium. However, it will not continue on its straight-line path. The light ray will travel in a straight line, make a sharp bend at the surface of the medium, and then travel in a straight line through the medium. It might be refracted again when it leaves the medium or encounters a new medium. When light is traveling through the medium, its speed is slower than its speed in a vacuum. Both the amount of slowing and the amount the light ray is refracted depend on the optical properties of the medium. The light path being bent by a magnifying lens is an example of refraction.
When light is reflected, it bounces off the surface of the medium. If we measure the angles of the incident light ray and the reflected light ray from a line perpendicular to the surface of the medium, the incident angle equals the reflected angle. Pool players use this principle when they bounce a shot off the wall of the pool table. Using a mirror to groom yourself is an example of light reflection.
Sometimes light is absorbed when it strikes a medium. The amount of absorption usually depends on the color, or wavelength, of the light. For example, the dye in your favorite pair of blue jeans will reflect blue light and absorb other colors of light.
When light encounters a medium consisting of a large number of small particles rather than a single large object, it will often be scattered. Essentially, individual light rays are reflected off individual particles. The effect is more random, however, than normal reflection because the individual small particles will have different shapes and orientations. When you see a beautiful red sunset or Carolina blue sky, you are seeing the effects of scattering. Particles in our atmosphere scatter blue light more effectively than red light. So when the sun is low in the sky even more blue light than normal is scattered, and the sun appears red. When the sun is overhead, blue light is scattered. Some of the blue light will be scattered a second time off a different part of the sky and into our eyes. Hence the entire sky will appear blue.
When a light wave encounters a very sharp boundary, it will often seem to bend around the boundary. This effect is called diffraction. Diffraction effects in sound waves are what allow us to hear a person talking when he is behind an obstacle. The sound waves diffract around the boundary of the obstacle. A diffraction grating is a flat piece of glass or plastic with hundreds to thousands of small straight grooves (like a plowed field) on the surface. The diffraction effects of these grooves depend on the wavelength. So the diffraction grating will, like a prism, split white light into its component colors and produce a groovy rainbow effect.
Sometimes more than one of these will occur. For example, a beam of light might strike a glass lens, and some of the light will be refracted while some is reflected.
When you look at a spoon in a glass of water, the part of the spoon above the surface of the water is slightly displaced from the part below the surface of the water. This effect occurs because the light waves are
The correct answer is A. The light waves coming from the underwater portion of the spoon are refracted when they pass from the water into the air. This refraction causes them to appear to originate from a slightly different position. The underwater portion of the spoon, therefore, appears to be displaced.
Other types of waves besides electromagnetic waves must have a medium to propagate. When these waves are traveling through the medium and encounter obstacles or changes in the medium, they can display the same effects that were just described for light.
Some examples follow. Surfers ride waves on the top surface of the ocean. The wave propagates along the surface of the water; the individual water molecules travel in a small circular path. A sound wave travels through air or another medium as individual molecules in the medium oscillate. When tectonic plates slip and vibrate the earth, seismic, or earthquake waves are generated. When these seismic waves travel along the surface, they can cause damage.
When seismic waves travel through the earth’s interior, scientists can study the effects, such as refraction, that occur and learn about the interior structure of the earth. An oscillating slinky also produces waves that will be easily seen by your students.
Oscillating the slinky sideways, perpendicular to the slinky, will produce a transverse wave traveling through the slinky. Oscillating the slinky by gathering the coils together and then releasing them, parallel to the slinky, will produce a longitudinal wave traveling through the slinky.
All these types of waves propagating through matter will display the same sorts of behavior that light waves display when they interact with matter.