What you'll learn
Energy is the ability to do work or cause change. In this lesson you will learn:
- the main forms of energy and how to spot them;
- the law of conservation of energy: energy changes form, but the total never changes;
- the three ways heat moves: conduction, convection and radiation;
- how waves carry energy, including sound and the electromagnetic spectrum;
- the difference between renewable and nonrenewable energy sources.
How it shows up on the GED: Physical science is about 40% of the Science test, and energy is one of its three big topics. Expect several questions built on a short experiment, a diagram, a data table or a line graph. Some ask you to name an energy transformation; others ask you to read a graph or do a one-step calculation. On the GED, the formula you need is usually given in the question or on a reference sheet, so your job is to pick the right numbers and plug them in.
Key terms
- energy — the ability to do work or cause change, measured in joules (J).
- kinetic energy (KE) — energy of motion (a rolling bowling ball).
- potential energy (PE) — stored energy due to position or condition (a book on a high shelf; a stretched rubber band).
- chemical energy — energy stored in the bonds of substances (food, gasoline, batteries).
- thermal energy — energy of the random motion of particles; we feel it as heat (a hot stove).
- radiant energy — energy carried by electromagnetic waves such as light (sunlight).
- electrical energy — energy carried by moving electric charges (current in a wire).
- law of conservation of energy — energy cannot be created or destroyed, only changed in form.
- efficiency — the percentage of input energy that becomes useful output.
- conduction — heat moving through direct contact (a metal spoon in hot soup gets hot).
- convection — heat moving by currents in a liquid or gas (warm air rising from a heater).
- radiation — energy moving as electromagnetic waves, even through empty space (feeling the Sun's warmth).
- wavelength — distance from one crest of a wave to the next.
- frequency — number of waves passing a point each second, measured in hertz (Hz).
- amplitude — the height of a wave; bigger amplitude means more energy (a louder sound).
- renewable / nonrenewable — replaced naturally (wind) / limited supply (coal).
Forms of energy and conservation
Energy comes in many forms, but they fall into two big families:
| Family | Examples |
|---|---|
| Kinetic (motion) | moving cars, flowing water, wind, sound, thermal energy (moving particles), electrical energy (moving charges) |
| Potential (stored) | gravitational (height), elastic (stretched spring), chemical (food, fuel, batteries), nuclear (inside atoms) |
The most important rule is the law of conservation of energy: energy is never created or destroyed. When something seems to "use up" energy, the energy has only changed form. Follow the chain:
- Car engine: chemical energy in gasoline → thermal energy → kinetic energy of the car (plus a lot of waste heat and sound).
- Solar panel: radiant energy from the Sun → electrical energy.
- Hydroelectric dam: gravitational potential energy of high water → kinetic energy of falling water → electrical energy in a generator.
In every real transformation, some energy becomes thermal energy that spreads out and is hard to use. Efficiency measures how much is useful. If a light bulb takes in 100 J of electrical energy and gives off 15 J of light, it is 15% efficient. The other 85 J still exists, mostly as heat.
Potential and kinetic energy trade places. A ball thrown up slows as kinetic energy becomes potential energy; on the way down, the trade reverses. On roller coaster and pendulum diagrams, the lowest point has the most kinetic energy (the fastest speed) and the highest point has the most potential energy.
How heat moves: conduction, convection, radiation
Heat is thermal energy moving from a warmer object to a cooler one. It always flows from hot to cold, never the other way, until both reach the same temperature. A cold drink does not "give off cold"; it warms up because heat flows into it from the air.
| Type | How it works | Everyday example |
|---|---|---|
| Conduction | Particles bump into neighbors and pass energy along; needs direct contact. Best in solids, especially metals. | A metal pan handle gets hot; your hand feels cold on a steel railing. |
| Convection | A warm liquid or gas becomes less dense and rises; cooler fluid sinks to take its place, making a current. | Boiling water rolls; warm air rises from a heater; sea breezes. |
| Radiation | Energy travels as electromagnetic waves (mostly infrared). Needs no matter at all. | Sunlight warms Earth across space; you feel heat beside a campfire. |
Conductors (metals) let heat pass quickly. Insulators (foam, wool, plastic, trapped air) slow it down. A foam cup keeps coffee hot and keeps a cold drink cold, because it slows heat flow in either direction.
Measuring heat. The heat gained or lost by a substance depends on its mass, what it is made of, and how much its temperature changes: Q = mc\Delta T. Water has a high specific heat (c \approx 4.18 J/g·°C): it takes a lot of energy to warm up, which is why oceans keep coastal cities mild.
Waves: sound, light and the electromagnetic spectrum
A wave carries energy without carrying matter. Every wave has a wavelength, a frequency and an amplitude.
Two kinds of waves:
- Mechanical waves need a medium (solid, liquid or gas). Sound is a mechanical wave: it is a series of compressions in air, water or solids. That is why there is no sound in outer space. Sound travels about 343 m/s in air, faster in water and fastest in solids like steel. Higher frequency = higher pitch; bigger amplitude = louder sound.
- Electromagnetic (EM) waves need no medium. They all travel at the speed of light, about 3.0 \times 10^8 m/s in a vacuum. Bigger amplitude means brighter light.
The EM spectrum, from long wavelength to short: radio → microwave → infrared → visible → ultraviolet → X-ray → gamma. A memory trick: "Raging Martians Invaded Venus Using X-ray Guns." As you move from radio to gamma:
- wavelength gets shorter;
- frequency gets higher;
- energy per wave gets greater (which is why UV, X-rays and gamma rays can damage living cells, and radio waves cannot).
Visible light is a tiny slice in the middle: red has the longest wavelength and violet the shortest.
Energy sources for society
Power plants change one form of energy into electrical energy. Most turn a turbine connected to a generator.
| Source | Renewable? | Trade-offs |
|---|---|---|
| Coal, oil, natural gas | No | Reliable; release carbon dioxide and pollution |
| Nuclear (uranium) | No | No carbon dioxide while running; radioactive waste |
| Solar, wind | Yes | Clean; depend on weather |
| Hydroelectric | Yes | Steady; dams change rivers |
| Geothermal | Yes | Only in certain places |
| Biomass (wood, crop waste) | Yes | Regrows, but burning pollutes |
On the GED, a source is renewable if nature replaces it within a human lifetime. Notice that "renewable" and "pollution-free" are not the same thing: biomass is renewable but releases smoke, and nuclear power is nonrenewable but releases no carbon dioxide while running. Watch for answer choices that mix up these ideas.
Formulas you may see
On the GED, the formula you need is usually given in the question or on a reference sheet, so your job is to pick the right numbers and plug them in.
Kinetic energy: KE = \dfrac{1}{2}mv^2 — m = mass in kg, v = speed in m/s, KE in joules (J).
Gravitational potential energy: PE = mgh — m = mass in kg, g = 9.8 m/s² on Earth, h = height in m.
Heat: Q = mc\Delta T — Q = heat in joules, m = mass in g, c = specific heat (water: 4.18 J/g·°C), \Delta T = change in temperature (final − starting).
Wave speed: v = f\lambda — v = speed in m/s, f = frequency in Hz, \lambda (lambda) = wavelength in m.
Efficiency: \text{efficiency} = \dfrac{\text{useful energy out}}{\text{total energy in}} \times 100\%
Check units first: 1 MHz = 1,000,000 Hz.
Worked example 1: Name the transformation
Passage: A skateboarder stands at the top edge of a U-shaped ramp, then drops in. She speeds up to the bottom, then rolls up the other side and slows to a stop just below the opposite edge.
Question: Which statement best describes the energy changes from the top edge to the bottom of the ramp?
A. Kinetic energy changes into potential energy. B. Potential energy changes into kinetic energy. C. Thermal energy changes into potential energy. D. Energy is created as she speeds up.
Think-aloud:
- Top: high and still (potential energy). Bottom: low and fast (kinetic energy). So potential → kinetic: B.
- A is the climb up the far side. C is backward. D breaks the law of conservation of energy, so it can be crossed out immediately.
Answer: B
Worked example 2: Which kind of heat transfer?
Passage: A camper does three things at a campsite: (1) holds her hands near the fire to warm them; (2) stirs soup with a metal spoon and feels the handle get hot; (3) notices smoke and hot air rising above the fire.
Question: Which list matches each observation to the correct type of heat transfer?
A. (1) conduction, (2) radiation, (3) convection B. (1) radiation, (2) conduction, (3) convection C. (1) convection, (2) conduction, (3) radiation D. (1) radiation, (2) convection, (3) conduction
Think-aloud:
- Her hands do not touch the fire, and the warmth reaches them even sideways. Energy arriving as waves without contact = radiation.
- Energy passes along the metal spoon by contact, particle to particle = conduction.
- Hot air becomes less dense and rises, carrying heat upward = convection.
- Only choice B has all three right. Tip: start with the match you are surest of (spoon = conduction) to eliminate A and D at once.
Answer: B
Worked example 3: A potential and kinetic energy calculation
Passage: A 2.0 kg rock is held 5.0 m above the ground and then dropped. Ignore air resistance. Use PE = mgh, where g = 9.8 m/s².
Question: How much kinetic energy does the rock have just before it hits the ground?
Think-aloud:
- The formula is given, so find the numbers: m = 2.0 kg, h = 5.0 m, g = 9.8 m/s².
- Potential energy at the top: PE = 2.0 \times 9.8 \times 5.0 = 98 J.
- Kinetic energy at the top is 0 (the rock is not moving yet).
- With no air resistance, all of that potential energy becomes kinetic energy by the time it reaches the ground (where h = 0). Conservation of energy tells us KE = 98 J.
- Trap: some students try to use KE = \tfrac{1}{2}mv^2, but we do not know v. Conservation of energy is the shortcut.
Answer: 98 J
Worked example 4: Reasoning from wave data
Passage: A class looked up the speed of sound in three materials at room temperature.
| Material | State | Speed of sound (m/s) |
|---|---|---|
| Air | gas | 343 |
| Water | liquid | 1,480 |
| Steel | solid | 5,960 |
Question: Which conclusion is best supported by the data?
A. Sound travels fastest through gases. B. Sound travels faster through water than through air. C. Sound can travel through empty space. D. Sound travels at the same speed as light.
Think-aloud:
- Read the table: speed rises from air (343) to water (1,480) to steel (5,960).
- A is the opposite of the data: the gas is the slowest.
- B matches the table directly: 1,480 is greater than 343. Keep it.
- C cannot be tested with this table (no vacuum is listed), and science says it is false: sound needs a medium.
- D: light travels about 300,000,000 m/s, far faster than any number here.
Answer: B
Common mistakes
- "Energy gets used up." Wrong: a dead battery's energy is gone. Right: it became electrical energy, then light and heat.
- "Cold flows into things." Wrong: ice sends cold into a drink. Right: heat flows from the drink into the ice.
- Mixing up KE and PE on a diagram. Wrong: the highest point has the most kinetic energy. Right: highest point = most potential energy; lowest point = most kinetic energy.
- Thinking sound and light are the same kind of wave. Wrong: sound can travel through space. Right: sound needs matter; light and other EM waves do not.
- Reversing wavelength and energy. Wrong: longer waves carry more energy. Right: shorter wavelength = higher frequency = more energy for EM waves (gamma has the most).
- Using the wrong temperature change. Wrong: in Q = mc\Delta T, using the final temperature as \Delta T. Right: \Delta T = final − starting (use the size of the change).
GED test strategy
- Read the question first, then the passage.
- Trace the energy chain (chemical → electrical → light + heat), then match it to the choices.
- Check graph axes and units before you read any value.
- Use conservation as a filter. Any choice that says energy is created, destroyed or "used up" is wrong.
- Watch absolute words such as always, never and only.
- For calculations, list the numbers with units, substitute, and ask "Is my answer reasonable?"
- Time: aim for about 1 to 1.5 minutes per question; flag long calculations and come back.
Calculator tip (TI-30XS)
Use parentheses for the temperature change in Q = mc\Delta T: 200 × 4.18 × ( 80 − 38 ) enter. For 3.0 \times 10^8, type 3 × 10 ^ 8.
Check yourself
Keeping Coffee Hot
A student wanted to know which kind of cup keeps a hot drink warm the longest. She poured 200 g of water at 80 °C into each of three cups: a foam cup, a ceramic mug and a thin steel mug. The cups had the same shape and no lids, and the room stayed at 22 °C. She recorded the water temperature in each cup every 4 minutes for 20 minutes.
Thermal energy always moves from a warmer object to a cooler one. In this setup, energy moved from the hot water, through the cup walls, into the cooler room air. Materials that let thermal energy pass through easily are called conductors; materials that slow it down are called insulators.
According to the graph, the water in which cup lost thermal energy the fastest?
Hint
All three cups start at the same temperature. Which line drops the most in the same amount of time?
Step-by-step solution
Evidence: The graph shows all three cups starting at 80 °C. After 20 minutes the foam cup is at 63 °C, the ceramic mug at 49 °C and the steel mug at 38 °C.
Step by step:
- Find the temperature drop for each cup over the same 20 minutes: foam 80 - 63 = 17 °C, ceramic 80 - 49 = 31 °C, steel 80 - 38 = 42 °C.
- All cups held the same 200 g of water, so the biggest temperature drop means the most thermal energy lost in the same time.
- The steel line is also the steepest on the graph from the very first reading (66 °C at 4 minutes).
The water in the steel mug lost thermal energy the fastest. Steel is a metal, a good conductor, so heat passed through its walls quickly.
Tip: On a temperature–time graph, a steeper line means a faster change. Compare lines over the same time interval.
Check yourself
Energy on a Roller Coaster
A roller coaster has no engine after the first hill. A chain lifts the car to the top of hill A, and from then on gravity does the work. The diagram shows the track. The car has a mass of 500 kg and starts at rest at point A.
As the car rolls down, its gravitational potential energy (stored energy due to height) changes into kinetic energy (energy of motion). As it climbs the next hill, kinetic energy changes back into potential energy. The total amount of energy stays the same, but friction with the track and air resistance turn a small part of it into thermal energy and sound along the way. That is why each hill on a real coaster must be lower than the first one.
Look at the roller coaster diagram in the passage. Click the point where the car has the greatest kinetic energy.
Click on the figure to mark your answer (or focus it and use the arrow keys).
Hint
Kinetic energy is energy of motion. Where on the track would the car be moving fastest?
Step-by-step solution
Evidence: The passage says that as the car rolls down, "its gravitational potential energy (stored energy due to height) changes into kinetic energy." The diagram shows point B at 0 m, the lowest point on the track.
Step by step:
- The car starts at rest at A (40 m), so all its energy begins as potential energy.
- The lower the car goes, the more potential energy has changed into kinetic energy.
- Point B (0 m) is the lowest point, so almost all the energy is kinetic there and the car moves fastest.
- Point A has the most potential energy but zero kinetic energy (it starts at rest). Points C (25 m), E (15 m) and D (5 m) are all higher than B, so some energy is still stored as potential energy there.
The answer is point B.
Tip: On roller coaster and pendulum questions, lowest point = fastest = most kinetic energy; highest point = most potential energy.
Summary
- Energy is never created or destroyed; it only changes form. Some always becomes thermal energy that is hard to use, so no machine is 100% efficient.
- Potential energy (height, chemical, elastic) and kinetic energy (motion) trade places; the lowest point of a fall or track has the most kinetic energy.
- Heat flows from hot to cold by conduction (contact), convection (fluid currents) and radiation (EM waves, no matter needed).
- Waves carry energy, not matter. Sound needs a medium; EM waves do not. From radio to gamma, wavelength shrinks while frequency and energy grow.
- Renewable sources (solar, wind, hydro, geothermal, biomass) are replaced naturally; fossil fuels and uranium are not.