Energy: Movement, Heat, Light and Sound

Watch a playground for a moment. A swing moves, shoes scrape, and sunlight warms a bench. Each gives you a sign of energy. Energy is what is needed to make things happen.

Name what energy does

Scientists name energy by what it does or where it is found.

Motion energy is also called kinetic energy. You cannot hold energy like a handful of sand. You find it by noticing movement, warmth, light, sound, or a working electrical device.

Find more than one form

Think about a campfire. The wood holds chemical energy before burning. The flames glow, warm nearby things, and crackle. Those changes show light, heat, and sound energy. One object or event can involve several forms at once.

Food holds chemical energy too. Your body releases that energy to help you move, grow, and keep warm. Green plants use sunlight to make their food. A machine also needs energy: a toy robot gets it from batteries, while a car may get it from fuel.

Energy can be ready for later

A log, a slice of bread, and a battery can sit still. They still hold chemical energy. You do not need to see movement for energy to be present. When you connect the battery to a toy, energy can be passed to the toy's moving parts.

A force is a push or pull. When you push a cart and it speeds up, you pass energy to it. The force is the push; motion energy is the energy the moving cart has. If you push a wall that stays still, it gains no motion energy. Your working muscles still release energy from food and become warmer.

Roll a ball gently, then roll the same ball faster. The faster ball has more motion energy. A still ball has no motion energy. When you speed something up, you give it energy.

Use a collision as evidence

A class releases one toy car from different ramp heights. It hits the same paper cup each time. These are their results:

Read the first row: the low start sends the cup 6 cm. Next, compare the other rows. Higher starts make the car faster at the bottom. The cup moves farther after those hits. The pattern is evidence that the faster car has more energy to pass on.

Make the comparison fair

Change only the car's starting height. Keep the car, ramp, cup, and floor surface the same. Release the car without an extra push. Measure from the cup's starting point to its finishing point with a ruler.

Repeat each height three times. If one run gives an odd distance, the other runs help you notice it. A cup that tips over or a car that hits sideways may give a different result. Record what happens instead of hiding that run.

Think about mass as well

Mass tells how much matter an object has. At the same speed, a bowling ball has more motion energy than a tennis ball. It has more mass. But mass alone cannot settle every comparison. A very fast tennis ball can have more motion energy than a slowly rolling bowling ball.

Speed makes a big difference. The same object moving twice as fast has four times the motion energy. That helps explain why fast crashes can cause so much damage.

When you brake a bike, it slows as motion energy becomes heat in the brakes and wheel rims. Some also becomes sound. The energy must go somewhere as the bike stops.

A collision happens when a moving object bumps into something. The other object may be moving or still. During the bump, the objects push on each other and pass energy between them.

Watch before and after

A rolling marble hits a still marble of the same size, straight on. The first nearly stops. The second rolls away almost as fast as the first was moving. The second marble has gained motion energy. The click tells you some energy also became sound.

Now picture a toy car hitting a wooden block. The car stops, the block slides, and you hear a bang. Follow the evidence in order:

1. Before the hit, the moving car has motion energy.
2. During the hit, the car pushes the block and passes energy to it.
3. After the hit, the moving block has motion energy. Some energy has also spread as sound and heat.

A faster hit with the same car and block can move the block farther. Keep the floor and the kind of hit the same when comparing.

A collision can change more than speed

Things can stop, speed up, slow down, or change direction. A ball hitting a wall can bounce back. Soft things may squash, and hard things may dent. A lump of clay changes shape and bounces less than a rubber ball.

A bouncing ball reaches a lower height after each bounce. Some energy warms the ball and floor a tiny amount. Some travels away as sound. Less is left for the next bounce. Stopping does not mean the energy has disappeared.

Use squashing to protect people

A bike helmet, a car's crumple zone, and a soft playground surface change shape during a hit. Squashing makes the stop take longer and takes in energy. This helps reduce harm to your body.

Collisions also help you play sports. A swinging bat passes energy to a baseball. A faster swing can send it away faster and farther. The crack you hear is another destination for some of that energy.

You can feel sunlight warming your face without touching the Sun. You can hear a drum across a room. Light and sound carry energy from their sources to other places.

Light delivers energy

Light travels very fast and in straight lines. It can cross empty space. Sunlight reaches Earth even though most of the space between them has no air.

When an object absorbs light, it takes in energy and warms up. In one class test, two thermometers start at 20 °C (68 °F). One is covered with black paper, the other with white paper. After 20 minutes in sunshine, they read 31 °C and 25 °C.

Both papers receive sunlight. The black paper absorbs more of its energy. The white paper reflects more away. The different temperatures show what happened to the arriving energy.

Light can do other jobs too. A solar panel changes it into electrical energy. Green plants use it to make food holding chemical energy. A solar oven absorbs it to heat food.

Sound delivers energy through matter

Sound starts with a vibration: a quick back-and-forth movement. A drum skin vibrates when you strike it. It passes vibrations to nearby air. Each bit of air passes the movement to the next, carrying energy toward your ear.

The air does not rush all the way from the drum to you. Think of a row of people passing a message. The message moves down the row while the people stay near their places. In sound, vibrations and energy are passed along.

Sound can travel through air, water, and solids. It cannot cross empty space because there is no material to pass vibrations along.

Look for movement caused by sound

A nearby loud drum can make rice grains jump on another drum skin. A loud truck can make a window rattle. These movements show that sound has carried energy to something else.

Sound becomes quieter with distance as its energy spreads over a larger area. Eventually that energy becomes tiny amounts of heat in the surroundings.

Flip a switch and a lamp lights up. Energy has reached the lamp through an electric current. A current is a flow of electric charge through a material, such as a metal wire.

Close the loop

In a battery circuit, wires connect the battery and device in a complete loop. A switch can open a gap or close it. With a closed loop, current flows and carries energy to the device. Open the switch and the device stops receiving energy from the battery.

The battery's chemicals hold energy. When the circuit is complete, the battery pushes current around the loop. A bulb receives electrical energy and gives out light and heat. The current continues around the circuit; the bulb does not use it up.

A battery goes flat after its chemicals have changed. It can no longer keep pushing the current. Its energy has been passed on to the device and surroundings.

Identify what each device does

Some devices make several changes at once. In a hair dryer, a motor spins a fan. A wire coil heats the moving air. You also hear a whir. You can follow the energy by looking at the job of each part: motion, heat, and sound.

Follow electricity to your home

Power plants, wind farms, solar farms, and dams supply electricity. Power lines carry energy to buildings. A lamp plugged into a wall outlet receives energy when its circuit is complete. The switch controls the circuit; it does not make the energy.

For your own circuit experiments, use small batteries with an adult. Never put anything into a wall outlet, and keep electrical devices away from water. Wall electricity can badly hurt you.

A device converts, or transforms, energy when it changes its form. A flashlight changes chemical energy in a battery into electrical energy, then light and heat. Your design needs an energy input and a job to do.

Decide what success looks like

Engineers start with a problem. A solar oven could solve the problem of warming food with sunlight. Its criteria describe success: for example, melt a piece of chocolate within 30 minutes.

Its constraints are limits. You might have only a pizza box, foil, black paper, plastic wrap, tape, and one class period. A successful design must do the job within those limits.

Give each part a job

In a solar oven, foil on the lid flap reflects sunlight into the box. Black paper on the bottom absorbs light and becomes warmer. Plastic wrap over the opening lets light enter and keeps warm air inside. These parts help energy reach the chocolate and stay near it.

Make a drawing, choose your materials, and build a prototype, or test model. Record the oven's starting temperature. Read its thermometer every 5 minutes and check whether the chocolate melts in time.

Improve using evidence

If the chocolate stays solid, change one feature. You could tilt the foil flap to send more sunlight inside. Test again with the same size chocolate piece and similar sunlight. Changing one feature helps you connect a better result to that change.

A different device could use motion. In a hand-crank flashlight, your arm turns a generator. The generator changes motion energy into electrical energy. The bulb then changes it into light. A motor works the other way: electrical energy makes it move.

You could also design a door alarm. A battery, wires, and buzzer form a circuit. Two foil pieces under a mat act as the switch. Stepping on the mat brings them together, so current flows and the buzzer sounds.

A device changes energy that enters it. It does not make energy from nothing. Most devices also send some energy into unwanted heat or sound. For a room heater, though, making heat is the useful job.

The Sun warms a path. A toaster warms bread. Your palms warm when you rub them together. A heat source gives off heat, and heat can be produced in several ways.

Find where the heat comes from

Burning wood or gas changes chemical energy into heat. A stove or electric heater changes electrical energy into heat. Friction, the rubbing between surfaces, changes motion energy into heat. Your body releases energy from food and gives off heat too.

A sweater feels warm, but it makes no heat of its own. It slows heat leaving your body. A thermometer wrapped in a sweater on a table will not become warmer just because of the sweater.

Measure hot and cold

Temperature tells how hot or cold something is. Use a thermometer to measure it. Scientists use degrees Celsius, written °C. US weather reports and home thermostats often use degrees Fahrenheit, written °F.

Useful reference temperatures are:

• Pure water freezes at 0 °C (32 °F).
• Pure water boils at 100 °C (212 °F) at sea level.
• A healthy body's temperature is about 37 °C (98.6 °F).
• A comfortable room is about 20 °C (68 °F).

For a liquid thermometer, read the top of the liquid at eye level. First check the scale. If the marks go up by 2 degrees, halfway between 24 and 26 is 25 °C. Looking down from above can make the level appear wrong.

Temperature does not tell the whole story

A bathtub of warm water can melt more ice than a cup of hot tea. There is far more water in the tub. The amount of thermal energy depends on how much material there is as well as its temperature.

Taking in heat usually raises temperature. But melting ice can take in heat while staying at about 0 °C. The energy is helping it melt. Boiling water can also keep the same temperature while heat enters.

Touch is a poor way to measure temperature. The same lukewarm water can feel warm to a chilled hand and cool to a warm hand. A thermometer gives you a reading you can compare.

Hold a cool spoon in your hand. Your hand loses heat to the spoon, and your skin feels cooler. Heat is energy moving from a warmer object or place to a cooler one.

Use temperatures to find the direction

Hot cocoa at 70 °C (158 °F) stands in a room at 20 °C (68 °F). Heat leaves the cocoa for the cooler cup, air, and table. Its temperature falls toward room temperature.

Ice water in that room receives heat from the warmer surroundings. It warms as heat enters. Cold is not a substance flowing out of the glass. You can decide the heat direction by comparing temperatures before thinking about how anything feels.

Read a warming-and-cooling pattern

A small cup holds 200 mL of water at 60 °C. It sits in a bowl holding 600 mL at 20 °C. These readings show a model of heat exchange between the two amounts of water, with no heat entering or leaving the pair.

Heat goes from the warmer cup water to the cooler bowl water. The gap between their temperatures shrinks. The bowl warms less than the cup cools because it holds more water. They reach the same temperature, 30 °C.

At matching temperatures, neither keeps warming the other overall. In a real room, both can still exchange heat with the surrounding air. If the room is cooler, both will eventually cool toward its temperature.

Apply the rule to ice and metal

Ice cream melts when heat enters from warmer air or sunlight. Water freezes when it loses heat to colder surroundings. Neither change needs cold to flow into anything.

A metal spoon and a wooden spoon left in the same room reach the same temperature. The metal feels colder because it carries heat away from your hand faster.

A refrigerator uses electrical energy to move heat from its cold inside to the warmer room. That needs a working device; heat moving on its own goes from warmer to cooler.

Heat can travel through a spoon, around a room, or across space. You can tell the three ways apart by asking what moves and whether material is needed.

Conduction: through touching material

Conduction passes heat through a material or between things that touch. Tiny particles in the warmer part jiggle faster and pass energy to neighbors. The material does not travel along with the heat.

Stand a metal spoon in hot soup. Heat moves along it toward the cooler handle. The spoon stays in place as energy travels through it.

Convection: carried by moving air or water

Convection happens when a moving liquid or gas carries heat. Air warmed by a heater spreads out. A cupful of it has less mass than a cupful of cooler air. Cooler, heavier air sinks and pushes the warmer air up.

That movement makes a loop called a convection current. Warm air rises, cools, and sinks; cooler air moves toward the heater. Water heated in a pan can circulate in a similar loop.

Radiation: carried by rays

Radiation carries energy as rays, including visible light and invisible infrared. It can cross empty space. The Sun warms Earth by radiation. Warm objects give off infrared rays too, so a fire can warm your face from the side.

Dark surfaces absorb more sunlight than light-colored ones. Light colors reflect more sunlight away. Shiny metal reflects infrared well and gives off little of it. A shiny emergency blanket helps slow heat leaving a person's body.

Check one scene in three ways

Picture water warming in a pan on an electric stove.

People say, "Heat rises." Warm air and water rise in convection currents. Heat can also travel downward through metal or sideways by radiation.

You want a pan to pass heat into food quickly. You want its handle to pass heat slowly. Different jobs need different materials.

Choose a conductor or insulator

A thermal conductor lets heat pass easily. Most metals, including copper, aluminum, and steel, are good conductors. A thermal insulator slows heat passing through it. Wood, plastic, cork, wool, cardboard, foam, and still air are good insulators.

Wool and fleece hold air in small spaces. Foam does too. Trapping the air keeps it from moving around easily and carrying heat away. Double-pane windows use a gap between panes to help slow heat transfer.

Keep cold things cold too

A coat slows heat leaving your body for colder air. A picnic cooler slows heat entering cold drinks from warmer air. The same idea works in either direction: insulation slows the heat flow.

Wrap an ice cube in a wool scarf. The scarf adds no heat. It slows heat arriving from the room, so the ice melts more slowly. An oven mitt also slows heat, helping protect your hand near a hot pan.

Compare materials fairly

A class puts identical ice cubes on boards of equal thickness. All start together in the same room. These are their results:

The aluminum passes heat from the room into the ice fastest. Foam slows it most in this test. Use ice cubes from the same freezer, equal board thicknesses, the same room, and matching checking times. Change only the board material.

Use insulation in homes

A furnace, boiler, heat pump, wood stove, or electric heater supplies warmth to a home. Insulation in walls and attics slows heat leaving in winter. Double-pane windows and weatherstripping help too. In summer, insulation slows heat entering from hotter outdoors.

Insulation reduces the fuel or electricity needed to keep rooms comfortable. It slows heat transfer; it does not stop it forever. Ice in a cooler will eventually melt.

A rolling toy slows and stops. Where did its energy go? Friction passed it into heat in the toy, floor, and air. Some became sound. The energy changed form and place, but none disappeared.

Follow the arrows

An energy chain shows where energy comes from and where it goes. Name a source, then the next object or form, and connect the steps with arrows. The arrow means energy is being passed on or changed.

For a flashlight, follow these steps:

1. The battery holds chemical energy.
2. A complete circuit carries electrical energy to the bulb.
3. The bulb gives out light and some heat.
4. Light reaching a wall is absorbed and warms the wall a tiny amount.

You can write the main changes as: battery's chemical energy → electrical energy → light and heat. A chain can branch because energy can go to more than one place.

Start a longer journey at the Sun

Sunlight reaches grass. The grass makes food holding chemical energy. A cow eats the grass. Milk from the cow is made into cheese, which you eat. Your muscles use energy from that food when you kick a ball.

The kick passes energy into the ball's motion. As it rolls and slows, friction warms the ball, grass, and air slightly. The kick makes sound too. Follow both the movement and the less obvious warming.

Explain why a new supply is needed

Energy cannot be made from nothing or destroyed. This is conservation of energy. It can be transferred or changed into another form.

At the end of most everyday chains, energy is spread as heat through the surroundings. That spread-out energy is hard to gather for another useful job. You still need fresh food, and a toy still needs a charged battery.

Saying a battery is "used up" means it can no longer supply energy in the way you need. It does not mean energy has been destroyed.

At home, "conserving energy" means saving electricity or fuel. In science, "conservation of energy" means the total stays the same. Saving electricity reduces the demand for fuel even though the energy itself never disappears.

A toaster should make heat. A reading lamp should make light. Whether an energy change is useful depends on the job you want the device to do.

Name the job first

Useful energy does the wanted job. Wasted energy goes somewhere you do not need it, often as heat or sound in the surroundings.

A warm phone charger sends some input energy into heat. That warming is not the charger's job. But heat warming a room is the useful output of an electric room heater.

Compare two kinds of bulb

An old-style incandescent bulb has a thin wire that glows. About 90 percent of the electrical energy it receives becomes heat. Only about 10 percent becomes light. For a lamp, most of the energy goes into an unwanted form.

An LED bulb sends much more of its energy into useful light. For the same brightness, it uses at least 75 percent less electricity than an incandescent bulb. The useful job stays the same while less input energy is needed.

Reduce unwanted transfers

Rubbing parts can warm up and squeak. Oiling a squeaky bike chain reduces friction. Less energy goes into unwanted heat and sound, leaving more for moving the bike.

To compare designs, keep the useful job the same. Two lamps should give the same brightness. Two heaters should warm the same room to the same temperature. Otherwise a lower electricity use might simply mean less work is being done.

Wasted energy still counts. Add the useful and wasted parts and you have all the energy that entered the device. A warm laptop has changed some electrical energy into heat; it has not created extra energy. That heat spreads through its surroundings, where it is hard to use again.

An energy source is something we get energy from. Sources include sunlight, fuels, wind, flowing water, and heat inside Earth. Electricity carries energy from a source to where we need it.

Trace food and fuels back to sunlight

The Sun supplies most energy for life on Earth. Plants use its light to make food. Animals eat plants or other animals, so you can trace your breakfast's energy back to sunlight.

A fuel releases energy when it is used. Wood, coal, oil, natural gas, and propane release heat when burned. Burning needs oxygen from the air. Food fuels your body, but your body releases its energy slowly without flames.

Fossil fuels are coal, oil, and natural gas. They formed over millions of years from buried remains of living things. Their chemical energy began as sunlight captured long ago. Oil can be made into gasoline, diesel, and heating oil.

Follow the steps in a power plant

Many fuel-burning power plants follow this sequence:

1. Burning fuel heats water.
2. The water becomes steam.
3. Steam spins a turbine, a wheel with blades.
4. The turbine turns a generator, changing motion into electrical energy.
5. Power lines carry electricity to homes, schools, and factories.

A nuclear plant uses energy from the centers of uranium atoms to heat water. It does not burn uranium like coal or wood.

Wind and water can turn generators without making steam. Wind pushes a wind turbine's blades. Flowing water spins turbines in a hydroelectric dam. Solar panels work differently: they change sunlight directly into electrical energy, with no moving parts.

Find which sources depend on the Sun

The Sun heats air unevenly, making winds blow. It also evaporates water. That water later falls as rain and fills rivers. Wind and river energy can therefore be traced back to the Sun.

Some sources have other beginnings. Geothermal energy is heat from deep inside Earth. Nuclear energy comes from uranium. Tides are caused mainly by the Moon's pull. These sources do not begin with sunlight.

A windy hill can supply energy again tomorrow. A lump of coal cannot grow back after burning. You can sort energy sources by how nature replaces them.

Ask whether the supply is renewed

A renewable source is naturally replaced as fast as we use it, or does not run out on human timescales. Sunlight, wind, flowing water, waves, tides, and geothermal heat are renewable sources.

Biomass is material from living things, such as wood and crops. It can be renewed when new plants replace those used. Trees need years to grow. For a wood supply to last, growth must keep up with use. Cutting a forest without replacing it does not renew the wood.

A nonrenewable source has a limited supply that nature cannot replace within a human lifetime. Coal, oil, and natural gas take millions of years to form. Uranium is also a limited resource in the ground.

Separate the source from the carrier

Electricity is a way to carry energy. To sort its source, find out how the electricity was made. Wind-generated electricity comes from a renewable source. Coal-generated electricity comes from a nonrenewable source.

Nuclear power is nonrenewable even though it does not burn fuel like a fire. Uranium is dug from a limited supply. Smoke is not the test for whether a source is renewable.

Compare availability as well as supply

Sunlight and wind will remain available as resources, but a particular solar panel or wind turbine cannot deliver electricity all the time. Solar panels need light, and wind turbines need moving air.

A community can use batteries, water kept behind dams, or other sources when sunlight or wind is low. Fuel-powered plants can provide energy when needed while fuel is available. Their fuel supplies are limited, and burning fossil fuels pollutes the air.

Getting and using energy changes our surroundings. A source can help people and still have drawbacks. Compare the same features for each source so your choice has clear reasons.

Look at air, land, water, and wildlife

Burning fossil fuels releases carbon dioxide. This gas traps heat in the air around Earth and is warming the climate. Burning also releases air pollutants that harm health. Mines can damage land, and oil spills can harm water and living things.

Other sources have effects too:

• Dams can flood valleys and block fish traveling upriver.
• Wind turbines can harm birds and bats and change the view.
• Solar farms cover land. Making panels needs mined materials.
• Nuclear plants produce no smoke or carbon dioxide while running. Used nuclear fuel is radioactive and needs safe storage for thousands of years.

No source is free of environmental effects. Ask whether it will run out, when it is available, what pollution it causes, how it affects wildlife, and what it costs.

Make a fair comparison

A town compares coal power with a wind farm. Coal can supply electricity day and night while fuel is available. But its supply is limited, and burning it releases carbon dioxide and air pollution.

Wind needs no fuel to burn and gives off no smoke while running. Its source will not run out. On calm days, however, its output is low. The town must plan another supply or use stored electricity. It also needs to consider wildlife and the view.

Save energy without losing the useful job

Switch off lights and screens you are not using. LED bulbs need less electricity for the same brightness. Close doors and windows while heating is on. A sweater can help you stay comfortable with a lower heat setting. Walking, biking, or taking a bus can reduce fuel use for trips.

Insulation slows heat leaving a house. The furnace runs less to keep the same temperature. That saves fuel and money and reduces pollution from burning fuel.

Energy itself is never destroyed. Saving it means using less electricity or fuel before the energy spreads into the surroundings. Fuel that is not burned remains available for another time.