ENERGY – PHYSICAL SCIENCE

Turning on a lightbulb and running a mile both require energy, but not the same kind of energy. Energy is simply described as the ability to do work. It exists in many forms, including:

• Chemical energy — stored in a substance’s chemical bonds
• Electrical energy — carried by electrical charges
• Radiant energy — carried by electromagnetic waves, such as light
• Mechanical energy — related to an object’s motion and position
• Nuclear energy — stored in the nuclei of atoms
• Thermal energy — related to a substance’s temperature

Mechanical Energy

Mechanical energy depends on two characteristics of an object:

• Potential energy — energy an object has because of its position; often described as stored energy
• Kinetic energy — energy an object has because of its motion

An object’s total mechanical energy can be calculated by adding its potential and kinetic energy

Energy Transformations

Energy cannot be created or destroyed, but it can be converted (transformed) from one form to another.

Photosynthesis: plants convert sunlight (radiant energy) into food (chemical energy).

Movement: when you walk or run, chemical energy from food is converted into kinetic energy.

Body heat: some of the chemical energy is also converted into thermal energy as your body releases heat.

Waves

Energy can be transported through oscillations (repetitive movements) called waves. Waves can travel through substances or empty space.

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Waves

Energy can be transported by oscillations (repetitive movements) called waves. Waves can travel through substances (like water or air) or even empty space.

The following diagram shows two types of wave motion.

Types of Waves

Both types of waves in the diagram travel from left to right, but they differ in how particles in the medium move.

Transverse Waves

• Particle Motion: Up and down
• Wave Direction: Perpendicular to particle movement Example: A jump rope—if one person whips one end up and down while the other end is held steady, a transverse wave ripples through the rope.
• Energy Example: Light energy is transferred by transverse waves.

  Longitudinal Waves

• Particle Motion: Backward and forward
• Wave Direction: Parallel to particle movement Example: A toy spring—if one person quickly compresses one end while the other end is held steady, a longitudinal wave moves through the spring.
• Energy Example: Sound energy is transferred by longitudinal waves.

Parts of Waves

Two of the most important characteristics of a wave are wavelength and frequency.

Wavelength: Wavelength is the distance between corresponding parts of a wave.

Transverse Waves: Measured from crest to crest, trough to trough, or between any two corresponding points. Longitudinal Waves: Measured from compression to compression.

The  diagram shows how wavelength is measured in both wave types.

Frequency: Frequency is the number of wavelengths that pass a specific point per second.

In general, higher frequency = more energy.

> Example (from diagram): Top graph: 6 wavelengths per second Bottom graph: 3 wavelengths per second The top wave has twice the frequency and carries twice as much energy.

Relationship Between Wavelength and Frequency

Shorter wavelength = higher frequency = more energy

Longer wavelength = lower frequency = less energy

Waves with shorter wavelengths tend to carry more energy than waves with longer wavelengths.

Types of Electromagnetic Radiation

Electromagnetic radiation comes in many forms, with different levels of energy and potential risks. In general, the shorter the wavelength, the more dangerous the radiation. Radio waves and microwaves are considered relatively safe. High doses of infrared, visible light, and ultraviolet radiation can cause burns. X-rays and gamma rays can cause DNA mutations, cell destruction, and cancer.

When using these types of radiation, precautions are taken to minimize exposure. Prolonged exposure to any type of electromagnetic radiation should be avoided.

Heat

The particles that make up any substance are constantly moving. Temperature is a measure of the average speed (or kinetic energy) of these particles. Particles move slowest in a solid. Particles move faster in a liquid. Particles move fastest in a gas.

Example: Ice (solid) has a lower temperature than liquid water. Liquid water has a lower temperature than water vapor (gas).

Heat is the transfer of energy between substances because of a difference in their temperatures. Energy always moves from a substance with higher temperature to one with lower temperature. Example: When an ice cube melts, energy from the warmer air flows into the cold ice cube. This extra energy causes the ice cube’s particles to move faster. As a result, its temperature rises, and it changes from solid to liquid.

Energy and Work

Remember: Energy is the ability to do work. When heat is transferred to a substance; Work is done on its particles. The particles move faster. The substance’s temperature increases.

Heat Transfer

The following diagram shows the three main ways that heat can be transferred:

•  Conduction: Conduction transfers heat between two substances that are directly touching. Example: When you touch a hot pot handle, heat moves directly from the handle to your hand. Heat transfer continues until both substances reach the same temperature.
• Convection: Convection transfers heat by the movement of a liquid or gas.

 Example (diagram):As water at the bottom of a pot heats up, it becomes less dense. The warm, less dense water rises to the top. The cooler, denser water sinks to the bottom. This creates currents that circulate heat throughout the water.

• Radiation: Radiation transfers heat by electromagnetic waves—not through direct contact or movement of substances.

Examples (diagram): Feeling heat from a fire without touching it. Microwave ovens heating food. The sun heating the Earth.

Energy in Reactions

When chemical reactions occur, the total amount of energy in the products is often different from the energy in the reactants.

Chemical reactions can be classified based on whether they absorb or release energy:

Endothermic Reactions

In an endothermic reaction, the products contain more total energy than the original reactants.

The diagram shows the energy change for an endothermic reaction.

Key points about endothermic reactions: They must absorb energy (heat) from the environment. Because they pull in heat, they make their surroundings feel cold. Example; Instant cold packs contain two chemicals separated by a barrier. When the barrier is broken, the chemicals mix and undergo an endothermic reaction, absorbing heat and cooling the pack.

Exothermic Reactions

Exothermic reactions release energy into the environment.

The diagram shows the energy change for an exothermic reaction.

Key points about exothermic reactions: The products contain less total energy than the original reactants. Energy is released to the surroundings, often as heat or light. Examples: Burning fuels Combustion reactions Many oxidation processes

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Exothermic reactions make their surroundings feel warm because they release energy (usually heat) into the environment.

 Example: When wood burns in a fire, it undergoes an oxidation reaction, which is a type of exothermic reaction. The heat you feel from the fire is the released energy from this reaction.

Sources of Energy

Energy is involved in nearly every part of daily life: Using kitchen appliances, Heating and cooling buildings, Powering vehicles and devices. All of these activities require energy, which comes from a variety of sources.

Common Energy Sources

> The following table lists common sources of energy and briefly outlines their benefits and drawbacks.

Comparing Energy Sources

Every energy source comes with both advantages and disadvantages:

• Fossil Fuels (coal, oil, natural gas): Provide a large amount of energy. Release pollutants like smog and greenhouse gases (e.g., carbon dioxide); oil spills can damage ecosystems
• Nuclear Energy: Produces a large amount of energy, creates radioactive waste, which must be stored safely
• Solar Energy:  Clean, renewable, produces little to no pollution. Produces a limited amount of energy depending on weather and location

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