Particles in Motion Chapter Notes | Science for Grade 7 PDF Download

Introduction

Everything around us, like wood, metal, water, and air, is made up of tiny pieces called particles. This chapter, "Particles in Motion," helps us understand how these particles move in solids, liquids, and gases. We will learn why some things feel colder or hotter, how particles carry energy, and how their movement changes with temperature. By studying this, we discover how particles affect the world around us, from the smell of food to the size of a balloon!

How do particles move?

• Particles are tiny bits of matter that make up solids, liquids, and gases.
• Even when an object looks still, its particles are moving.
• Particles in different materials move in different ways, affecting how the material feels or behaves.

Movement and Collisions

• In liquids like water, particles are always moving and bumping into each other.
• This movement causes things like food coloring to spread out in water, a process called diffusion.
• Diffusion is when particles move from an area with more particles to an area with fewer particles.
• Diffusion takes time and continues until particles are evenly spread in a container.
• When food coloring spreads out completely, the liquid becomes one color.

What determines how much energy particles have?

• Particles have energy because they are moving, just like a rolling ball has energy.
• The faster particles move, the more energy they have.
• This energy is called kinetic energy, which is the energy of motion.
• Adding energy, like heat, makes particles move faster, increasing their kinetic energy.

Movement and Energy

• Scientists study how quickly a substance's particles move by observing diffusion.
• The faster the diffusion, the faster the particles are moving.
• In an experiment, when energy was added using a hot plate, the water and dye particles gained energy.
• This added energy increased their motion energy, also called kinetic energy.
• As kinetic energy increased, the speed of the particles also increased.
• The faster particles move, the more kinetic energy they have.

How to Model Movement

• In still images, motion lines are used to show how particles move.
• Because particles move at different speeds, they are shown with different numbers of motion lines.
• More motion lines mean the particle is moving faster.

Did You Know?

James Prescott Joule

• In the late 1700s and early 1800s, scientists began studying heat, called thermodynamics.
• James Prescott Joule, an English scientist, studied particle motion in matter.
• His ideas were not always accepted, but they were correct and important.
• The unit of energy, called the joule (J), is named after him.
• Joule said particles in all materials move very fast, even if the object looks still.
• These tiny particles, or atoms, are too small to see, even with strong microscopes.
• Joule explained that faster particle movement increases temperature.
• Heat can spread between objects by particles passing motion to each other, called conduction.
• If particles move too fast, they may try to move apart due to a force called centrifugal force.

What happens to a liquid when kinetic energy changes?

Energy and Volume

• When the temperature of a material increases, its particles move faster.
• These fast-moving particles collide more often and push each other farther apart, which causes the material to expand in volume.
• This process is called thermal expansion.
• The opposite happens when a substance loses kinetic energy:
  • The particles move more slowly.
  • They collide less often.
  • The material takes up less space.
• This decrease in volume is called thermal contraction, which occurs when particle motion decreases and the volume gets smaller.

Energy and Temperature

• Thermal expansion and contraction can be used to help measure temperature.
• Temperature is the measure of the average kinetic energy of particles in a material.
• The more kinetic energy the particles have, the higher the temperature.
• The less kinetic energy, the lower the temperature.
• One way to estimate particle speed or energy is to measure how much a substance expands or contracts.

Temperature Scales

• Temperature is measured using scales like Celsius, Fahrenheit, and Kelvin.
• A scale uses two fixed points, like the freezing and boiling points of water.
• In the Celsius scale, water freezes at 0°C and boils at 100°C.
• The Kelvin scale starts at 0 K, called absolute zero, where particles stop moving completely.
• Scientists have not cooled anything to absolute zero, so particles always have some motion.
• 0°C (Celsius) equals 32°F (Fahrenheit).
• Scientists use Celsius and Kelvin scales for experiments.

How do particles in a gas behave compared to particles in a liquid?

• Gas particles move faster than liquid particles and have more kinetic energy.
• Gas particles move in straight lines until they hit something, like another particle.
• Collisions change the speed and direction of gas particles.
• Gases can expand and contract, like liquids.
• The smell of food spreading through a room shows gas particles are moving.
• People closer to the source smell it first because gas particles take time to spread.

What evidence is there that particles in a solid move?

• Solids, like wood and metal, have a fixed shape and do not flow like liquids or gases.
• Particles in solids do not move freely; they vibrate in place.
• Solid particles have less kinetic energy because they only vibrate.
• Solids can expand and contract, but it is less noticeable than in liquids or gases.

How does the total amount of a substance affect its energy?

• Kinetic energy is one part of a substance’s total energy.
• Two substances at the same temperature have the same average kinetic energy per particle.
• A substance with more particles (more mass) has more total energy.
• For example, 100 grams of water has more particles and more total energy than 20 grams of water at the same temperature.
• More particles mean more total energy in a substance.

A Closer Look: Thermal Expansion in Solids

• Thermal expansion happens in solids, liquids, and gases when particles gain energy.
• In solids, thermal expansion causes small changes that you can see.
• Engineers use expansion joints in bridges and sidewalks to allow for expansion and contraction.
• For example, the Golden Gate Bridge has metal grates that let it expand in heat and contract in cold.
• Concrete in sidewalks or roads can crack if there are no expansion joints to handle thermal expansion.
• Thermal expansion is used in thermometers to measure temperature.
• Hot air balloons float because heated air expands and becomes less dense.