Temperature (1) - Oxford University Press

More documents

Recommendations

Info

98 THERMAL EFFECTS 5.01 � Water can exist in three forms: solid, liquid, and gas. (The gas is called water vapour, and it is present in the air.) Like all materials, water is made up of tiny particles. Which form it takes depends on how firmly its particles stick together. Solid Particles vibrate about fixed positions. Liquid Particles vibrate, but can change positions. Gas Particles move about freely. Moving particles Solids, liquids, and gases Every material is a solid, a liquid, or a gas. Scientists have developed a model (description) called the kinetic theory to explain how solids, liquids, and gases behave. According to this theory, matter is made up of tiny particles which are constantly in motion. The particles attract each other strongly when close, but the attractions weaken if they move further apart. Solid A solid, such as iron, has a fixed shape and volume. Its particles are held closely together by strong forces of attraction called bonds. They vibrate backwards and forwards but cannot change positions. Liquid A liquid, such as water, has a fixed volume but can flow to fill any shape. The particles are close together and attract each other. But they vibrate so vigorously that the attractions cannot hold them in fixed positions, and they can move past each other. Gas A gas, such as hydrogen, has no fixed shape or volume and quickly fills any space available. Its particles are well spaced out, and virtually free of any attractions. They move about at high speed, colliding with each other and the walls of their container. What are the particles? Everything is made from about 100 simple substances called elements. An atom is the smallest possible amount of an element. In some materials, the ‘moving particles’ of the kinetic theory are atoms. However, in most materials, they are groups of atoms called molecules. Below, each atom is shown as a coloured sphere. This is a simplified model (description) of an atom. Atoms have no colour or precise shape. oxygen atom hydrogen atom Iron atoms Water molecules Hydrogen molecules
Brownian motion: evidence for moving particles Smoke is made up of millions of tiny bits of ash or oil droplets. If you look at smoke through a microscope, as on the right, you can see the bits of smoke glinting in the light. As they drift through the air, they wobble about in zig-zag paths. This effect is called Brownian motion, after the scientist Robert Brown who first noticed the wobbling, wandering motion of pollen grains in water, in 1827. The kinetic theory explains Brownian motion as follows. The bits of smoke are just big enough to be seen, but have so little mass that they are jostled about as thousands of particles (gas molecules) in the surrounding air bump into them at random. View through microscope zig-zag paths of smoke bits Energy of particles The particles (atoms or molecules) in solids, liquids, and gases have kinetic energy because they are moving. They also have potential energy because their motion keeps them separated and opposes the bonds trying to pull them together. The particles in gases have the most potential energy because they are furthest apart. The total kinetic and potential energies of all the atoms or molecules in a material is called its internal energy. The hotter a material is, the faster its particles move, and the more internal energy it has. If a hot material is in contact with a cold one, the hot one cools down and loses internal energy, while the cold one heats up and gains internal energy. The energy transferred is known as heat. The term thermal energy is often used for both internal energy and heat. Q 1 Say whether each of the following describes a solid, a liquid, or a gas: a Particles move about freely at high speed. b Particles vibrate and cannot change positions. c Fixed shape and volume. d Particles vibrate but can change positions. e No fixed shape or volume. f Fixed volume but no fixed shape. g Virtually no attractions between particles. Related topics: energy 4.01; fusion and vaporization 5.11; atoms and elements 11.01 THERMAL EFFECTS microscope lamp smoke glass cell Kinetic energy Energy because of motion. Potential energy Energy stored because of a change in position or shape. glass cover 2 Smoke is made up of millions of tiny bits of ash or oil droplets. a What do you see when you use a microscope to study illuminated smoke floating in air? b What is the effect called? c How does the kinetic theory explain the effect? 3 If a gas is heated up, how does this affect the motion of its particles? 4 What is meant by the internal energy of an object? ! 99
Page 2 and 3: Now even more comprehensive - and w
Page 4 and 5: Electricity 8.1 Electric charge (1)
Page 8 and 9: 100 probe contains thermistor THERM
Page 10 and 11: 102 THERMAL T EFFECTS ice point 0
Page 12 and 13: 104 invar (metal) Pyrex glass plati
Page 14: Title Pub Date ISBN Price I/C* Qty

Temperature (1) - Oxford University Press

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?