Sci Chap 10

Question 1

What is temperature?

Answer 1

Temperature is the degree of hotness or coldness of an object. For an object of a given mass, the temperature of the object provides a quantitative description of the energy in its internal (thermal) store of the object. The SI unit of temperature is kelvin (K). Other common units include degrees Celsius (°C), degrees Fahrenheit (°F). A thermometer can be used to measure temperature.

Question 2

What is internal (thermal) store?

Answer 2

Internal (thermal) store refers to the energy store that is made up of the total energy associated with the random motion of the particles in the system. This energy, stored kinetically, that is associated with the movement of the particles, is thus often referred to as its “microscopic kinetic energy”.

Question 3

What does a change in temperature indicate?

Answer 3

A change in temperature indicates a change in energy in the internal (thermal) store: a decrease in temperature indicates a transfer of energy out from the internal (thermal) store; an increase in temperature indicates a transfer of energy into the internal (thermal) store of the object.

Question 4

What happens when a substance is heated?

Answer 4

When a substance is heated, the energy in the kinetic store of the particles increases. The particles move faster and there is an increase in the average kinetic energy. This is indicated by an increase in the temperature of the substance.

Question 5

what happens when a sucstance is cooled?

Answer 5

When a substance cools down, the energy in the kinetic store of the particles decreases. The
particles move slower and there is a decrease in the average kinetic energy. This is indicated by a decrease in temperature of the substance.

Question 6

What is the direction of energy transfer (temp)?

Answer 6

Energy is transferred from a region of higher temperature to a region of lower temperature.

Question 7

When a substance is heated, its temperature increases quickly.
(a) What causes the temperature to increase? Explain in terms of kinetic particle theory.

Answer 7

When a substance is heated, energy is transferred to the internal (thermal) store of the substance due to the temperature difference, increasing the average kinetic energy of the particles. As the average kinetic energy of the particles increases, the temperature of the substance increases.

Question 8

The microscopic kinetic energy of the particles is different from the kinetic energy of an object. An object can be stationary and still have non-zero energy in its internal store.

Answer 8

noted w thanks.

Question 9

When a substance is heated, its temperature increases quickly.
(b) Which substance, helium gas or oil, will have a greater increase in temperature when heated? Why?

Answer 9

Particles in helium gas are held together by weaker intermolecular forces of attraction. Hence, they are able to move faster, increasing the average kinetic energy of the particles more, hence helium gas will have a greater increase in temperature.

Question 10

What effects does the transfer of energy via heating have on an object?

Answer 10

• When a substance is heated, it undergoes expansion and increases in volume. Density decreases.
• When a substance is cooled, it undergoes contraction and decreases in volume. Density increases.

Question 11

How do the relative strengths of the intermolecular forces between particles differ between the different states of matter?

Answer 11

The strength of the intermolecular force of the particles of a gas is the weakest, and it is the strongest in solids.

Question 12

How do the relative distances between the particles differ between the different states of matter?

Answer 12

The distance between the particles in a solid are the closest, whereas the distance between the particles in a gas are the furthest apart.

Question 13

Why does expansion and contraction occur? Provide examples.

Answer 13

Expansion is due to increase in distance between particles.
- Water pipes and oil pipelines can expand on hot days, causing them to burst. Hence, expansion bends are made in these pipes. This allows the pipes.

• Hot air balloon: When the air in the balloon is heated, it expands and becomes less dense than the surrounding air, causing the balloon to rise.

to expand without any damage.
Contraction is due to decrease in distance between particles
- Rivets are used to join steel plates
and girders.

Question 14

What happens during contraction and expansion?

Answer 14

The contraction or expansion of an object gives rise to large forces. Expansion and contraction may cause damage to structures that are constantly exposed to the environment.

Question 15

What are bimetallic strips?

Answer 15

A bimetallic strip consists of two different metals that expand at different rates when heated. For example, a strip of brass can be riveted to a strip of iron. As brass expands and contracts more than iron in a fixed period of time, the heated or cooled strip bends with the brass on the outside of curve when heated and bends with the iron outside of the curve when cooled.

Question 16

State the 3 processes through which energy can be transferred via heating from one place to another.

Answer 16

Conduction, convection, radiation.

Question 17

Define conduction with examples.

Answer 17

Conduction is the process of energy transfer through the passing on of vibrational motion from one particle to another without any movement of the medium or material.
It may take place in gases, liquids, and solids, but not in vacuums.
e.g. When a metal rod is heated, after some time, the other end will also heat up. Energy is transferred from one end of the metal rod to the other by conduction.
e.g. your hands feel cold as ice melts on them. Heat energy from your hands is conducted away from your hands by the ice.

The working principle of conduction includes:
1. Vibration of particles (applied to both metals and non-metals)
* Energy is transferred to one end of the object by
heating
* Particles nearer to the heat source vibrate more vigorously about their fixed positions
* Particles with more kinetic energy collide with neighbouring particles, transferring energy into their kinetic store.
* Neighbouring particles vibrate more vigorously about their fixed positions
* Process continues, and energy is transferred from the heat source to the other end, causing the
temperature of the cold end to increase.

and 2) Electron diffusion (only applicable to metals)
Electron diffusion can take place in metals as they contain free-moving electrons.
* Thermal energy is supplied to one end of the metal
* Free electrons at the end nearer to the heat source gain energy from interacting with the particles
* The electrons move quickly to the cooler end and transfer energy to the particles there by collision.
* The particles at the cold end gain energy and
vibrate more vigorously about their fixed positions
* The temperature of the cold end increases as a result.

Question 18

Define convection.

Answer 18

Convection is the process of energy transfer by means of convection current (bulk movement) of fluids (liquids and gases), due to a difference in density.

• Convection takes place due to the expansion of a fluid. When it is hot, its volume increases and its density decreases.
• The less dense fluid then rises while the denser fluid flows in to take its place.

e.g. convection currents when rice is boiled in a pot of water.
- As the water is heated, the hot, less dense water at the bottom rises to the top, causing the rice grains to rise towards the surface of the water.
- The cold, denser water from the top sinks to the bottom of the pot.
- The movement of the water itself carried warmer liquid away from the heat source and cooler liquid towards it. This process allows energy to be transferred to the entire liquid.

Convection in gases.
e.g. A hot air balloon rising. The heat source heats the air trapped within the balloon. The air in the balloon becomes hotter, expands in volume and becomes less dense, rising to lift the balloon off the ground.

e.g. Sea breeze.
During the day, the land heats up more readily than the sea. Air above the land becomes hotter, expands in volume, becomes less dense and rises. Air which is not as hot and above the sea gets drawn towards the land to fill the space left by the rising hot air, to form a sea breeze.

Question 19

What are liquids and gases known as collectively?

Answer 19

fluids.

Question 20

Define radiation.

Answer 20

Radiation is the process of energy transfer by electromagnetic waves.

It does not require a medium, and can take place in vacuums.
An object can emit and absorb radiation at the same time.

e.g. we feel warm sitting next to a campfire. Heat transfer by radiation takes place in all directions.

e.g. The black pipes on solar water heaters are good absorbers of radiation from the sun. Energy from the Sun is transferred to the pipes quickly. The water flowing in the pipes heat up more quickly.

Question 21

What does rate of energy transfer mean?

Answer 21

The amount of energy transferred from one system to another in one second.

Question 22

What are the factors that affect the rate of conduction?

Answer 22

1. Material of substance
   Good thermal conductors include most metals. Poor thermal conductors include non-metals such as wood, rubber, asbestos, plastic, glass, water and air. They are used to reduce the rate of energy transfer by conduction from one place to another. Most liquids are also poor thermal conductors, except for mercury, which is a metal.
2. State of matter
   The rate of transfer of energy through conduction occurs the fastest in solids, especially metals, at the particles are closely packed as compared to the particles of liquids and gases. Hence, the transfer of heat by colliding into less energetic, neighbouring particles in solids can take place very efficiently.

Water, a liquid, is a poor conductor of heat as the particles r are spaced further apart than the particles of solids. Hence, heat transfer by conduction in water is not as effective when compared to solids.

Question 23

What test can be carried out to identify good and poor conductors?

Answer 23

The touch test. * In a warm environment, good thermal conductors feel warmer to touch than poor conductors.
* In a cold environment, good thermal conductors feel colder to touch than poor conductors.

Question 24

Explain the heat pipe thermal cycle.

Answer 24

1) Working fluid boils and turns into vapour after absorbing thermal energy
2) Vapour migrates along cavity to lower temperature end
3) Vapour condenses back to fluid and is absorbed by the wick, releasing thermal energy
4) Working fluid flows back to higher temperature end

Question 25

suggest why a heat pipe is a better conductor of heat than a copper pipe.

Answer 25

In addition to particle collision and free electron diffusion, the vapour in the heat pipe migrates quickly to the other end of the heat pipe transfers energy to the other end quickly when it condenses back to fluid.

Question 26

Suggest and explain one application of such a heat pipe.

Answer 26

Heat pipes can be used in cooling systems of computers. As the computer is heated up after use, thermal energy needs to be transferred from the computer to the surrounding via the heat pipe very quickly, preventing the computer from being overheated.

Question 27

What factors affect radiation?

Answer 27

1. **Types of surfaces** - dark-coloured, dull, and rough surfaces are better emitters and absorbers of radiation than light-coloured, shiny, smooth surfaces. 2. **Temperature of object** - The higher the temperature of the object, the higher the rate of energy transfer by radiation from the object. e.g. the blue flame from a Bunsen burner is used for heating as it is hotter than the orange flame, and is able to transfer energy by radiation at a faster rate. 3. **Surface area of object** - An object with a larger surface area has a higher rate of energy transfer by radiation. e.g., An elephant's large ears help to lose heat energy at a faster rate via radiation than the small ears of a fox.

Question 28

State a use of poor thermal conductors.

Answer 28

Rubber and plastics are poor thermal conductors. These materials are used in many items such as oven gloves, table mats, sunshields, toilet seats, picnic food carriers and ground mats to minimize the transfer of thermal energy by conduction.

Question 29

State each part of a vacuum flask and their materials and the part they play in reducing the rate of heat transfer.

Answer 29

- The cap is made of a poor thermal conductor, such as plastic. - The mouth of the flask is stopped using a poor thermal conductor. It also minimize energy transfer through convection to the surroundings. - The wall is painted silver on the vacuum side to reduce radiation. - A vacuum is created between the glass walls by removing air. Conduction and convection cannot take place in a vacuum. Additionally, the vacuum flask also prevents heat from entering the flask, thus allowing a cold liquid to remain.

Question 30

Would the vacuum flask work as well in keeping its contents hot if its inner glass wall has cracked?

Answer 30

No. The vacuum between its double walled vessel would be no longer exist. Thermal energy would be lost through conduction and convection.

Question 31

How are buildings built to keep cool?

Answer 31

- Air-conditioners Convection currents set up by air-conditioners cool the rooms in the buildings. -Glass windows Shiny windows are poor absorbers of radiation. Glass is also a poor thermal conductor.

Question 32

Energy is transferred frm the electrical heater to the water in a kettle mainly thru: a. conduction b. convection c. convection and radiation d. conduction and convection

Answer 32

A

Question 33

If the heating element of the electric kettle is placed near the surface of the water, the surface water will be heated quickly. The water at the bottom takes a long time to be heated because: a. water does not allow convection currents to be set up. b. water is a poor thermal conductor. c. radiation cannot take place. d. conduction cannot take place.

Answer 33

B

Question 34

Metal cup A and styrofoam Cip B are cups to identical size shape. They contain the same volume off hot water at the same temperature and are left to cool in an air conditioned room. Which cup will the hot water cool down faster? (fixed ans. temp!)

Answer 34

Metal is a better thermal conductor than styrofoam. The rate of transfer of thermal energy from hot water in cup A to cooler surrounding via conduction is faster. Hence, the hot water in cup A will cool down faster than cup B.

Question 35

In terms of transfer of energy by heating , explain how we can keep warm by wearing many layers of clothes.

Answer 35

Air is trapped in between the layers of clothing. Since air is a poor thermal conductor, this will decrease the rate of transfer of thermal energy from our bodies to the surrounding via conduction.

Question 36

In terms of transfer of energy by heating , explain how the handle of a frying pan is made of plastic.

Answer 36

Plastic is a poor conductor of thermal energy. This will decrease the rate of transfer of thermal energy from the pan to the handle via conduction. Hence, the plastic handle will not become too hot during cooking and scald our hands.

Question 37

State on modification to the design of the electric kettle to make it safer to use.

Answer 37

Add a layer of insulation around the metal casing or change the handle of the kettle to a thermal insulator (e.g. plastic, rubber)

Question 38

Temperature and Heat are not the same. Why?

Answer 38

Heat is a form of energy, temperature is a measure of how much heat energy an object contains.

Question 39

Do particles move from a region of higher temperature to a region of lower temperature during conduction?

Answer 39

No. Heat energy is transferred to the particles through the collision of particles with neighbouring particles IN their FIXED POSITIONS.

Question 40

During a hot day, a student uses an umbrella. She finds that the metal rod is cooler than the wooden handle, although both are at the same temperature. Why is this so?

Answer 40

Metal is a better conductor of thermal energy than wood. Therefore, the metal rod would transfer thermal energy away from the hand at a HIGHER rate than the wooden handle. As the rate of thermal energy lost by the hand is greater, the hand on the metal rod would feel cooler.

Question 41

Where should the AC be installed? Explain why. __________________________________________ | ceiling | |_____ | | A | | |_____| | |_____ | | B | | |_____| | | | |_____ ______| | C | | D | |_____| floor | _____| |________________________________________|

Answer 41

A. AC will cool the surrounding air, causing it to sink due to higher density. By placing the AC as close to the ceiling as possible, the cooler air will travel a longer distance from the position of the AC to the floor, and push more warmer air towards the AC, forming convection currents.