SPM Form 4 Physics - Heat

Physics (Form 4)

Chapter 4: HEAT

4.1 Understanding Thermal Equilibrium

DefinitionDefinition

Temperature The measure of degree of hotness of an object.SI unit: Kelvin, K

Heat A form of energy.SI unit: Joules, JEg: Heat is transferred from a hotter object (higher temperature) to a cooler object (lower temperature).

Thermal contact

Two objects are in thermal contact when heat energy can be transferred between them.

Heat transfer When two objects with different degree of hotness come into thermal contact, heat energy is transferred between these two objects.

Mechanism of thermal equilibrium

• Heat energy is transferred at a faster rate from the hotter object to the cooler object.

• Energy is also transferred from the cooler object to the hotter object, but at a much slower rate.

• There is a net flow of energy from the hotter object to the cooler object.

• The hotter object cools down, while the cooler object warms up.

• After some time, energy is transferred at the same rate between the two objects.

• There is NO net heat transfer between the objects.

• These two objects are said to be in thermal equilibrium.

Thermal equilibrium

• When two objects are in thermal equilibrium, there is ___________________________ between them.

• Two objects in thermal equilibrium have the _________ temperature.

Example of application of thermal equilibrium in daily lives

Wet towel & fever• Wet towel is places on your forehead when you

have fever.• Initially, temperature of the cloth is lower the your

body temperature.• Heat energy is transferred from ____________ to

______________ until ____________________________ is reached.

• The towel is then rinsed and the procedure repeats.• This way, heat energy is removed from you.

Cooling drinks• Ice cubes are added to a hot drink.• Heat from the ______________ is transferred

to the ____________ until ________________________ is reached.

• The final temperature of the drink is the same as the final temperature of the ice cubes.

Liquid-in-glass thermometer

• Characteristics of liquid used:– Easily seen– Expand and contract rapidly and uniformly over a

wide range of temperature– Does not stick to the glass wall of the capillary

tube

How does it work?

• The bulb of the thermometer contains a fixed mass of mercury. The volume of mercury increased when it absorbs heat.

• The mercury expands and rises in the capillary tube.

• The length of the mercury column in the capillary tube indicates the magnitude of the temperature.

How is a thermometer calibrated?

• A temperature scale is obtained by choosing 2 temperature, called fixed points.

Fixed point Definition ValueIce point Temperature of

pure melting ice00C

Steam point Temperature of steam from boiling water under standard atmospheric pressure.

1000C

• When two fixed points have been marked on the stem of the thermometer, the range between them is divided equally into 100 divisions. The thermometer now has a scale.

Working principle of a thermometer

• When the thermometer is placed in contact with hot water, heat is transferred from the hot water to the thermometer.

• Thermal equilibrium between the thermometer and the hot water is reached when the net rate of heat transfer is _______.

• The thermometer and the water are at the same temperature.

• At this point, the thermometer reading shows the temperature of the water.

Characteristics of mercury

1. Good heat conductor.2. High boiling point, 3570C.3. Expands uniformly when heated.4. Opaque and can be easily seen.

**Freezing point of mercury is -390C. Therefore is is not suitable for measuring temperatures below this temperature, like in the north pole.

How to increase sensitivity of mercury thermometer?

1. Thin capillary tube.2. Glass bulb with thin wall.3. Large bulb.

4.2 Understanding specific heat capacity

Definitions• Heat capacity, C:– The amount of heat required to change the temperature

of an object by 10C.• Specific heat capacity, c:– The amount of heat required to increase 1kg of

substance by 10C.

Quantity of heat absorbed or lost by a substance:

What does specific heat capacity of aluminium 900 J/kg 0C mean?

900 J of heat energy needs to be supplied to 1kg of aluminium to produce a 10C temperature

increase.

What does specific heat capacity of water 4200J kg-1 0C-1 mean?

Answer:

Physical meaning of specific heat capacity, c

• When two objects of the equal mass are heated at equal rates, the object with smaller specific heat capacity will have a faster temperature increase.

• When two objects of the equal mass are left to cool down, the temperature of the object with smaller specific heat capacity will drop faster.

Substance with small value of specific heat capacity

1. Heats up and cools down at a faster rate.– Eg: Metals like iron, steel, copper and aluminum

are used as pots and pans between they can be quickly heated up when there is only small heat absorption.

2. Sensitive to temperature change.– Eg: A thermometer has low specific heat capacity.

It enables heat to be easily absorbed and released even when small quantities of heat are involved.

Substance with high value of specific heat capacity

1. Heats up and cools down at a slower rate. Requires more heat to raise its temperature by a specific amount.– Poor heat conductor handle of pot/pan

2. Can absorb a great amount of heat without a high increase in temperature.– Water: used as a cooling agent in a car radiator.

Application of specific heat capacityCooking pot:1. Copper base:– Low specific heat capacity. The pot becomes hot very quickly,

enables quick cooking of food.– High density. Heavier base ensures that the pot is stable and

will not topple easily.2. Wooden handle:– Large specific heat capacity. Poor heat conductor: handle will

not become too hot when heat is absorbed.3. Aluminum body:– Relatively low specific heat capacity. Pot becomes hot very

quickly.– Low density.– Does not react with food in the pot when heated.

Sea breeze: (daytime)• Land has smaller specific heat capacity than

the sea.• In daytime, land has a faster increase in

temperature. Land is warmer than the sea.• Air above the land is heated up and rises.• Cooler air from the sea moves towards the

land as sea breeze.

Land breeze: (Nighttime)• At night, heat is lost from the land and sea.• The sea has a larger specific heat capacity,

release heat slower, therefore the sea is warmer than the land.

• Warm air above the sea rises.• Cooler air from the land moves to the sea as

land breeze.

Cooling system of car engine:

Water is cheap and has a ____________________________________. Therefore, it is a preferred cooling agent.A water pump circulates the water inside the engine. Heat produced by the engine is __________________ by the water that flows along the space in engine walls. The hot water then flow to the radiator where heat is lost to the cooler air that flows through the cooling fan.

A boy drinks the hot soup with a spoon. If he accidentally spills a spoonful of soup onto his hand, he would only experience slight pain. But if he spills the whole bowl of soup onto himself, he would suffer serious injury.Why?

• The mass of the spoonful of soup is smaller than the mass of a bowl of soup, although both have the same temperature and the same specific heat capacity.

• .

• Mass is directly proportional to the quantity of heat.• Soup in the bowl contains more heat.

Example 1:

• Calculate the total heat that is absorbed by a copper block of mass 500g which has been heated from 31˚C to 80˚C.[Specific heat capacity of copper = 390 J kg-1 ˚C-1]

Example 2:• When an electric heater is supplied with an electric

power of 2kW to heat 4kg of water for 1 minute, calculate the increase in temperature of the water.

[specific heat capacity of water = 4200 J kg-1 ˚C-1] Assume there is no heat loss to the surrounding.

Example 3:• A lead bullet moves horizontally with a velocity of 130 ms-1

and embedded into a cement wall after collision. If the specific heat capacity of lead = 130 J kg-1 ˚C-1 and all the heat produces is absorbed by the bullet, what is the increase in temperature of the bullet?

Example 4:• An aluminum block of mass 1 kg is heated by an electric

heater for 3 minutes and a temperature rise of 15˚C is recorded. If the electric heater is connected to a voltmeter which gives a reading of 30V and an ammeter which gives a reading of 2.5A, calculate the specific heat capacity of aluminum.

Example 5:

• 300g of water at temperature of 40˚C is mixed with 900g of water at temperature of 80˚C. Assuming there is no heat loss to the surrounding, what is the final temperature when thermal equilibrium is achieved by the mixture of water?

4.3 Understanding specific latent heat

Latent heat

• Definition: The heat absorbed or the heat released at constant temperature during

change of phase.

4 main changes of phase

Melting:When a solid melts, latent heat of fusion is absorbed but the temp remains constant at its melting point. Solidification:

For a liquid to solidify at its freezing point, latent heat of fusion has to be removed.

Boiling:When a liquid is boiling, latent heat of vaporization is absorbed but the temp remains constant at its boiling point.

Condensation:When vapour condenses back into liquid, latent of vapourization is released.

A

B

C

D

E

Heating graph

A

• PQ: Solid heated to its melting point.• Heat energy: Absorbed• Temperature: Increases• State of matter: No change• Contents: Solid

B

• Q: Solid begins to melt• QR: Solid is melting• S: Entire solid has melted• Heat energy: Absorbed• Temperature: Constant• State of matter: Solid to liquid• Contents: Solid and liquid

C

• RS: Liquid heated to its boiling point• Heat energy: Absorbed• Temperature: Increases• State of matter: No change• Contents: Liquid

D

• S: Liquid begins to boil• ST: Liquid is boiling• T: Entire liquid has boiled away• Heat energy: Absorbed• Temperature: Constant• State of matter: Liquid to gas• Content: Liquid and gas

E

• TU: Gas is heated• Heat energy: Absorbed• Temperature: Increases• State of matter: No change• Contents: Gas

Cooling graphA

B

C

D

E

A

• PQ: Gas cools down to its boiling point• Heat energy: Released• Temperature: Decreases• State of matter: No change• Content: Gas

B

• Q: Gas begins to condense• QR: Gas condenses• R: Entire gas has condensed• Heat energy: Released• Temperature: Constant• State of matter: Gas to liquid• Content: Gas and liquid

C

• RS: Liquid cools down to its freezing point• Heat energy: Released• Temperature: Decreases• State of matter: No change• Content: Liquid

D

• S: Liquid begins to freeze• ST: Liquid is freezing• T: Entire liquid has freeze• Heat energy: Released• Temperature: Constant• State of matter: Liquid to solid• Content: Liquid and solid

E

• TU: Solid cools down• Heat energy: Released• Temperature: Decreases• State of matter: No change• Content: Solid

Common characteristics of the 4 processes in the change of phase

1. Substance undergoes a change of phase at a particular temperature (melting point, boiling point, freezing point)

2. Heat energy is transferred during change of phase.

3. During change of phase, TEMPERATURE REMAINS THE SAME even though there is transfer of heat.

Notes

**Temperature of a substance is proportional to the average kinetic energy of its particles.**1. Temp increases when the average kinetic

energy of the particles increase.2. Temperature ___________ when the average

kinetic energy of the particles ___________.3. Temperature remains constant when the

average kinetic energy does not change.

Why does temperature remain constant during change of phase??

• During change of phase, the transfer of heat does not cause a change in the kinetic energy of the molecules.

• During melting, the heat absorbed is used to break up the bonds between the particles. The particles are freed from their fixed positions and are able to vibrate and move.

• During boiling, heat absorbed is used to break the bonds between the particles and to work against the atmospheric pressure when gaseous vapour expands into the atmosphere.

Specific latent heat, l

• Definition: Amount of heat required to change the phase of 1kg of a substance at a constant temperature.

Specific latent heat of fusion

• The amount of heat required to change 1kg of a substance from solid to liquid

phase without a change in temperature.

Specific latent heat of vaporization

• The amount of heat required to change __1kg___ of substance from ___liquid___

to ___gas____ phase without __change______ in temperature.

Specific latent heat of fusion of ice is 33600 J kg-1.

Explanation:

33600 J of latent heat is needed for 1kg of ice to melt to become water at 0˚C.

Specific latent heat of vaporization of water is 2.26x106 J kg-1.

Explanation:

• When the heat added or removed changes the temperature of an object, heat is calculated using:

• When the heat added or removed changes the phase of an object at CONSTANT TEMPERATURE, heat is calculated using:

• When heat is supplied electrically:– Q = Electrical energy = Pt– P = power of heater in Watt, W– t = time, in seconds, s

Can also be written as, Pt = ml

Example 1:

• The specific latent heat of fusion of ice is 33600 J kg-1. What is the quantity of heat required to melt 2.5kg of ice at 0˚C?

Example 2:

• An electric kettle contains 3kg of water. Calculate the amount of heat required to boil away all the water after the boiling point has been reached. [Latent heat of vaporization of water = 2.26x106 J kg-1]

Example 3:• What is the quantity of heat that is required to convert 4g of

ice into steam at 100˚C? [specific latent heat of vaporization of water is 2.26x106 J kg-1 ; specific latent heat of fusion of ice is 33600 J kg-1 ; specific heat capacity of water = 4.2x103 J kg-1 ˚C-1]

Application of specific latent heat1. Drinks can be cooled by adding ice cubes. When ice melts, a

large amount of heat is absorbed, this lowers the temperature of the drink.

2. Freshness of fish and meat is maintained by keeping them with ice. Ice absorbs heat from the fish as it melts, thus food can be kept longer.

3. Cooking by steaming. Water has a large specific latent heat of vaporization. When steam condenses on the food, latent heat is released directly onto the food, enabling food to be cooked at a faster rate.

4. Our bodies are cooled down after sweating. Latent heat of vaporization is absorbed from the body when sweat evaporates. Heat is removed from the body.

5. When water is boiling in a pot, always be careful when opening the lid. Water has a large specific latent heat of vaporization. When steam

condenses on our skin, large amount of latent heat release can cause serious burn.

4.4 Understanding GAS LAW

1. Boyle’s law2. Charles’ law3. Pressure law

Boyles’ law• States that for a fixed

mass of gas, the pressure of the gas, P is inversely proportional to its volume, V when the temperature, T is kept constant.

• When the volume of a gas in decreased, the number of molecules per unit volume increase.

• The same number of molecules move in a smaller space.

• The molecules collide more frequently.• The increase of rate of collision increases the

pressure exerted by the gas.

Charles’ law

• States that for a fixed mass of gas, the volume of the gas, V is directly proportional to its absolute temperature, T when its pressure, P is kept constant.

• When a gas is heated, the average kinetic energy of the molecules increases. The temperature of the gas increases.

• The rate of collision between the molecules and the walls will increase if the volume is constant.

• If the gas is allowed to expand, the faster molecules move in a bigger space.

• Therefore, the rate of collision between molecules and the wall remain constant and the pressure remain constant also.

Pressure’s law

• States that for a fixed mass of gas, the pressure of gas, P is directly proportional to its absolute temperature, T when its volume, V is kept constant.

• When a gas is heated, the average kinetic energy increases. The temperature of the gas increases.

• The faster moving molecules strike the walls of the container more frequently.

• Thus, the pressure of the gas increases.

Universal gas law

Absolute temperature• Temperature measured in Kelvin, K.• Convert ˚C to K: x + 273• Convert K to ˚C: x - 273

Absolute zero• The lowest possible temperature which is -273˚C or 0 K. • 0 K = -273˚C• At this point:– Volume and pressure of gas is zero– Kinetic energy of gas molecules is zero– Gas molecules are stationary.

Example 1:

• The air in a foot pump has an initial volume of 2800 cm3 and pressure 100kPa. The outlet of the pump is closed and the piston pushed inwards until the volume of the air becomes 700cm3. What is the pressure of the compressed air in the pump?

Example 2:

• The pressure of a bubble under the sea is 120 cm Hg. When the bubble rises to the surface of the sea, its volume becomes 25.0 cm3. Assuming that the atmospheric pressure is 76 cm Hg, what is the original volume of the bubble?

Example 3:

• A cylinder contains 200 cm3 of gas at a temperature of 27˚C. The gas is heated until its temperature increases by 30˚C. If the piston of the cylinder expands under constant pressure, what is the final volume of the gas?

Example 4:

• A fixed mass of gas in an enclosed metal container has a pressure of 2.5x105 Pa. If the has is heated from 27˚C to 87˚C, calculate the final pressure of the gas.

Universal gas law: