Exam #2 Specific Heat

Thermal Equilibrium Examples

-If two objects of different temperatures are placed in contact with each other, energy ALWAYS flows from the hotter to the colder object. -A cup of hot coffee becomes cool because the heat from the coffee flows out into the surrounding cooler air. -A glass of cold water becomes hot because the heat from the surrounding air flows into the cold water. Heat will not flow if the temperatures of the two are equal. This is thermal equilibrium.

Convection

-In liquids and gases, heat can be transferred by a movement of the substances from a hotter region to cooler region. -Convection of heat is dependent on the change in density of the substance when it becomes hotter. -We have seen that hot air rises. It rises because it is less dense -Less dense objects float on top of more dense objects.

Conduction in Gasses

-Particles in a gas are further apart than solids or liquids -They collide less often than solids and liquids, thus they conduct heat slower than solids or liquids.

Conduction in Liquids

-They are further apart. -They collide less often than solids thus they conduct heat slower than solids.

A 12.0 oz can of coke (355 mL) at 25.0 oC is placed into a freezer to chill. How much heat must be removed from the coke to reach a cool 4.0 oC? (Assume the specific heat capacity of coke is the same as water)

0.355 kg x 4190 J/kg oC x 21.0 oC = 31200 J Change of temperature 25o-4o=21.0 oC

A stainless steel spoon with a specific heat capacity of 450.0 J/kg oC is heated in a 300.0 oC oven. Then, 100.0 mL of 20.0 oC water is added to a bowl. The spoon is then taken out of the oven and placed in the bowl of water to cool. The water temperature in the bowl increases to 38.0 oC. What is the mass (in grams) of the spoon?

1ml=1g so 100.0 mL = 100.0g 100.0g = .1000Kg Heat released by the spoon = Heat gained by the water Q spoon = Q water Mass of spoon (kg) = 𝒎 𝒙 𝑪 𝒙 𝜟𝑻 (𝒘𝒂𝒕𝒆𝒓) / 𝑪 𝒙 𝜟𝑻 (𝒎𝒆𝒕𝒂𝒍) = 64.0 g

Another form of energy between Joule and calorie

4.18 joules= 1 calorie

Heating substances

As we heat a substance the particles increase their velocities and the spaces between particles increases. Therefore it occupies a greater space. In other words, its volume increases. Notice, that as the volume of the substance increases while its mass remains constant, the density decreases.

Sand and water

Both substances reach its highest temperature at the same time.

Temperature Celsius scale

Celsius Scale, where freezing point of water is 0o and boiling is 100o.

Which of the following substances could be heated from 20°C to 100°C most quickly? Assume identical heat sources are used on all substances

Gold

The specific heat of gravel is 890 J/kg oC and the specific heat of water is 4190 J/kg oC. You start the experiment with 200.0 grams of each substance at 20.0 oC and place them on a hot plate. After heating for 10 minutes, the temperature of the water is 32.0 oC and the gravel is76.5 oC. How much heat energy did they absorb on the hotplate? (Q = m C T) Are they equal?

Gravel 0.2000 kg x 890 J/g oC x 56.5 oC = 10100 J Change of Temp= 76.5-20.0= 56.5 Water 0.2000 kg x 4190 J/kg oC x 12.0 oC = 10100 J Change of Temp= 32.0-20.0= 12.0 The heat absorbed by the gravel and water is equal. Because of the higher specific heat capacity of water, the temperature did not increase as much as the gravel.

Thermal Equilibrium

Heat flow continues until the temperatures of the two substances are equal.

Heat

Heat is energy -Heat is the flow of thermal energy between two objects.

Conduction

Heat is transferred through the objects. -Heat is transferred by the atoms of the metal colliding with each other. -It is a transfer of kinetic energy from one particle to the other. -There is no movement of the particles from one spot to the other. -The rate at which objects conduct heat is dependent upon the substance. -Particles in a solid are closer together. They collied more often thus they can transfer kinetic energy faster.

How much heat energy (in joules) is used to raise the temperature of 25.0 g of water 10.0 o C?

Heat used (joules) = (mass H2O) x (specific heat of water) x (change in temp. o C) # joules = (0.0250 kg) x (4190 j/kg o C) x (10.0 o C) = 1048 joules

In the old days, on a cold winter night, it was common to bring a hot object to bed with you to keep you warm. Which would be better to keep you warm through the cold night - a 10 kilogram brick (similar in composition to sand) or a 10 kilogram jug of hot water at the same temperature? Explain your reasoning using appropriate scientific terms.

I pick the water because it has a higher specific heat capacity. When heating the brick and the water, the water required much more heat to raise its temperature than the brick. Therefore, under the covers, water has more heat to release, taking the entire night to cool off and transfer its heat to you.

If a ball is dropped from a height of 2 m, does it bounce back to its original height?Where did the energy go?

No! The difference between the height dropped and the height of the bounce is the energy lost. -Energy to compress the ball. -Energy to overcome air resistance. -And, the temperature of the ball increased (energy lost as heat) because of friction of the ball coming into contact with the floor

Heat Energy formula

Q=mass(kg) x C (j/kg c) x Change in Temp.

Is the amount of thermal energy absorbed by each substance equal, or did one substance absorb a greater amount of thermal energy than the other? Explain your reasoning.

Since the sand and the water were on the hot plate the same amount of time, both substances absorbed the same amount of thermal energy.

Temperature proportionality

Since we have twice as much water, then it takes twice as much energy to increase its temperature the same amount. Therefore, since we have double the water, we get half the change in temperature. We have twice as many particles, thus they get half the energy of the smaller container.

Value of Cmetal formula

Specific Heat (metal) = Mass (water) x Specific Heat (water) x Temp. (water) / Mass (metal) x Temp (metal)

Aluminum has a larger specific heat capacity than copper. If equal masses of aluminum and copper wire are placed in a flame, which metal will have the greatest increase in temperature?

Specific heat capacity and change in temperature are inversely proportional. The copper will undergo the greatest change in temperature.

Temperature and Kinetic Energies

Temperature is related to the kinetic energies of the particles of the substances. At the boiling point, the energies of the particles are equal. They have the same velocities. However, there are more particles in the gallon of water. There are more particles able to transfer energy to another object. There is greater energy flow or heat flow.

Which has a greater amount of heat?

The gallon of boiling water! Heat is the AMOUNT of heat flow. The gallon of boiling water can transfer MORE heat because it has MORE mass.

What is the relationship between the mass of water and the heat energy transferred to raise its temperature?

The greater the mass of water the greater the amount of heat energy needed to raise the temperature of the water.

a statement that describes the relationship between the temperature of water and the heat energy transferred to raise its temperature?

The greater the temperature change the greater the amount of heat energy absorbed when heated or released when cooled.

During a summer visit to the beach, you notice that the sand burns your bare feet but the wooden boardwalk does not. What must be true about the sand and the wood?

The sand has a lower specific heat value than the wood.

Thermodynamics

The study of Heat

Heat Capacity Substance or Inertia- objects resisting a change in motion.

The tendency of a substance to resist a change in temperature after absorbing or releasing some heat is the specific heat capacity of the substance.

By the Law of Conservation of Mechanical Energy

The work we put into a system is equal to the work we get out.

In the summer I like to bring my family to the beach and have a picnic. The night before I make sandwiches (ham & cheese) and watermelon slices and place them in containers stored in the refrigerator. At the beach the next day, a couple of hours after we arrived, I notice that the sandwiches are warm but the watermelon slices are still cold. Can you provide an explanation for these observations?

There is more water in the watermelon than the sandwiches. Because it will require more heat to raise the temperature of the water, the watermelon will remain cold while the sandwiches, which have a lower specific heat capacity than water, will warm faster. So eat the sandwiches first.

Take a gallon of boiling water and a cup of boiling water. Which is at a higher temperature?

They are equal (100o C

Heat formula proportinality

When Heat and mass are constant, then heat capacity and temperature are inversely proportional.

Thermal insulators

objects that transfer heat poorly

Thermal conductors

objects that transfer heat readily.