Lesson 3

Internal EnergyLiceo Da Procida

Review

Last time: Heat propagation

•Conduction (two things touching, thermal conductivity)

•Convection (fluids flowing)

•Radiation (two things not touching, electromagnetic waves)

Today: Effects of heat propagation inside a material

Reminder Question

What is thermal conductivity?

1)A quantity measured in degrees Celsius

2)How much energy it takes to increase the temperature of a material

3)Energy transfer due to a temperature difference

4)A measure of how well a material conducts heat

Reminder Question

What is thermal conductivity?

1)A quantity measured in degrees Celsius

2)How much energy it takes to increase the temperature of a material

3)Energy transfer due to a temperature difference

4)A measure of how well a material conducts heat

Heating Different Materials

During the summer time, when you walk from the sand into the ocean, which is hotter?

Why could this be?

Sand vs. Water• The sun transfers heat to the sand and water

• What kind of heat propagation is this?

• The sand and water each get the same amount of heat from the sun

• Why does one get hotter than the other?

vs.

Heat Capacity• The sand and water are at different

temperatures because they have different heat capacities

• Heat capacity tells us how quickly a material’s temperature will increase for a given amount of heat transfer

• Think of a pie: the crust and filling get the same amount of heat from the oven, but which part do you burn your mouth on?

Heat Capacity vs. Thermal Conductivity

• Thermal conductivity = how well a material will conduct heat (heat passing, heat transfer)

• Heat capacity = how quickly something will heat up (heat staying, temperature)

• Heat going through the walls vs. temperature of the walls

vs.

Heat Capacity vs. Thermal Conductivity

PracticeAre each of these examples of heat capacity (1) or thermal conductivity (2)?

•I make a pizza and the tomato sauce gets hotter than the cheese

•I burn my mouth when I eat the pizza

•A pot doesn’t feel hot if I use a glove to hold it

•Gold increases its temperature more quickly than silver

•Walking barefoot across the street feels hotter than walking on the sidewalk

Specific Heat CapacityIf we look at the heat capacity per mass, we call this specific heat capacity.

A high specific heat capacity means that something takes a long time to heat up, while a low specific heat capacity means something heats up quickly.

Water has an extremely high specific heat capacity, while metals do not.

Calculating Specific Heat Capacity

We have a formula that relates:•heat (Q) •mass (m)•specific heat capacity (c)•change in temperature (ΔT)

Q = m c ΔT

Concept Question

Remember, Q = m c ΔT

If I increase the specific heat capacity, will it take more or less heat to increase the temperature by 1 degree?

1)More heat

2)Less heat

Concept Question

Remember, Q = m c ΔT

If I increase the specific heat capacity, will it take more or less heat to increase the temperature by 1 degree?

1)More heat

2)Less heat

Concept Question

Remember, Q = m c ΔT

Remember the beach question. Does sand or water have a higher specific heat capacity?

1)Sand

2)Water

Concept Question

Remember, Q = m c ΔT

Remember the beach question. Does sand or water have a higher specific heat capacity?

1)Sand

2)Water

VideoSpecific heat capacity/global warming

Specific Heat Capacity of Water

The specific heat capacity of a material is usually not a constant - it changes with temperature.

However, we generally only work around 15 deg. C, so we take the specific heat capacity of water as 1 C/g, or 4.184 J/g.

Internal Energy

• Remember, heat is a kind of ENERGY

• When you add heat energy to an object, where does that energy go?

• The energy is used to increase the temperature, and it is stored in the object

• Since it is stored in the object, we refer to this energy as internal energy

• Change in internal energy = heat added to the object

• ΔU = Q

Practice Problems

Remember, Q = m c ΔT

•How much heat is absorbed when 500. g of water , c = 4.184 J/goC, goes from 25.0oC to 35.0oC?

•What is the change in internal energy when 500. g of copper, c = 0.385 J/goC ,  goes from 25.0oC to 35.0oC?

•I have a cup filled with 0.150kg of coffee (c=4187J/kgC) at 70C. I add 0.01kg of milk (c=3800J/(kgC)) at 5.0C. What is the final temperature of the coffee and milk mixture?

•I made a bowl of minestrone soup (c=4187J/kgC). I then put 0.6kg of soup at 90C in a 0.2kg glass bowl (c=840J/kgC) that is initially at 20C, what will be the temperature of the soup and the bowl when they have reached equilibrium?