Lesson 3

Specific HeatDa Procida: Year 4

ReviewLast week:

Internal Energy (U) : total energy of a system.

Heat (Q) : heat added to the system (positive) or taken away from the system. (negative)

Work (W) : Work done by the system (negative) or work done on the system (positive).

Concept QuestionWhat is heat?

1) A measure of how hot or cold something is.

2) A basketball team from Boston.

3) Energy transferred due to a temperature difference.

4) A quantity measured in degrees Kelvin.

AnswerWhat is heat?

1) A measure of how hot or cold something is.

2) A basketball team from Boston.

3) Energy transferred due to a temperature difference.

4) A quantity measured in degrees Kelvin.

Units of Heat

Heat is a form of energy, and so we measure it with either Joules or Calories.

4.184 Joules = 1 Calorie

When do we often talk about calories?

Food!We talk about kilocalories when we talk about food.

Why?

Our bodies require a certain amount of energy to function - about 2000 kcal a day.

What is a “Calorie,” exactly?

1 Calorie

is defined to be the amount of energy that it takes to heat 1 gram of water up by 1 degree.

Heating Different Materials

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

Why? Is there a different amount of heat being applied?

Heat CapacityEvery single substance has a quantity that we call heat capacity.

This quantity refers to how quickly or how slowly something will heat up.

Although the same amount of heat is applied to the water and sand, the sand heats up more quickly.

Pizza: tomato sauce heats up faster than cheese, so we burn our mouths from the tomato sauce!

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

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

Q = m c T

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.

Problems

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

How much heat is absorbed when 500. g of copper, Cp = 0.385 J/goC ,  goes from 25.0 oC to 35.0 oC?

Casey’s insulated foam cup is filled with .150kg of coffee (c=4187J/kgC) that is too hot to drink so she adds .01kg of milk (c=3800J/(kgC)) at 5.0C. If the coffee has an initial temperature of 70 C, how hot is the coffee after the milk is added? (What is the final temperature of the coffee and milk mixture?)

Rosetta has made a bowl of tagliolini (c=4187J/kgC) which she ladles from a pot into a glass bowl. If .6kg of soup at 90C is placed in a .2kg glass bowl (c=840J/kgC) that is initially at 20C, what will be the temperature of the soup when the soup and the bowl have reached equilibrium?