TODAY’S OUTCOMES:
HEAT
- Review heat capacity and how it relates
to thermal energy
- Study how different materials affect the
transfer of thermal energy
Reminder: Course Evaluation Window Is
Open until Dec. 7th
http://mercury.pa.uky.edu/~evaluation/
8. The wind certainly makes you feel cold. Yet a fan blowing on a thermometer does not
change the reading! (See for yourself! It has been set up at the front of the room). Even
though both thermometers and people have red noses, they differ in the effect that the
wind has on them. Discuss the other respects in which you differ from a thermometer that
that might explain this.
If a thermometer is at a steady temperature, it is in thermal equilibrium with its surroundings.
A human, being warm-blooded, is not in thermal equilibrium with its surroundings.
When wind blows, it carries thermal energy with it! Wind blowing against your body speeds up
the rate at which your body transfers thermal energy to its surroundings. A thermometer, being
in thermal equilibrium, isn’t transferring thermal energy to its surroundings, so there is not
transfer of energy to “speed up” - the temperature stays the same.
9. Dana likes to take long showers. Her mom complains that this uses up all the hot water. “Heating water
is expensive” she says.
A) If the tank holds 200 kg of water, how much energy is needed to make it “hot”?
“Hot” = about 50°C; room temperature is about 20°C, so the change in temperature is about
30°C.
Energy used/°C = 4184 Joules/kg/°C ✕ 200 kg = 836800 Joules/°C
Total energy = 836800 Joules/°C ✕ 30°C = 25,104,000 Joules
B) Electricity costs about 10¢ per kilowatt-hour (which is 3,600,000 Joules). How much does Dana’s
shower cost?
energy = 25,104,000/3,600,000 kilowatt-hours = 6.97 kilowatt-hours
at 10¢ per kilowatt-hour, this is just under 70¢.
HEAT CAPACITY
The heat capacity of a material is the amount
of energy required to raise 1 kg by 1°C.
Heat capacity of water = 4184 Joules/kg/°C
Heat capacity of aluminum = 897 Joules/kg/°C
Water has a much higher heat capacity
than aluminum.
1 kg of water can hold much more thermal
energy than 1 kg of aluminum!
HEAT CAPACITY
An equation you need to know how to use:
Thermal energy = mass ✕ heat capacity ✕ temperature change
thermal energy (like kinetic and potential
energy) is measured in joules
( If you took high school chemistry,
you might have seen this written as
Q = mc∆T
)
Thermal energy contains a lot of joules:
from the Unit on Force and Motion:
1m
1 joule is the amount of energy needed
to lift a 1 kg mass 1 meter high
1 liter of water has a mass of 1 kg
It takes over 4000
joules to raise the
temperature of 1 liter
of water by 1°C!
1 kg
WHAT YOU ARE EXPECTED TO KNOW:
- The definition of heat capacity
- What a differing heat capacity means for the
amount of thermal energy stored in a material
- How to use heat capacity to calculate the
amount of thermal energy transferred
TODAY’S OUTCOMES:
HEAT
- Review heat capacity and how it relates
to thermal energy ✓
- Study how different materials affect the
transfer of thermal energy
Reminder: Course Evaluation Window Is
Open until Dec. 7th
http://mercury.pa.uky.edu/~evaluation/