13-2: Energy Transfer
Monday, January 04, 2016
10:14 AM
Key Terms:
1. Thermal conduction - the transfer of energy as heat through a material.
2. Convection - the movement of matter due to differences in density that are caused by temperature
variations.
3. Convection current - the vertical movement of air currents due to temperature variations.
4. Radiation - the energy that is transferred as electromagnetic waves, such as visible light and infrared
waves.
5. Specific heat - the quantity of heat required to raise a unit mass of homogeneous material 1 K or 1⁰C in
a specified way given constant pressure and volume.
The three pictures all show examples of energy transfer. Answer the questions about what
happens in each picture, and identify how the heat got from one object to another in each
case.
1. Why is it a bad idea to drink hot cocoa out of a tin cup? Explain the energy transfers on
the atomic level.
2. What happens to your hand when you place it above a lighted candle? (Assume you are
not touching the flame. Explain the energy transfers on the atomic level. Hint: Remember
that warm air rises.)
3. When you sit near a fire, you can feel its warmth on your skin, even if you are in cool air.
Does this sensation depend upon the fact that warm air rises?
Ch13 Heat and Temperature Page 1
I. Methods of Energy Transfer
The transfer of heat energy from a hot object can occur in three ways:
○ Thermal conduction is the transfer of energy as heat through a material.
○ Convection is the movement of matter due to differences in density that are
caused by temperature variations.
○ Radiation is the energy that is transferred as electromagnetic waves, such as
visible light and infrared waves.
Thermal Conduction
Conduction involves objects in direct contact.
Conduction takes place when two objects that are in contact are at unequal
temperatures.
Convection
Convection results from the movement of warm fluids.
During convection, energy is carried away by a heated fluid that expands and
rises above cooler, denser fluids.
A convection current is the vertical movement of air currents due to temperature
variations.
Radiation
Radiation is energy transferred as heat in the form of electromagnetic waves.
Unlike conduction and convection, radiation does not involve the movement of
matter.
Ch13 Heat and Temperature Page 2
Radiation is therefore the only method of energy transfer that can take place in a
vacuum.
Much of the energy we receive from the sun is transferred by radiation.
Which type of heat transfer is taking place in the following examples:
1. The sun heats a car.
2. You touch the door handle on a hot car.
3. The hot air from inside the car warms your face as you open the door.
II. Conductors and Insulators
Any material through which energy can be easily transferred as heat is called a
conductor.
Poor conductors are called insulators.
Gases are extremely poor conductors.
Liquids are also poor conductors.
Some solids, such as rubber and wood, are good insulators.
Most metals are good conductors.
III. Specific Heat
Specific heat describes how much energy is required to raise an object’s
temperature.
Specific heat is defined as the quantity of heat required to raise a unit mass of
homogenous material 1 K or 1°C in a specified way given constant pressure and
volume.
Specific Heat Equation:
energy = (specific heat)  (mass)  (temperature change)
Practice Problem
Specific Heat How much energy must be transferred as heat to the
420 kg of water in a bathtub in order to raise the water’s temperature
from 25°C to 37°C?
Ch13 Heat and Temperature Page 3
1. List the given and the unknown values.
Given:
t = 37ºC – 25ºC = 12ºC = 12 K
T = 12 K
m = 420 kg
c = 4186 J/kg• K (from table in textbook)
Unknown: energy = ? J
2. Write down the specific heat equation.
energy = cmt
3. Substitute the specific heat, mass, and temperature change
values, and solve.
energy =
energy = 21,000,000 J = 2.1
104 kJ
Quiz: Determine what type of energy transfer is described by each phrase below.
1. Transfer of energy by electromagnetic waves
2. Transfer of energy between colliding particles
3. Transfer of energy by the movement of fluids
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