# Section 10.2 The Flow of Energy

Mechanics

Oct 27, 2013 (4 years and 8 months ago)

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Section 10.2

The Flow of Energy

1.
To understand how heat energy flows and how it is
measured

2.
To understand how substances differ in their capacity
to absorb and release heat

3.
To solve problems involving heat energy

Learning Objectives

Section 10.2

The Flow of Energy

Recall: Exothermic vs. Endothermic processes

Heat flows when there
is a

from
higher to lower
temperature

An
exothermic

process
gives off heat (system
loses energy)

An
endothermic

process
gains energy)

Section 10.2

The Flow of Energy

A.
Thermodynamics

Thermodynamics

study of energy

First law of thermodynamics

Energy of the universe is constant:

just as matter is conserved, energy is conserved

Section 10.2

The Flow of Energy

Internal energy, E

sum
of kinetic and potential
energies of all the
“particles” in a system

A.
Thermodynamics

Internal energy can be changed
by two types of energy flow:

Heat (q)

Work (w)

E = q + w

Section 10.2

The Flow of Energy

Thermodynamic quantities always consist of 2 parts:

A.
Thermodynamics

A number (magnitude of the change)

A sign (indicates the direction of flow)

Reflects the system’s point of view

X

X

Q =
energy
flowing

(heat)

Section 10.2

The Flow of Energy

B. Measuring Energy Changes
-

Units

Common energy units for heat

Calorie (c)

the amount of energy (heat) required to
raise the temperature of one gram of water by 1
o
C.

* Note:
Food calorie
(C) =
1000 calories
or 1kCal *

Joule

(J)

1 calorie = 4.184 joules

Section 10.2

The Flow of Energy

B. Measuring Energy Changes

The energy (heat) required to change the temperature of a
substance depends on:

1.
The
amount
of substance being heated (mass in g)

2.

The
temperature change
(no. of degrees,
∆T)

∆T =
T
final

T
initial

cont.

Section 10.2

The Flow of Energy

B. Measuring Energy Changes

3.

The
type of substance
, because different substances
react differently to heat:

1 calorie raises a gram of water by 1
o
C

1 calorie raises a gram of gold by 32
o
C!

Section 10.2

The Flow of Energy

Water has a relatively high specific heat

it absorbs a
lot of heat for a small change in temperature…

Section 10.2

The Flow of Energy

Great Britain

Chicago Latitude: 41.9
o

London Latitude: 51.5
o

Section 10.2

The Flow of Energy

Measuring Energy Changes

Ex.: Water

How much energy is needed to raise the temperature of
7.40 g water from 29.0
o
C to 46.0
o
C?

4.184 joules (1 cal) are needed to raise one gram of water

7.40 g water require
7.40g x 4.184 joules
to be heated by

Since we want an
increase of 17 degrees
,

we must also multiply by 17.0:

4.184 J / g
o
C x 7.40 g x 17.0
o
C = 526 J

Section 10.2

The Flow of Energy

B. Measuring Energy Changes

Specific heat capacity
is the
energy required to change
the temperature of one gram
of a substance by one
Celsius degree.

* KNOW THE VALUE FOR WATER! *

Section 10.2

The Flow of Energy

B. Measuring Energy Changes

To calculate the energy required for a reaction:

Q = s

m

t

Q
lost

=
-
Q
gained