Chapter 5
Energy
Energy
o
Energy is the ability to do work
o
There are many different forms
n
Kinetic Energy: energy of motion
o
Depends on mass and velocity
n
Potential Energy: energy of an object due to
its position or condition
n
Mechanical Energy: energy of lifting, bending,
or stretching
o
The sum of all potential energy and kinetic
energy
How to Measure Energy
o
All energy in measured in the units called
joules (J)
o
To calculate work:
n
The transfer of energy as a result of motion
Work = Force x Distance
W = F x d
Types of Energy
o
Thermal energy: heat energy
n
Temperature: measure of average kinetic energy
of the particles in matter
o
Chemical, Electrical, Magnetic, Radiant, and
Nuclear energy are other forms discussed in several other chapters in physical
science
How to Measure Heat
o By using the specific heat of a substance you can
calculate thermal energy changes
o Specific Heat: the amount of energy that is needed to
increase the temperature of 1 kg of any substance 1° C.
n Every substance has a different specific heat
To find the change in thermal energy
o
Use the formula: Q = (∆ T) x (m) x (Cp)
Where: Q = change in thermal energy
∆ T = change in temperature
T final T initial or
T initial T final
m = mass in kg
Cp = specific heat
Example problem
An iron pipe with a mass of 510g cools from
20° C to 10° C. What is the change in thermal energy?
Your Data: m
= 510 g (.51kg)
T final
= 10° C
T initial
= 20° C
Cp = 450 J/kg x C
Q = ?
Show your work
Q = (∆ T) x (m) x (Cp)
Q = (20 C 10 C) x (.51kg) x 450
J/kg x C
Q= 10 C x (.51kg) x 450 J/kg x C
Q = 2295 J
Practice problems for Work
A students full
back pack weighs 10N. She lifts it from the floor to the shelf 1.5 meters high.
How much work is done?
A carpenter lifts
a 45 kg beam 1.2 meters. How much work is done?
A dancer lifts a
400N ballerina overhead a distance of 1.4 meters. How much work is done if she
is held for several seconds?
Practice problems for Heat
A 3.1g ball of aluminum foil cools from 30 ° C to 15 ° C. What is the
change in thermal energy?
A copper block with a mass of 250g is heated
from 15 °
C to 95 °
C. How much energy is gained by the block?