IES MACARENA
PHYSICS AND CHEMISTRY 2º ESO – UNIT 6
UNIT 6 ENERGY I: BASIC CONCEPTS
1. ENERGY
Vocabulary of this section: Joule, calorie, kilowatt-hour.
So far we have studied matter. Now we are going to study energy.
Energy is what makes matter suffer physical and chemical changes.
Energy is a scalar physical quantity. The unit of energy in the SI is joule (J). Other units of
energy are calorie (cal) and kilowatt-hour (kWh).
1 cal = 4.18 J
1 kWh = 3.6·106 J
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Activity 1: Change the following magnitudes into joules with conversion factors:
a) 100 cal
b) 0.5 kWh
2. ENERGY PROPERTIES.
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Energy can be transformed. For example, solar energy can be transformed into
electric energy.
Energy can be transmitted. For example, sound is energy transmission by vibrational
waves.
Energy cannot be created or destroyed. This means that the total amount of energy
remains the same.
3. TYPES OF ENERGY.
Vocabulary of this section: kinetic energy, potential energy, mechanical energy.
Kinetic energy is the kind of energy that a body has when it is at motion. The symbol of
kinetic energy is EK. The kinetic energy of a body depends on body’s mass and velocity, and
can be calculated by:
𝟏
EK = 𝟐·m·v2
Where m means mass and v means velocity.
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Activity 2: A 50-kilogram student runs at a constant speed of 5 m/s. Find out its
kinetic energy.
Gravitational potential energy.
Gravitational potential energy is the kind of energy that a body has when it is placed at
certain height. The symbol of potential energy is EP. This energy depends on mass m and
height h. It can be found out by:
EP = m·g·h
The value of acceleration of gravity g is 9’8 m/s2.
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Activity 3: A 50-kilogram student has climbed a 2-meter wall. Find out its potential
energy.
Mechanical energy.
Mechanical energy is the result from adding both kinetic and potential energy: EM = EC + EP
4. PRINCIPLE OF MECHANICAL ENERGY’S CONSERVATION.
Vocabulary of this section: friction, heat.
“Mechanical energy of a body remains constant if there is no friction”.
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Activity: See activity Educaplus – Física – Energía – Conservación de la energía en
el péndulo. What does the blue bar mean? And the orange one? And the green one?
When friction is produced then a certain amount of energy is transformed into heat and then
mechanical energy does not remain constant.
Remember that if you rub your hands together, the friction between them causes heat. This
can be understood when a meteorite enters the atmosphere: friction with gases makes
meteorites lose kinetic energy and then meteorites get hotter and hotter.
5. WORK.
Vocabulary of this section: work, force, displacement.
There are two ways of exchanging energy between two bodies: by heating and by working.
Heat is energy that goes from a body to another which is at less temperature. (We will study
heat in another unit).
In Physics, work is produced when a force acts on an object and causes a displacement.
Work produces a transmission of energy. For example:
- If you push a supermarket cart you are doing work. This work is transmitted as kinetic
energy.
- Wind makes blades of a windmill spin.
- Falling water in a hydroelectric station makes a turbine spin.
In order to calculate work we use this formula:
W = F·d
where F is the force that
has been exerted and d is the resulting displacement. Its unit in the SI is Joule as well.
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Activity 4: A man pushes a supermarket cart with a force of 20 N and causes a
displacement of 4 m. Find out the work done by the man. What kind of energy did this
work change into?