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Name:
Date:
Subject:
Electromotive Force (EMF)
Objectives
Objective 1: ASWBAT describe what electromotive force (EMF) is and how it is measured.
Objective 2: ASWBAT name 3 sources of EMF that we use in our lives and describe what
form of energy is being used to make the electricity.
Objective 3: ASWBAT describe what alternating current (AC) and direct current (DC) are
and how we use each of them in our lives.
On a scale of 1 – 3 how well can you meet this objective now.
(1 – Not at all
2 – I know it a little bit
3 – I know it very well)
Highlight your choice.
Objective: 1
Objective: 2
Objective: 3
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Key Vocabulary Words
On a scale of 1 – 3 rate how well you know the vocabulary words below.
(1 – Not at all 2 – I know it a little bit 3– I know it very well)
Electromotive Force
Volts
Alternating Current
Direct Current
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Pre-quiz
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1. What is electric current? What is it measured in?
Answer:
2. What is a circuit? What is the difference between and open and closed circuit?
Answer:
3. What is resistance? What is it measured in?
Answer:
Electromotive Force (EMF)
We also need to know something about the force that causes the electrons to move in an
electrical circuit. This force is called electromotive force, or EMF. It is the force that
makes the electrons move. It is the force that creates electrical current. Sometimes it is
convenient to think of EMF as electrical pressure. In other words, it is the force that makes
electrons move in a certain direction within a conductor. Electromotive force is measured
in volts. Because it is measured in volts it is sometimes referred to as voltage.
1. What is electromotive force?
Answer:
2. What is electromotive force measured in?
Answer:
Electrical Sources
Electromotive force doesn’t come from nowhere. Voltage has to have a source. Some of the
most common power sources are batteries, generators and photovoltaic cells.
1. Name three common sources of electromotive force.
Answer:
Batteries
A battery is a device that converts chemical energy directly to electrical energy. If you look
at any battery, you’ll notice that it has two terminals. One terminal is marked (+), or
positive, while the other is marked (-), or negative. In an AA, C or D cell (normal flashlight
batteries), the ends of the battery are the terminals. In a large car battery, there are two
heavy lead posts that act as the terminals. Electrons collect on the negative terminal of the
battery. If you connect a wire between the negative and positive terminals, the electrons
will flow from the negative to the positive terminal as fast as they can (and wear out the
battery very quickly — this also tends to be dangerous, especially with large batteries, so
it is not something you want to be doing). Normally, you connect some type of load to the
battery using the wire. The load might be something like a light bulb, a motor or an
electronic circuit like a radio.
1. Where does the energy come from to make electricity in a battery?
Answer:
Electrical Generators
A generator is simply a device that moves a magnet near a wire to create a steady flow of
electrons. They convert the mechanical energy of the movement of the magnets into
electrical energy. The force that moves the magnet or copper wires can come from a
variety of energy sources. Coal plants and gas-powered generators use chemical energy
while nuclear power plants use nuclear energy. Hydroelectric dams use the force of
gravity to move the magnets and wind generators use the movement of air.
One simple way to think about a generator is to imagine it acting like a pump pushing
water through a pipe. Only instead of pushing water, a generator uses a magnet to push
electrons along.
Simple Generator
1. What type of energy does an electrical generator use to produce electricity?
Answer:
Photovoltaic Cells (Solar Panels)
Photovoltaic cells directly convert light energy into electricity at the atomic level. Some
materials exhibit a property known as the photoelectric effect that causes them to absorb
photons of light and release electrons. When these free electrons are captured, an electric
current results that can be used as electricity. Photovoltaic cells are a clean source of
energy because they don’t produce CO2!
Video: Solar Roadways
1. What does a photovoltaic cell use to make electricity?
Answer:
2. Why are photovoltaic cells environmentally friendly?
Answer:
Alternating Current (AC)
In alternating current, the charges move in one direction for a very short time, and then
they reverse direction. This happens over and over again. Why do we use AC power all
over the world? The biggest advantage of AC is that you can transmit high voltages over
long distances more efficiently. AC power incurs less resistance when it is flowing through
a conductor so less energy is lost when it travels to your house. Because the power plants
that we have can be very far away, the power grid that supplies our homes uses AC.
Alternating Current
1. What is alternating current?
Answer:
2. Why do we mostly use AC power to power our homes?
Answer:
Direct Current (DC)
The current in DC circuits is moving in a constant direction. The amount of current can
change, but it will always flow from one point to another. You will probably use direct
current power whenever you carry something around that uses electricity. This is because
batteries produce DC current. Almost everything we use needs DC power. So before the
power in our houses can be used it must be converted to DC. This unfortunately results in
a loss of power and therefore it is really inefficient.
Direct Current
1. What is direct current?
Answer:
2.What do we use DC power for?
Answer:
3. What has to happen to the power in our homes before our electronics use it?
Answer:
Video: AC vs. DC Power:
Quiz
Directions: Answer the following questions to see if you met the objectives.
1. What is electromotive force?
Answer:
2. What is electromotive force measured in?
Answer:
3. Name three common sources of electromotive force?
Answer:
4. Where does the energy come from to make electricity in a battery?
Answer:
5. What type of energy does an electrical generator use to produce electricity?
Answer:
6. What does a photovoltaic cell use to make electricity?
Answer:
7. Why are photovoltaic cells environmentally friendly?
Answer:
8. What is alternating current?
Answer:
9. Why do we mostly use AC power to power our homes?
Answer:
10. What is direct current?
Answer:
11.What do we use DC power for?
Answer:
12. What has to happen to the power in our homes before our electronics use it?
Answer: