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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 1 1 1 2 2 2 3 3 3 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 1 1 1 1 Pre-quiz 2 2 2 2 3 3 3 3 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: