SAM Teachers Guide Electricity

Transcription

1 SAM Teachers Guide Electricity Overview Students explore the role of electron voltage and density on electric current. They compare the movement of electrons in a conductor and an insulator. They derive Ohm s Law from their observations. They apply their understanding to circuits, the working of an incandescent light, and a fuel cell. Learning Objectives Students will be able to: Explain how voltage is the driving force behind electron movement. Define electric current as the number of electrons flowing through a wire over a given period of time. Explain how conductivity and resistivity relate to electric current. Infer the relationships between current, voltage, and resistance as described in Ohm s Law (I = V / R). Explain how electricity can be converted to other forms of energy. Possible Student Pre/Misconceptions Electrons in a circuit come from somewhere else, rather than the circuit itself. Electrons travel very quickly around a wire. This explains why a light bulb turns on as soon as someone turns on a light switch. Charge is used up as it flows through a circuit. The charge that flows through a circuit originates in the battery. Models to Highlight and Possible Discussion Questions After completion of Part 1 of the activity: Models to Highlight: Page 2 Voltage Model o Link to other SAM activities: Atoms and Energy. Discuss voltage by referring back to gravitational potential energy. Page 3 Electron Density and Flow Speed o Highlight the relationship between electron density and current of two materials. o Link to other SAM activities: Atomic Structure. Review the structure of an atom, its parts, and the charges. Page 4 Bottom Model of Conductivity and Resistivity o Review the differences between conductors and insulators. Use model to highlight the effect of temperature on current.

2 o Emphasize the idea that the electrons are randomly flowing in all directions, and not in a directed flow as was modeled on page 3. Page 5 Ohm s Law Models o Use both models to highlight the relationship between current, voltage, and resistance as shown in Ohm s Law. Possible Discussion Questions: What is the difference between current and voltage? What is the relationship between current and resistance? What is moving as an electric current flows? How many electrons are flowing through a typical wire? Refer to the caption on p.3. Discuss why you might receive an electric shock. Do the electrons themselves flow from a power plant to your house? Why or why not? What does get passed that distance? What are some real world applications that are rooted in the relationship between temperature on insulators and conductors? [Importance in designing space travel materials and for aircraft, satellite, and military applications.] What effect does an increase in temperature have on the ability of an insulator to insulate? On a conductor to conduct? Demonstration/Laboratory Ideas: o Use a circuitry board to connect a battery and a light. o Set up circuits and vary the wire (diameter, type of metal, etc.). o For those who have access: CPO 13A and/or CASTLE activities. After completion of Part 2 of the activity: Models to Highlight: o Page 6 Circuit Model o Review the flow of electrons through a circuit and how the law of conservation of energy plays a role. o Link to other SAM activities: Electrostatics. Review attraction / repulsion of charges and how this plays a role in a circuit. o Page 8 Incandescent Light Bulb Model o Highlight the concept of energy conversion using this model and the role of resistance in the incandescence of the light bulb. o Link to other SAM activities: Atoms and Energy. Review the idea of energy transfer vs. energy conversion to a different form. This also ties into the Law of Conservation of Energy.

3 Possible Discussion Questions: How does an incandescent light bulb work? What effect does adding more batteries have on the total current in a circuit? How would the knowledge of hydrogen fuel cells help us in the future? Can you explain how the brightness of the bulbs would change if you add or subtract parallel branches?

4 Connections to Other SAM Activities The focus of this activity is to understand the motion of electric charge. This activity is supported by Electrostatics where static charges create fields of positive and negative charge. Electricity shows the motion of these charged particles. Heat and Temperature helps students explore how current can be transformed into heat and light. Similarly, Excited States and Photons explores atoms in their excited states and describes how they can emit photons. The Electricity activity supports Diffusion, Osmosis and Active Transport, Cellular Respiration, and Photosynthesis. In each of these activities, a buildup of electric potential can be converted into chemical energy if current is allowed to flow back through the membrane. In Chemical Reactions and Energy, electricity is one of the forms of energy that can be created if electrons in a chemical reaction are forced through a wire.

5 Activity Answer Guide Page 1: No questions. Page 2: 1. In the model above, describe the relationship between voltage and kinetic energy as the electron moves. (a) 2. Complete the analogy: If gravitational potential energy is the work done by gravity on a mass moving a certain distance then electric potential energy is... Electric potential energy is work required to move an electron from one position to another. Voltage is the electric potential energy. Page 3: 1. Does a material with a higher density of electrons or a lower density produce a greater current under the same voltage? (a) 2. Copper and iron have a lot of electrons that can flow freely. When a voltage is applied would that produce a lot of current or a little current? Explain. When a voltage is applied, a lot of current would be generated due to the high number of electrons that can flow freely. Page 4: 1. Electric conductivity measures: (c) 2. Describe why the current measurements are so low for an insulator. Insulators block or slow the flow of electric current. The electrons cannot move freely so current measurements are low. Page 5: 1. How does voltage affect the current passing through a material? (a) 2. Set the voltage to zero and observe both the model and the ammeter. The electrons are moving, but why doesn't the ammeter measure a current? The electrons are moving in both directions so there is no current. Therefore there is no electric force and the ammeter won t measure a current. 3. How does resistance affect the current passing through a material? (b) 4. Why is the current higher in a material with lower resistance than in a material with higher resistance under the same voltage? Under the same voltage the electron would feel some electric force. Material with lower resistance, mean that the attraction of the electron to the atom will be lower and will result in flow and materials with a higher resistance would have a greater attraction and the electrons don t flow. Page 6: 1. If there were no battery describe how the model above would change. The battery supplies the energy to create the electric potential difference. If there were no battery, there would be no electron flow. Page 7: 1. Imagine that the wire to the green resistor was broken at point A. Describe what you think would happen to the movement of the electrons. With the wire no longer connected at point A electrons would no longer flow to the green resistor. The electrons would instead only flow through the red resistor to complete only one branch of the circuit.

6 Page 8: 1. Based on your experience with the model, explain why a light bulb from a table lamp won't be lit if you connect it to an AA battery. An AA battery has a voltage of 1.5 V. A light bulb of a table lamp works under the voltage of 110 V, so the battery does not generate enough electricity to meet the demands of the resistor and effectively power the light bulb. 3. According to Ohm's Law, in any electrical circuit, the current increases: (Choose all that apply.) (a) (d) 4. What causes the electric energy to change to light energy in a light bulb? The motion of the electrons through the resistor increases temperature. When temperature increases enough due to the filament s resistance, visible photons of light are released. Page 9: No questions. Page 10: 1. Compared to conductive wires, insulators are considered to have: (a) 2. If two batteries were serially used in a circuit instead of one, would the electric current be different? Why or why not. Two batteries in series would increase the voltage or electric potential. Referring to Ohm s Law, if I = V/R, increasing the voltage would increase the flow of current through the circuit. This is true if the resistance is kept the same.

7 SAM HOMEWORK QUESTIONS Electric Current Directions: After completing the unit, answer the following questions to review. 0. Electric current can be defined as the flow of electric charges. How can you define voltage? 1. When there is a very low density of electrons in a wire, would you expect a large current or a small one? Why? 2. What is the difference between a conductor and an insulator? Give an example of each. 3. Ohm s Law describes the relationship between current, voltage, and resistance. What is the equation for Ohm s Law? Write out the mathematical equation and then explain it in words. Equation: Explanation: 4. Below is a picture of a simple parallel circuit. a) What is represented by V? b) Use an arrow to draw the flow of electrons through this circuit. c) Why is I = I 1 + I 2? 5. Explain how the presence of a resistor in a circuit can convert electricity into light? *Hint: Think back to the model on page 8 of the activity that demonstrated incandescence.* 6. Career connection: How is computer modeling of new smart electric grids going to make it easier to add renewable energy sources to our electric power supply?

8 SAM HOMEWORK QUESTIONS Electric Current With Suggested Answers for Teachers Directions: After completing the unit, answer the following questions to review. 1. Electric current can be defined as the flow of electric charges. How can you define voltage? Voltage is the difference in electric potential energy between two points in the path of an electron. 2. When there is a very low density of electrons in a wire, would you expect a large current or a small one? Why? You would expect a smaller current because there are fewer electrons flowing through a cross section of wire at a given time. 3. What is the difference between a conductor and an insulator? Give an example of each. A conductor allows electric current to flow through easily, such as copper. An insulator resists the flow of electricity, such as plastic. 4. Ohm s Law describes the relationship between current, voltage, and resistance. What is the equation for Ohm s Law? Write out the mathematical equation and then explain it in words. Equation: I=V/R Explanation: I = current, V = voltage, and R = resistance Ohm s Law states that electric current is directly proportional to voltage and indirectly proportional to resistance. 5. Below is a picture of a simple parallel circuit. a) What is represented by V? Voltage source such as a battery. b) Use an arrow to draw the flow of electrons through this circuit. c) Why is I = I 1 + I 2? The circuits are parallel. 6. Explain how the presence of a resistor in a circuit can convert electricity into light? *Hint: Think back to the model on page 8 of the activity that demonstrated incandescence. Resistance causes an increase in temperature that is high enough to cause visible photons to be emitted. 7. Career connection: The electrical grid in our country is outdated and poorly managed. Computers controlling the flow of electric current would better manage supply and demand of electricity. Models of a new smart grid predict better management and make it easier to incorporate more sporadic sources of power such as solar or wind.