10/30/14 Series and Parallel Science video: Plant-e: plants generate electricity Finish one bulb circuits Challenge items Resistance and Ohm’s Law Series and Parallel circuits Start building series and parallel circuits Circuit activity Homework: Electrical Resistance , Series Circuits, worksheets (first 2 pages), associated readings Learning objectives • Given a one wire, one bulb, one battery circuit, determine whether or not the bulb will light. If it will not light, explain why. • Describe the concept of electric potential and determine where in a circuit a charge has the greatest electric potential or electric potential energy. • Describe/diagram how electric potential and potential energy varies throughout a circuit. Gravitational PE Gravitational Potential Energy GPE depends on height and mass Gravitational Potential GP depends on height only Electric Potential Electric Potential Electric Potential depends on distance only EP in a circuit E is the electric field direction, the “natural” direction for a positive charge Battery as a charge pump Electric Potential in circuits The electric potential drops across each light bulb. The drop depends on a characteristic called resistance Discuss Electric Potential Difference worksheet Basic electricity activity • Sketch the battery and bulb shown on the right in your notebook. • Add a single wire to your sketch such that the bulb would light if these were real objects. • Briefly explain why your bulb would light. Basic electricity activity Suppose you had to explain to a friend over the telephone how to light a bulb. Your friend has one battery, one bulb, and one wire, but has never lit a bulb before. 1. Write a description of how to light the bulb. (Your description should be general enough to work for all the ways that light the bulb.) 2. Read your description to a partner. Your partner should try to follow your directions exactly to light the bulb, pretending that s/he has never lit a bulb before. Basic electricity activity explanation The bulb lights when there is an uninterrupted path from one end of the bulb, through the battery and through the wire back to the other end of the bulb. The bulb will light when: 1) one end of the battery is connected to one metal part of the bulb. 2) one end of a wire is touching the other metal part of the bulb. 3) the other end of the wire is touching the end of the battery not touching the bulb. This results in a complete circuit. Lighting a bulb Go over Electric circuits worksheet Challenge questions • What does the inside of a bulb look like? (The part behind the metal threads.) • What happens to the brightness of the bulb when you add another bulb? o How many ways can you add another bulb? • What happens to the brightness of the single bulb when you add another battery? o How many ways can you add another battery? A naked bulb Ways to add two bulbs Ways to add two batteries Resistance demo How will the resistance change if I … • Double the length? • Cut the length in half? • Triple the area? • Decrease the resistivity by a factor of five? Resistance and Ohm’s Law The resistance of a long, thin conductor of length L, cross sectional area A, and resistivity ρ is: The unit of resistance is the ohm. 1 ohm=1 Ω = 1 V/A. The current through a conductor is determined by the potential difference ΔV along its length: Two types of circuits – an analogy Added tollbooths further restrict flow Added tollbooths increase flow Series Circuits Parallel Circuits Circuit activity • Material: battery holders, one battery, 2 identical bulbs, 2 bulb holders, about 4 wires. • Construct 3 different circuits. In each case, note how the bulb brightness changes as you add a bulb o 1 bulb o 2 bulbs in series o 2 bulbs in parallel • Compare the brightness of the bulbs in each case • Describe what happens when you take out a bulb in each of the three circuits One minute reflection • Take a minute to write a brief reflection of what you learned for the past couple of weeks. Let these questions guide you. o What new content did you learn? o What new skills did you learn o How can you use what you learned in the future? o What questions do you still have? • You need not answer all of these questions. Use them if you are having trouble thinking of something to write.