1. Controlling the Flow of Electrical Current Section 2.1-Day #2

2. Homework Check and Reflect #5 and 7 (p. 303). 5. Conductivity is related a substance’s atomic structure. Conductors allow electrons to flow, while insulators do not allow electrons to flow freely. 7. Metal A provides more resistance to current flow.

3. The Plan Discussion on resistance, demo, animation, video clips, and discussion on switches and variable resistors. I can identify electrical conductors and insulators and compare the resistance of different materials. I can identify and explain resistors and switches.

4. Engineers need to know how well different materials conduct electricity so they can design both safe and effective devices.

5. A superconductor walks into a store, the store clerk says, “Hey get out, we don’t serve your kind here!” The superconductor leaves without resistance.

6. Resistance Resistance is how much current a material will block.  Low resistance = lots of current through (little stopped).  High resistance = little current through (most is stopped).

7. How Does the Length and Width of a Wire Relate to Resistance? How Does the Length and Width of a Wire Relate to Resistance? http://nuweb.neu.edu/bmaheswaran/phyu121/data/ch05/fig/fig0514.htm

8. Resistor A resistor allows electrons to flow through it, but provides resistance to it (slows them down or restricts the flow of electricity). Resistance is measured in Ohms.  ohms =  resistance.

10. Resistors The volume control on a stereo is a special kind of resistor called a variable resistor or rheostat. A resistor is a device having resistance to the passage of electrical current, often used to control current in a circuit.

11. Is Resistance Good or Bad? Usually the electrical energy is converted to heat so resistors work at high temperatures.  (E.g., light bulbs-the filament is a resistor, so is the burner on your stove).

12. Examples of Devices that Use Resistors

13. Resistors Solutions can also be resistors. What type of water would provide the most resistance? 1. Distilled water 2. Tap water 3. Sugar water

14. Resistors Solutions can also be resistors. What type of water would provide the most resistance? 1. Distilled water 2. Tap water 3. Sugar water

15. Polygraph (Lie Detector) Polygraph (Lie Detector) http://www.polygraphsingapore.com/

16. Switch The basic switch works by maintaining or breaking the circuit. ON- resistor relays the electrons (complete circuit). Can say it is a closed circuit. OFF- resistor stops the electrons (broken circuit). Can say it opens a circuit.

17. Variable Resistor (Rheostat) A variable resistor is a resistor whose resistance can be changed by adjusting current flow. Rheostats can increase or decrease the amount of current in a circuit by adjusting the portion of the resistor that the current travels through.

18. Variable Resistor (Rheostat) Variable Resistor (Rheostat) The longer a current passes through a resistor (like a nichrome wire), the more energy is absorbed, and the less gets through to the load.

19. Example

20. Chart

21. Homework Check and Reflect #3, 8, and 9 (p. 303) 3. When a switch is off, conductors are separated to prevent current flow. When a switch is on, conductors are pressed together to allow current to pass.

22. Homework 8. Technically, your friend is correct, pure water does not conduct electricity. However, it is very likely that the tap water that has flooded the basement has dissolved ions in it, allowing it to conduct electricity. This would make using the electric drill very dangerous in the flooded basement, with a significant risk of electrocution.

23. Homework 9. This would be a perfect application for a variable resistor (rheostat). Using one would allow you to control the amount of current flowing to the pump, thereby controlling how hard the pump sprays water.