Electric Current and Ohm’s Law

1. Electric Currentand Ohm’s Law Sec 20.2

2. Electric Current • electric current – continuous flow of electric charge • measured in ampere (A) – 1 coulomb per second • two types • direct current – charge flows one way • Found in devices like flashlights and calculators • alternating current – charges vibrate back and forth around a fixed position • Type of current in households

3. Conductors and Insulators • conductor – material through which charge can easily flow • examples: metals, salt water, gases in fluorescent light • conductors transmit charge because they have free electrons in their structure • insulator – material through which charge cannot easily flow • Examples: plastic, foam, rubber, wood

4. Challenge Question Q: If a lightening storm occurs, you are relatively safe in a car. Is this because of the car’s conducting properties or insulating properties? A: You are safe because of the car’s conducting properties. If lightening strikes the car, the metal conducts the electricity around the outside of the car and to the ground. Since you are inside the car, the electric current never reaches you.

5. Resistance • opposition to the flow of electrons • measured in ohms (Ω) • occurs because electrons moving through a conductor collide with other electrons, ions and atoms • depends on material’s length, thickness and temperature • Longer wire increases resistance • Thicker wire decreases resistance • Increasing temperature increases resistance

6. Quick Check Determine which of the following in each pair would have the lower resistance: • A wire 10 mm thick or 5 mm thick • A spoon at 25°C or at 30°C • A fluorescent bulb 20 cm long or 120 cm long.

7. Voltage • need source of electrical energy in order for charges to flow • batteries, solar cells and generators are all sources of electrical energy • charges flow from areas of high potential to areas of low potential in much the same way as objects fall from areas of high gravitational potential energy to low gravitational potential energy

8. Voltage (continued) • electrical potential energy of a charge depends on its position in an electric field just like the gravitational potential energy of an object depends on its height above the earth • potential difference = the difference in electrical potential energy of two places in an electric field • measured in volts (V) • also called voltage

9. Ohm’s Law • relationship between current, resistance and voltage in a circuit • first discovered by Georg Ohm (1826) • Voltage = current x resistance or V = I x R Ohm's Law Simulation

10. Summary of Simulation • If you increase the voltage and keep the resistance constant, what happens to the current? • If you increase the resistance and keep the voltage constant, what happens to the current?

11. Challenge Question Q: A simple lie detector consists of an electric circuit, one part of which is part of your body. A sensitive meter shows the current that flows when a small voltage is applied. How does this technique indicate that a person is lying? A: When people lie their skin tends to get moist from increased sweating. Moist skin has a lower resistance than dry skin. As the resistance of the skin goes down, the current will increase which is detected by the meter.

12. Challenge Question Q: If power lines have high voltages and are so dangerous, why don’t birds get electrocuted when they land on the lines? A: In order for the current to flow through the bird, their needs to be a potential difference. When both feet are on the same line there is no potential difference between their feet and no current flows. You also would be safe hanging on the power line as long as you were not touching anything else. If you touch the line and touch another object or the ground there would then be a potential difference and you would be electrocuted.