2. ELECTRIC Current  Identify 3 sources of electric current  Distinguish between current, voltage, & resistance  Ohm’s Law  How resistance changes in a wire

3. Info you must know You remember that Static Electricity is where electrons have gathered and are not moving. Give these electrons a wire to travel in and they become known as Electric Current.

4. Sources of ELECTRIC CURRENT Electrochemical Cells - Electricity can be gene- rated by chemical reactions within an enclosed container commonly called a battery. Two types of Batteries Wet cell an automobile battery is an example of a wet cell. A wet cell contains lead and lead oxide plates called electrodes and a liquid con- ductor called an electrolyte. (usually H 2 SO 4, sulfuric acid)

5. SOURCES (CONTINUED) -- - - - - - - - - - - - - - - Lead plate Liquid electrolyte H 2 SO 4 (sulfuric acid) The chemical reaction between the acid and the lead plates releases electrons to be used as electrical current. + pole- pole Wet Cell Battery

6. SOURCES (CONTINUED) Dry cell batteries work in much the same way as wet cells except that the electrodes in a dry cell are made of zinc and carbon. The electrolyte is usually a dry or semi-moist chemical such as ammonium chloride. Dry cells come in varying sizes and are used in things like flashlights, radios, remote controls, toys, etc.

7. SOURCES (continued) Thermocouples A thermocouple is a device capable of generating electric current due to a difference in temperatures. This simple device consists of an iron wire and a copper wire with the ends joined together to form a loop. Electric current is produced when one iron/copper junction is heated and the other is cooled.

8. The THERMOCOUPLE Copper wire Iron wire Junction being heated Junction being cooled Ammeter Device which detects electrical current

9. SOURCES (CONTINUED) Thermocouples are used in things like : 1. Automobile temperature gauges 2. Safety device (switch) in furnaces or ovens.

10. TYPES OF CURRENT There are two types of current. Direct Current (DC) is electrical current which comes from a source which delivers a steady flow of electrons such as an electro- chemical cell (battery) Alternating Current (AC) is when the elec- trons in the current change directions. This direction change occurs about 120 times per second. The current in your home is (AC).

11. MEASURING CURRENT Flow rate The rate at which electricity flows through a wire can be compared to water flowing through a hose. A small diameter wire allows less flow A larger wire allows more flow. This can be compared to water flowing through a garden hose and a fire hose.

12. How is Flow Rate measured? The number of electrons that pass a given point in a circuit in one second indicates the flow rate of electric current. The > the number of electrons the > the current. Symbol for current is a capital I Current is measured with an instrument called an AMMETER The unit of measurement is called the ampere (amps) symbol is a capital A

13. MEASURING CURRENT (CONTINUED) VOLTAGE Voltage is the amount of energy available to move electrons through a wire. The > the voltage the > the ability of the electrons to do work. (i.e. the brighter the light in the flashlight) Voltage is measured with an instrument called a voltmeter. The symbol for voltage is the capital letter V

14. VOLTAGE I n order to keep electrons flowing through a wire there must be a source of electrons. This source must also provide a difference in in the charges present at each end of the wire. One end of the wire must be + positively charged and the other end must be - negatively charged. The difference in the charges at each end of the wire is called POTENTIAL DIFFERENCE or VOLTAGE.

15. RESISTANCE RESISTANCE The best way to show resistance is to talk about the Electricity flows freely through the copper wire leading to the light bulb Once the current gets to the smaller wire filament in the bulb it slows down because of the resistance of tungsten metal to the flow of electricity. When the resistance slows the flow down a glow is produced in the wire as the electrons pass through it.

16. RESISTANCE (CONTINUED) The symbol for resistance is the capital letter R. The unit for resistance is the ohm. The symbol for the ohm is the Greek letter “omega:

17. Ohm’s Law Ohm’s Law states the current in a circuit is equal to the voltage divided by the resistance. I = VRVR I - Current V - Voltage R - Resistance V IR

18. Using Ohm’s Law 1. The electric current in a hair dryer measures 10 amps. The resistance is 12 ohms. What is the voltage? V = I x R V = 10 A x 12 ohms V = 120 V

19. Ohm’s Law AGAIN 2. Two dry cells in a flashlight produce 3 V. The dry cells carry a current (I) of 0.5 A. What is the resistance of the flashlight? R = V I R = 3V 0.5 A R = 6 ohms

20. ONE LAST OHM’S LAW 3. What is the current if 120 V passes through a radio with a resistance of 1800 ohms? I = V R I = 120 V 1800 ohms I =.066 A (amps)

21. Quiz 1. Electrons moving along a wire are called ____________ __________. 2. The two main components of electro- chemical cells are ________ & ______. 3. A device that generates electrical current because of a difference in temperature is a __________________. 4. Electrochemical cells deliver _________ current. 5. The current used in homes is _________ current. 6. Current is measured with a device called a _______________. 7. The symbol for current is ________.

22. Quiz continued 8. The unit for current is the__________. 9. The amount of energy available to move electrons is called ______________. 10. The difference in charges at each end of a wire is called ________ ________. 11. _________ is those “things” that elec- tricity causes to work. 12.The Ohm’s Law formula is __________.

23. THE ANSWERS 1. ELECTRIC CURRENT 2. ELECTRODES & ELECTROLYTES 3. THERMOCOUPLE 4. DIRECT 5. ALTERNATING 6. AMMETER 7. I 8. AMPERE 9. VOLTAGE 10. POTENTIAL 11. RESISTANCE 12. I = V / R 8 PTS FOR EACH X

24. The end !!