2. Electric Current and Circuits Review

3. Current CURRENT: a flow of charged particles (electrons) through a conductor Current, I, is measured in amperes, A, or “amps”. Andre Ampere- first defined current e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e-

4. Circuits ELECTRIC CIRCUIT: Charges moving in a closed loop A circuit requires a both a conductor, usually metal wires, and a “charge pump”. CHARGE PUMP: a device that provides a potential difference so that charges keep moving. Potential Difference = Voltage = “pressure” Alexander Volta- found that electrical current could be harnessed and made to flow Made a “voltaic pile”- the first battery

5. Resistance all conductors offer some resistance to the flow of charges, even metal wires. RESISTANCE = The unit for resistance is the OHM, . This equation is often called OHM’S LAW

6. George Ohm- first determined the math relationship now called Ohm’s Law Henry Cavendish, who found a value for “G”, also experimented with electricity. In playing with “Leyden Jars”, he stated that the “velocity” (the current that flowed out of the jar) varied directly with the “degree of electrification” (the voltage). His work laid the groundwork for George Ohm to write Ohm’s Law.

7. Resistance in Wires Thick wires have less resistance than thin wires. The filaments in light bulbs are very thin and offer a lot of resistance. The thinner the filament, the higher the resistance. AND, the higher the resistance, the brighter the bulb!! The filament in a 100W bulb is thinner than the filament in a 60W bulb!

8. Certain metals offer less resistance to the flow of charges than others. Example: Copper is a better conductor than iron The resistance of a wire of length L and cross sectional area A is given by RESISTANCE, R = where  is the resistivity of that particular metal.

9. Electric Power Power is the rate that work is done or energy is transferred, that is Power = Power is measured in Watts, W

10. Electric power delivered to a circuit by a power supply is given by Power = Current x Voltage P = IV This equation can be combined with Ohm’s Law, R = V / I in its different forms: V = IR, I = V / R

11. Examples How much energy does a 75 W light bulb give off in five minutes? Power = Energy / time Energy = Power x time Energy = 75 W x 5 x 60 seconds Energy = 22500 J What is the power output of a 3 A motor running on regular house voltage? P = IV P = 3A x 120V P = 360 W

12. “Power” lines The higher the current, the more the wires in the circuit heat up, thereby “wasting” energy. This is a big problem when electric companies must provide electricity at great distances away from the power plants. The solution: Electric lines that carry current great distances are at very high voltage, so the current can be small. P = IV

13. High Voltage / Low Voltage TRANSFORMERS: devices that “step- up” the voltage at the power plant and then “step-down” the voltage at the customers’ location.

14. P IV V I R

15. SERIES CIRCUITS The same current through each device. EQUIVALENT RESISTANCE: What is the net resistance? What one resistor could replace a group of resistors? For resistors wired in series, the equivalent resistance is given by: R eq = R 1 + R 2 + R 3 + …

16. PARALLEL CIRCUIT Same voltage drop across each device. For resistors wired in parallel, the equivalent resistance is given by: Use the 1/x or x -1 button on your calculator! But don’t forget to hit that same button AFTER you hit “EQUAL” in order to get the equivalent Resistance for parallel circuits!

17. Example: What is the equivalent resistance of a 10 , 20 , and 30  resistor wired in series? In parallel? Series: R eq = 10 + 20 + 30 = 60  Parallel 1 / R eq = 1/10 + 1/20 + 1/30 R eq = 5.45  There’s much less resistance if resistors are wired in parallel than if they’re wired in series. With less resistance, the charge pump will able to push much more current around the circuit.

18. As you add more and more resistors in series, the equivalent resistance INCREASES. For maximum resistance and minimum current- use series wiring. As you add more and more resistors in parallel, the equivalent resistance DECREASES. For minimum resistance and maximum- use parallel wiring.

19. Characteristics of Series and Parallel Wiring Series: If one component goes out, They all go out! As more resistors are added, the equivalent resistance Increases! which means that the current in that part of the circuit Decreases! Parallel: If one component goes out, They rest still work! As more resistors are added, the equivalent resistance Decreases! which means that the current in that part of the circuit Increases!

21. Schematic diagram of series circuit with three reisstors Schematic diagram of parallel circuit with three resistors

22. 2 resistors of 8  and 10  are wired in series with a 12 V battery. What is the total current? To find the current flowing through the circuit, you first must find the total (equivalent) resistance of the circuit! Total Resistance = 8 + 10 = 18  Total Current = V / R = 12 / 18 = 0.667amps What is the voltage drop across the 10  resistor? Since the resistors are in series, the total current runs through each resistor, so  V = IR = 0.67amps x 10  = 6.67V What is the voltage drop across the 8  resistor?  V = IR = 0.67amps x 8  = 5.33V V = IR

23. 2 resistors of 8  and 10  are wired in parallel and the combination is wired to a 12V power supply. What is the total current? 1 / Total resistance = 1 / 8 + 1 / 10 Total resistance = 4.44  Total current = V / R = 12 / 4.44 = 2.7amps

24. Electrical Safety Fuse: a short piece of metal that melts if current exceeds a set value (to protect device) Circuit breaker: an automatic switch that opens the circuit when current exceeds a set value. (uses a bimetallic strip)

25. Ground-fault circuit interrupter: opens a circuit if the current going into a device is not the same as the current coming out of the device. gfci

26. Short Circuit- a circuit that is formed when wires touch. This effectively shortens the path of the circuit because the current no longer passes through the resistor. The extremely low resistance in the circuit produces very high current, which could cause melt down and fire.

27. Ground wire: provides a path for high current to go in case of a short circuit- it is attached to metal bars that literally go into the ground.

28. Ammeter- device to measure amps (current), measure “in series” Voltmeter- device to measure voltage, measure “in parallel” Ohmmeter- device to measure resistance, measure with NO power supplied to the circuit!! Multimeter- does all three