Born: March 16, 1789, Erlangen, Germany Died: July 6, 1854, Munich, Germany Education: University of Erlangen-Nuremberg

Transcription

1 Ohm s Law

2 Georg Simon Ohm was a German physicist and mathematician. As a school teacher, Ohm began his research with the new electrochemical cell, invented by Italian scientist Alessandro Volta. Born: March 16, 1789, Erlangen, Germany Died: July 6, 1854, Munich, Germany Education: University of Erlangen-Nuremberg

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13 Calculate: Total R, I and V for each resistor

14 Resistors in Parallel

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16 2 resistors in Parallel

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20 1. Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply. a. decrease the voltage b. decrease the resistance c. increase the voltage d. increase the resistance

21 Answers: A and D The current in a circuit is directly proportional to the electric potential difference impressed across the circuit and inversely proportional to the resistance of the circuit. Reducing the current can be done by reducing the voltage (choice A) or by increasing the resistance (choice D).

22 2. A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the following would cause the bulb to shine less brightly? Choose all that apply. a. increase the voltage of the battery (add another cell) b. decrease the voltage of the battery (remove a cell) c. decrease the resistance of the circuit d. increase the resistance of the circuit

23 Answers: B and D The bulb will shine less brightly if the current in it is reduced. Reducing the current can be done by reducing the electric potential difference impressed across the bulb (choice B) or by increasing the resistance of the bulb (choice D).

24 3. You have likely been warned to avoid contact with electrical appliances or even electrical outlets when your hands are wet. Such contact is more dangerous when your hands are wet (vs. dry) because wet hands cause. a. the voltage of the circuit to be higher b. the voltage of the circuit to be lower c. your resistance to be higher d. your resistance to be lower e. the current through you to be lower

25 Answer: D Wet hands have less resistance and thus less hindrance to the flow of charge; the current would thus be increased. Touching an outlet with wet hands increases the risk of charge flowing through you and causing electrical shock or even electrocution.

26 4. If the resistance of a circuit were tripled, then the current through the circuit would be. a. one-third as much b. three times as much c. unchanged d.... nonsense! There would be no way to make such a prediction.

27 Answer: A Current is inversely proportional to the resistance. A threefold increase in the resistance would cause a threefold decrease in the current.

28 5. If the voltage across a circuit is quadrupled, then the current through the circuit would be. a. one-fourth as much b. four times as much c. unchanged d.... nonsense! There would be no way to make such a prediction.

29 Answer: B Current is directly proportional to the voltage. A fourfold increase in the voltage would cause a fourfold increase in the current.

30 7. Use the Ohm's law equation to provide numerical answers to the following questions: a. An electrical device with a resistance of 3.0Ωwill allow a current of 4.0 amps to flow through it if a voltage drop of Volts is impressed across the device. b. When a voltage of 120 V is impressed across an electric heater, a current of 10.0 amps will flow through the heater if the resistance is Ω. c. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60Ωwill have a current of Amps.

31 Use the equation V = I R to solve for the unknown quantity. a. An electrical device with a resistance of 3.0 Ω will allow a current of 4.0 amps to flow through it if a voltage drop of 12 Volts is impressed across the device. b. When a voltage of 120 V is impressed across an electric heater, a current of 10.0 amps will flow through the heater if the resistance is 12 Ω. c. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60 Ω will have a current of 0.05 Amps.

32 8. Use the Ohm's law equation to determine the missing values in the following circuits.

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34 6. A circuit is wired with a power supply, a resistor and an ammeter (for measuring current). The ammeter reads a current of 24 ma (milliamps). Determine the new current if the voltage of the power supply was... a.... increased by a factor of 2 and the resistance was held constant. b.... increased by a factor of 3 and the resistance was held constant. c.... decreased by a factor of 2 and the resistance was held constant. d.... held constant and the resistance was increased by a factor of 2. e.... held constant and the resistance was increased by a factor of 4. f.... held constant and the resistance was decreased by a factor of 2. g.... increased by a factor of 2 and the resistance was increased by a factor of 2. h.... increased by a factor of 3 and the resistance was decreased by a factor of 2. i.... decreased by a factor of 2 and the resistance was increased by a factor of 2.

35 A circuit is wired with a power supply, a resistor and an ammeter (for measuring current). The ammeter reads a current of 24 ma (milliamps). Determine the new current if the voltage of the power supply was... a. Inew = 48 ma (Current is directly proportional to voltage; a doubling of the voltage will double the current.) b. Inew = 72 ma (Current is directly proportional to voltage; a tripling of the voltage will triple the current.) c. Inew = 12 ma (Current is directly proportional to voltage; a halving of the voltage will halve the current.) d. Inew = 12 ma (Current is inversely proportional to resistance; a doubling of the resistance will halve the current.) e. Inew = 6 ma (Current is inversely proportional to resistance; a quadrupling of the resistance will reduce the current to one-fourth its original value.) f. Inew = 48 ma (Current is inversely proportional to resistance; a halving of the resistance will double the current.) g. Inew = 24 ma (Current is directly proportional to voltage; a doubling of the voltage will double the current. But current is also inversely proportional to the resistance; a doubling of the resistance will halve the current. These two factors offset each other and there is no overall change in the current.) h. Inew = 144 ma (Current is directly proportional to voltage; a tripling of the voltage will triple the current. But current is also inversely proportional to the resistance; a halving of the resistance will double the current. So the new current can be found by tripling and then doubling the old current of 24 ma.) i. Inew = 6 ma (Current is directly proportional to voltage; a halving of the voltage will halve the current. But current is also inversely proportional to the resistance; a doubling of the resistance will halve the current. So the new current can be found by halving and then halving again the old current of 24 ma.)

36 Ohm s Law - Summary