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11.3 RESISTANCE - Ohm’s Law Resistance is a term used in current electricity to describe things like light bulbs, motors, heaters, stereos, etc. All of these things are called Loads and convert the energy the electrons carry, into other forms. Things with a lot of resistance need a lot of energy to make them move. Things with little resistance don’t need very much energy at all to make move. In both models Resistance Resistance makes it hard for the electrons to flow. described as an opposition to the flow of current. Resistance is measured in units called is often ohms (Ω). Ohm’s Law Ohm’s Law shows how the voltage, current, and resistance affect each other. In a simple circuit, if the voltage increases, what would you predict would happen to the current.? If the resistance somehow was increased, and the voltage stayed the same, what would happen to the current through the circuit.? We can use mathematical equations to quantitatively predict what will happen in these cases. If we wish to know the measured values for the different variables, we have to use Ohm’s Law. In words, Ohm’s Law states: the potential difference between two points on a conductor is proportional ( directly related) to the electric current flowing through the conductor. This law puts three variables (V, I, R) into a mathematical formula. Read horizontally: multiplication Read vertically: division REMEMBER: Symbol Unit Measures V volts, V voltage/ potential difference I amperes, A current R ohms, Ω resistance or To problem solve using mathematical formulas: 1. Record your givens: Read problem carefully and record all given quantities, using correct symbols and units. 2. Equation: Write the formulas related to the problem. 3. Substitute: Ensure data is in the correct units. Substitute given quantities into the formula. 4. Calculate: Record with correct units. 5. Answer: Write an answer statement in sentence form. Page 1 Example Problem #1 Example Problem #2 Find: Use: Find: Given V=? I=3A R=2Ω V V = IR (from the triangle) Given V=3V I=? R=2Ω I Use: (from the triangle) =3Ax2Ω =6V = 1.5 A Ohm’s Law Problems Please complete on a separate sheet of paper. 1. What is the current produced by a potential difference of 240 volts through a resistance of 0.2 ohms? 2. What resistance would produce a current of 120 amps from a 6 volt battery? 3. What is the potential difference if a resistance of 25 ohms produces a current of 250 amperes? 4. What is the current produced with a 9 volt battery through a resistance of 100 ohms? 5. Find the current when a 12 volt battery is connected through a resistance of 25 ohms. 6. If the potential difference is 120 volts and the resistance is 50 ohms, what is the current? 7. What would be the current in Problem 3 if the potential difference were doubled? 8. What would be the current in Problem 3 if the resistance were doubled? 9. What resistance would produce a current of 200 amperes with a potential difference of 2000 volts? 10. A 9 volt battery produces a current of 2.0 amperes. What is the resistance? 11. An overhead wire has a potential difference of 2000 volts. If the current flowing through the wire is one million amperes, what is the resistance of the wire? 12. What is the resistance of a light bulb if a 120 volt potential difference produces a current of 0.8 amperes? 13. What voltage produces a current of 50 amps with a resistance of 20 ohms? 14. What voltage produces a current of 500 amps with a resistance of 50 ohms? Page 2 TOTAL RESISTANCE OF CIRCUITS Set up the following circuits. Use the multimeter as an Ohmmeter to measure their total resistance. Answer the questions on the following page. Total Resistance Series Circuits Total Resistance (Ohms) (Ω) ΩT = Parallel Circuits Total Resistance (Ohms) (Ω) ΩT = ΩT = ΩT = ΩT = ΩT = Use the results from the notes and your table to answer the following; 1)What is the resistance? _____________________________________________________________________ _________________________________________________________________________________________ 2) As more lights are connected in SERIES: circle one of the choices in the brackets A) the total resistance of the circuit becomes (high/low) so we say the load becomes (small/large) B) it is (easier/harder/the same) for the battery to push coulombs (electrons) through the circuit. C) the current in the circuit becomes (higher/lower). 3) As more lights are connected in PARALLEL: circle one of the choices in the brackets A) the total resistance of the circuit becomes (low/high) so we say the load becomes (large/small). B) it is (easier/harder/the same) for the battery to push coulombs (electrons) through the circuit. C) the (higher/lower) the current in the main circuit. Page 3