PHYSICS MR. BALDWIN Ohm’s Law 31 March 2014

1. PHYSICS MR. BALDWINOhm’s Law 31 March 2014 AIM: Upon what factors does resistance depend? DO NOW: QUIZ Take out a sheet of paper and answer the questions that follow

2. What is the name given to a charged atom or molecule (one that gains or loses electrons)? When an atom loses an electron, what happens to its net charge? What is the name given to such atom that loses electrons? When an atom gains an electron, what happens to its net charge? What is the name given to such atom that gains electrons? BALDWIN 2

3. CHARGED PARTICLES • ION: When an atom or molecule gains or loses an electron, it becomes charged. • ANION: negatively charged ion • When an atom gains electrons, its net charge is negative • CATION: positively charged ion • When an atom loses electrons, its net charge is positive

4. The Electric Battery Volta discovered that electricity could be created if dissimilar metals (electrodes) were connected by a conductive solution called an electrolyte. This is a simple electric cell. A battery transforms chemical energy into electrical energy.

5. The Electric Battery Electrode that attracts anions is called an ANODE (+) Electrode that attracts cations is called a CATHODE (–)

6. PHYSICS MR. BALDWINOhm’s Law April 1, 2014 AIM: Upon what factors does the resistance of a conductor depend? DO NOW: QUIZ Take out a sheet of paper and answer the questions that follow

8. Recall: Electric Current Electric current (I) is the rate of flow of positive charges through a conductor: where q is the charge and t is the time Unit of electric current: the ampere, A. 1 A = 1 C/s.

9. Recall: Charges The charge (q) is the number of positive charges : What is the magnitude of the charge of an electron? What is the typical UNIT of a charged object?

10. TEST YOURSELF • Calculate the current where 10 coulombs of charge pass a point in 5 seconds. • Calculate the current in a lightning bolt that delivers a charge of 35 coulombs to the ground in 1ms.

11. A current of 5.0 A passes through the bulb in 20 µs. If the charge of an electron is 1.6 x 10-19 C, how many electrons flow through the bulb? TEST YOURSELF 11 BALDWIN

12. Resistance & Resistivity The electrical resistance of an electrical conductor is the opposition to the passage of an electric current through that conductor. The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area: The constant ρ is called the resistivity and is a characteristic of the material. It’s unit is the m The conducting ability of a material is often indicated by its resistivity. The lower the resistivity, the better the conductivity.

13. Variation of resistance with length and area: the analogy to water flow How do you think the flow rate will be affected if you increase the cross-sectional area or length of the pipe along which the water has to flow? resistance increases with length resistance decreases with diameter or cross-sectional area

14. Resistivity

15. Observation • Looking at the table of Resistivity, which metal material is the most conductive? • Which metal is the least conductive? • Which substance is the least conductive and why?

16. Practice Determine the resistance of a 1-mile length of 12-gauge copper wire. (Hint:1 mile = 1609 m; diameter of 12 gauge copper wire = 0.2117 cm) • L = 1609 m, • A = πR2 • ρ= 1.7 x 10-8Ωm. First, find the cross-sectional area: A = π•r2 = (3.14) • [ (0.002117 m) / 2)]2 = 3.519 x 10-6 m2 Now, substitute into the above equation to determine the resistance. R = (1.7 x 10-8Ωm) • (1609 m) / (3.519 x 10-6 m2) R = 7.8  (7.7709 ohm)

17. PHYSICS MR. BALDWINOhm’s Law April 2, 2014 AIM: How does the temperature, area and length of a conductor affects the resistance? DO NOW: QUIZ Determine the resistance of a 1-m length of 12-gauge gold wire. (Hint: diameter of 12 gauge wire = 0.2117 cm)

18. Mathematical Reasoning • Two wires - A and B - with circular cross-sections have identical lengths and are made of the same material. Yet, wire A has four times the resistance of wire B. How many times greater is the diameter of wire B than wire A?

19. Resistivity & Temperature For any given material, the resistivity increases with temperature: where T is temperature in 0C and is the thermal coefficient with unit 0C-1 or /0C.

20. Resistivity

21. RECALL: Resistance & Resistivity The constant ρ is called the resistivity and is a characteristic of the material. It’s unit is the m

22. June 2011 A circuit contains a 12.0-volt battery, an ammeter, a variable resistor, and connecting wires of negligible resistance, as shown below. A variable resistor made of nichrome wire is maintained at 20.°C. The length of the nichrome wire may be varied from 10.0 centimeters to 90.0 centimeters. The ammeter reads 2.00 amperes when the length of the wire is 10.0 centimeters. Determine the resistance of the 10.0-centimeter length of nichrome wire. [1] Calculate the cross-sectional area of the nichrome wire. [Show all work, including the equation and substitution with units.] [2]

23. June 2010 A 3.50-meter length of wire with a cross-sectional area of 3.14 × 10–6 m2 is at 20° Celsius. The current in the wire is 24.0 amperes when connected to a 1.50-volt source of potential difference. Determine the resistance of the wire. [1] Calculate the resistivity of the wire. [Show all work, including the equation and substitution with units.] [2]

25. PHYSICS MR. BALDWINOhm’s Law 3 April, 2014 AIM: What is the relationship between the current and resistance in a circuit? (What is Ohm’s Law?) DO NOW: QUIZ • What is electrical resistance? • What variables(factors) do you think might affect electrical resistance in an electric conductor? HOMEWORK: Worksheet 13.3 Ohm’s Law

26. opposition to the flow/passage of an electric current through a conductor. • factors affecting resistance: • Length of conductor • Cross-sectional area • Substance • Temperature

27. Simple Circuit In order for current to flow, there MUST be a path from one battery terminal, through the circuit, to the other battery terminal AND a potential difference across the terminal. Which one of these circuits will work? (c) THIS ONE

28. Simple Circuit Schematic A complete circuit is one where current can flow all the way around. By convention, current is defined as the flow of positive charges from + to – (high potential to low potential). Electrons actually flow in the opposite direction. Simple Circuit must contain? Battery (power source) Conductor (wire) Device Switch

29. Ohm’s Law Experimentally, it is found that the current in a wire is proportional to the potential difference between its ends:

30. Ohm’s Law: Graphically Materials that do not follow Ohm’s law are called nonohmic.

31. Ohm’s Law: Mathematically The ratio of the voltage to the current in a circuit is called the resistance. Unit of resistance: the ohm, Ω. 1 Ω = 1 V/A.

33. Electric Hazards Even very small currents: 10 to 100 mAcan be dangerous, disrupting the nervous system. Larger currents may also cause burns. Household voltage can be lethal if you are wet and in good contact with the ground. SO…Be careful boys & girls!

34. Some clarifications: • Batteries maintain a (nearly) constant potential difference; the current varies. • Resistance is a property of a material or device. • Current is not a vector but it does have a direction. • Current and charge do not get used up. Whatever charge goes in one end of a circuit comes out the other end.

35. Summarize the following terms • Ion • Anion • Cation • Anode • Cathode • Current • Circuit

36. Resistors Standard resistors are manufactured for use in electric circuits; they are color-coded to indicate their value and precision.

37. Resistors

38. http://www.physicslessons.com/exp22b.htm

39. Electric Power Power, as in kinematics, is the energy transformed by a device per unit time. In electricity, recall

40. Electric Power The unit of power is the Watt, W. For Ohmic devices, we can make the substitutions:

41. Electric Power What you pay for on your electric bill is not power, but energy – the power consumption multiplied by the time. We have been measuring energy in joules, but the electric company measures it in kilowatt-hours, kWh.

42. Alternating Current Current from a battery flows steadily in one direction (direct current, DC). Current from a power plant varies sinusoidally (alternating current, AC).

43. Superconductivity In general, resistivity decreases as temperature decreases. Some materials, however, have resistivity that falls abruptly to zero at a very low temperature, called the critical temperature, TC.

44. PHYSICS Prof.BALDWINELECTRICITY 10 April 2014 AIM: How do we determine the equivalent resistance in series circuits? DO NOW: Draw a simple series circuit and describe its characteristics in terms of the current and voltages (potential differences). 44 BALDWIN

45. DC Circuits https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc-virtual-lab

46. Resistors in Series A series connection has a single path from the battery, through each circuit element in turn, then back to the battery.

47. Diagramming Circuits • An ammeter measures the current through a circuit component. • The same current going through the component must go through the ammeter, so there can be only one current path. • A connection with only one current path is called a series connection.

48. Resistors in Series The current through each resistor is the same; the voltage depends on the resistance. The sum of the voltage drops across the resistors equals the battery voltage.

49. Resistors in Series From this we get the equivalent resistance (that single resistance that gives the same current in the circuit).

50. Resistors in Series Determine the equivalent resistance