Regents Physics

1. Regents Physics 11/10/2008 Mrs. Nephew

2. Electric Current • The rate at which charge passes a given point in a circuit. • Electric Circuit=a closed path along which charged particles move.

3. Switch-device for making, breaking, or changing the connections in an electric circuit. The symbol for switch is part of the diagram above.

4. Check out this animation… • http://www.rkm.com.au/ANIMATIONS/animation-electrical-circuit.html

5. Unit of current • SI fundamental unit of electric current I, is the ampere, A. • The coulomb, C, is the unit of charge. Electric Current Formula: I = ∆q / t

6. Ammeter • A device used to measure current.

7. Conditions Necessary for An Electric Current In addition to a complete circuit, a difference in potential between 2 points in the circuit must exist for there to be an electric current.

8. The potential difference may be supplied by a: • Cell-device that converts chemical energy to electrical energy. • Battery-a combination of two or more electrochemical cells.

9. Potential Difference Can be Measured By: • Voltmeter These symbols are how these devices are represented in an electric circuit diagram.

10. MOST currents consist of electrons in motion. • Positive charges move from higher potential to points of lower potential . • Negative charges tend to move in the opposite direction. • The direction of a current in an electric circuit can be defined as either of these directions.

11. Conductivity in Solids • For a current to exist in an electric circuit, the circuit must consist of materials through which charge can move.

12. Conductivity • A property of a material that depends on the availability of charges that are relatively free to move under the influence of an electric field. • Ability of a material to conduct electricity depends on the number of free charges per unit volume and on their mobility.

13. Pure metals have many electrons and the e- aren’t bound to any particular atom… SO, metals are good conductors b/c their electrons move readily.

14. Nonmetallic elements or compounds- electrons are tightly bound and few are free to move. • These types of materials are called insulators-because they are poor conductors.

15. Remember what potential difference was? • It’s a scalar quantity • It’s unit is joule per coulomb (which is called the volt). • It exists between points in an electric field.

16. Electrical Resistance ® • The opposition that a device or conductor offers to the flow of electric current. • The resistance of a conductor is the ratio of the potential difference applied to its ends and the current that flows through it. (Ohm’s Law) R=V / I

17. R=V / I V= potential difference in volts I=current in amperes R=resistance in volts per ampere Ohm is a derived unit and it’s the SI unit for resistance equal to one volt per ampere. It’s symbol is:

18. R=V / I • This equation is true for entire circuits or for any portion of a circuit AS LONG AS the temperature does not change.

19. Sample Problem: • A student measures a current of 0.10 ampere flowing through a lamp connected by short wires to a 12.0 volt source. What is the resistance of the lamp?

20. Sample Problem: • A student measures a current of 0.10 ampere flowing through a lamp connected by short wires to a 12.0 volt source. What is the resistance of the lamp? Use R=V/I V=12.0 volt I=0.10A

21. Sample Problem: • A student measures a current of 0.10 ampere flowing through a lamp connected by short wires to a 12.0 volt source. What is the resistance of the lamp? Use R=V/I V=12.0 volt I=0.10A R=120

22. Sample Problem: • Try this sample problem involving resistance. This would be a part B type problem on the Regents.

23. Factors that Affect the Resistance of a Conductor • The nature of the material • The geometry of the conductor • The temperature at which the resistance is measured

24. Metallic substances are good conductors. • Low resistances. • Resistivity is the quantity that measures how well a substance resists carrying a current. It’s unit is the ohmmeter.

25. Resistance of a regularly shaped conductor is directly proportional to its length and inversely proportional to it’s cross sectional area. • Making a conductor longer makes it more likely that an e- will collide with the atoms of the conductor, thereby increasing the resistance. • Making it wider increases the number of paths the electrons can take and decreases the resistance.

26. Resistance of metallic conductor generally increases with rising temperature. • Increasing the temp of a conductor increases the vibrational kinetic energy of its atoms, making collisons with electrons more likely.

28. All of these factors can be expressed as: R=L/A (at a specified temperature) L=length of conductor A=cross sectional area of conductor =resistivity in ohm•meters As the temp of a conductor increases, its resistivity increases.

29. Before we end today’s Powerpoint, here’s a sample problem: • Determine the resistance of a 4.00-meter length of copper wire having a diameter of 2.00 millimeters. Assume a temperature of 20C. (Remember to convert mm to m and that the A is the cross sectional area in square meters). Formula for area of a circle?

30. You should get 2.19 x 10-2 .

31. Resistor-device designed to have a definite amount of resistance. Can be used in circuits to limit current flow or provide a potential drop.

32. Variable resistor- a coil of resistance wire whose effective resistance can be varied by sliding a contact point. • As you use more of the coil in circuit, the resistance increases and the current decreases. 11/12/08

33. YouTube Time! • http://www.youtube.com/watch?v=ymRwIUNlEL4 A short video about Electric Circuits. • http://www.youtube.com/watch?v=IZgWFFXgpUg&feature=related A short video about Series and Parallel Circuits.