Regents Physics

1. Regents Physics • Circuits Unit Part I • Resistivity and Ohm’s Law

2. Electric Current • Electric current is amount of charge that passes a given point in a circuit • Current is the flow of charge • Current moves in an Electric Circuit, which is a closed path along which charged particles move

3. Current • SI unit for current is the ampere (A) • 1 A = 1 C / s = the amount of charge that passes a point per second q = I t q = the amount of charge in Coulombs that passes a point when a current of one ampere flows for 1 second

4. What is Voltage? • Voltage is the potential difference between two points and represents the amount of work required to move one coulomb of charge from point A to point B • Without a potential difference, current will not flow • Example: The Pipe! • Voltage is measured with a voltmeter

5. How Does Charge Move? • Positive charges tend to move from points of higher potential to points of lower potential, • or • From positive potential to negative potential • Negative charges tend to move in the opposite direction • The direction of a current can be defined as either of these directions

6. How Does Charge Move? • Conventional current is the direction of positive charge flowing from positive to negative terminals • However… • Since most current consists of electron flow, in Regents Physics we say current is the flow of negative charge from negative to positive terminals

7. Conductivity in Solids • For a current to exist in an electric circuit, the circuit must consist of materials through which charge can move • The ability of a material to conduct electricity depends on the number of free charges per unit volume and their mobility • Conductivity – is 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

8. Conductivity in Solids • Different materials have difference conductivity values • Pure metals have many electrons, and these electrons are not bound, or are only loosely bound, to any particular atom • So metals are good conductors since their electrons move easily • Nonmetals are not since their electrons are tightly bound – called insulators

9. Resistance and Ohm’s Law • Electrical Resistance,R, is the opposition that a device or conductor offers to the flow of electrical current • The resistance of a conductor is the ratio of the potential difference applied to it ends and the current that flows through it • Resistance is measured in ohms  Voltage (V) Pipe R = Current (I)

10. Factors Affecting Resistance of a Conductor • The resistance of a wire increases with the increasing length of a wire.. • Because the current (electrons) encounter and collide with an increasing number of atoms • Resistance varies directly with length of wire R  L • Resistance varies inversely with thickness of wire or cross-sectional area

11. Resistivity p • Resistivity, p, is a characteristic of a material that depends on its electronic structure and temperature • The resistance of a wire is directly proportional to its resistivity R  p • Good conductors have low resistivities • Good insulators have high resistivities

12. Resistance in a Conductor click Summary Larger cross-sectional areas allow for move movement of current = less resistance Atoms stay still so current can pass Atoms vibrate slowing down current

13. Resistivity and Resistance • SI unit is the ohm – meter  - m • As the temperature of a conductor increases, its resistivity also increases • We can find the resistivities of selected metals at 20 Celsius in our RT pL L and A are in meters R is resistance R = A end