Regents Physics 11/10/2008 Mrs. Nephew

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

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

Check out this animation… nimation-electrical-circuit.html

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

Ammeter A device used to measure current.

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.

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.

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

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.

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

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.

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.

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.

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.

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

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:

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.

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?

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

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 

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

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

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.

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.

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.

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 ohmmeters As the temp of a conductor increases, its resistivity increases.

Before we end today’s Powerpoint, here’s a sample problem: Determine the resistance of a 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?

You should get 2.19 x .

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

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

YouTube Time! wIUNlEL4 A short video about Electric Circuits. wIUNlEL4 FFXgpUg&feature=related A short video about Series and Parallel Circuits. FFXgpUg&feature=related