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.

Quic kTime™ and a TIFF (Unc ompres sed) dec ompres sor are needed to see t his pic ture.

Switch-device for making, breaking, or changing the connections in an electric circuit.

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The symbol for switch is part of the diagram above.

Check out this animation…  http://www.rkm.com.au/ANIMATIONS/a 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.

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Qui ckTime™ and a TIFF (U ncompr essed) decompressor are needed to see thi s pi cture.

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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: Qui ckTime™ and a TIFF (U ncompr essed) decompressor are needed to see thi s pi cture.

 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

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: Qui ckTime™ and a TIFF (U ncompr essed) decompressor are needed to see thi s pi cture.

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 ohm•meters 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 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?

 You should get 2.19 x 10 -2  .

 Resistor-device designed QuickTime™ and a TIFF (U ncompr essed) decompressor are needed to see thi s pi cture.

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!

 http://www.youtube.com/watch?v=ymR wIUNlEL4 A short video about Electric Circuits.

 http://www.youtube.com/watch?v=IZgW FFXgpUg&feature=related A short video about Series and Parallel Circuits.