Chapter 34-35 - Electric Current

ELECTRIC POTENTIAL ENERGY In order to bring two like charges near each other work must be done. If monkey-girl brought 2 or 3 charges instead of one, then she would have had to do more work so she would have created more electrical potential energy.

ELECTRIC POTENTIAL Since the electrical potential energy can change depending on the amount of charge you are moving, it is helpful to describe the electrical potential energy per unit of charge

VOLTAGE

VOLTAGE CALC. EXAMPLE: The amount of work done by the person is 30J, this is also the amount of electrical potential energy that is possessed by all three charges together.  The electrical potential (not energy) is the amount of energy per unit of charge At the original position of the charges they have no energy, so they also have no electrical potential or 0 volts.  Once they are pulled apart, they have have an electrical potential of 10 volts.  We could say that the electrical potential difference from one point to the other is 10 volts.  Keep in mind that the electrical potential describes the amount of energy per unit of charge.  This means that when one of the charges is released, the electric field will do 10 Joules of work on the charge so it will have a kinetic energy of 10 Joules the instant before it strikes the negative charge.

VOLTAGE Voltage is an “electric pressure” that can produce a flow of charge, or current, within a conductor. The flow is restrained by the resistance it encounters.

Voltage Sources Charges flow only if there is a potential difference. Something that provides a potential difference is known as a voltage source. If a positively, and a negatively charged metals are next to each other, there will be a large voltage between them.

Voltage Sources In dry cells and wet cells, energy released in a chemical reaction occurring inside the cell is converted to electrical energy. Generators convert mechanical energy to electrical energy. The electrical potential energy produced is available at the terminals of the cell or generator.

Flow of Charge Charge flows from one end to the other. Charge flows when there is a potential difference, or difference in potential (voltage), between the ends of a conductor. The flow continues until it reaches a common potential. When there is no potential difference, there is no longer a flow of charge through a conductor.

Flow of Charge When there is no potential difference, there is no longer a flow of charge through a conductor. To attain the flow of charge continuously, some arrangement must be provided to maintain a difference in potential.

CURRENT: The flow of charge is called electric current When the flow takes place along one direction, it is called direct current (DC). When it flows to and fro, it is called alternating current (AC).

CURRENT: When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field. The free electron is therefore the current carrier in a typical solid conductor.

CURRENT: For an electric current of 1 ampere, 1 coulomb of electric charge (which consists of about 6.242 × 1018 electrons) drifts every second through any imaginary plane through which the conductor passes.

CURRENT: where Q is the electric charge in coulombs (ampere seconds) , AND t is the time in seconds

Electric Current Electric current is the flow of electric charge. In solid conductors the electrons carry the charge through the circuit because they are free to move throughout the atomic network. These electrons are called conduction electrons.

Electric Current Once again: Electric current is measured in amperes. An ampere is the is the flow of 1 coulomb (6.24 billion billion electrons) of charge per second. If there are 5 amperes in a current, then there are 5 coulombs.

Electric Resistance The current also depends on the resistance that the conductor offers to the flow of charge which is called electric resistance. The resistance of a wire depends on the conductivity of the material used in the wire and also on the thickness and length of the wire.

Electric Resistance Thick wires have less resistance than thin wires. Longer wires have more resistance than short wires. The greater the jostling about of atoms within the conductor, the greater resistance the conductor offers to the flow of charge.

Electric Resistance: This resistance depends on a few things: First, the conductivity of the material the wire is made of. If electrons can move through the material better, then there will be less resistance. Second, it depends on the thickness of the wire. The thicker the wire, the more paths it offers for flow of charge; therefore, the less resistance it gives to the movement of charge.

Electric Resistance: Third, it depends on the length of the wire. If the wire is longer it will have greater resistance to the flow of charge. Finally, it can depend on temperature. Generally as the temperature increases so does the resistance, however there are some exceptions to this rule.

Electric Resistance: The unit for resistance is the Ohm, which we abbreviate Ω. The ohm is the electric resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.

Ohm’s Law Ohm’s law is the relationship among voltage, current, and resistance. current = voltage resistance Relationship among the units of measurement 1 ampere = 1 volt ohm

Ohm’s Law A voltage source, V, drives an electric current, I , through resistor, R, the three quantities obeying Ohm's law: V = IR.

Electric Power Electric Power is the rate at which electrical energy is converted into another form such as mechanical energy, heat, or light.

Electric Power (Continued) Electric power is equal to the product of current and voltage. Ex: electric power = current x voltage Unit form: 1 watt = 1 amp x 1 volt

WATT

Power in Electric Circuits Power = W = E J/sec t t E = V I t Power = V I t = V I t

Power in Electric Circuits P = V I V = I R P = (I R) I I = V/R P = I2 R P = V (V/R) P = V2 / R

Ohm’s Law and Electric Shock The damaging effects of electric shock are the result of current passing through the body. From Ohm’s law we can see that this current depends on the voltage applied, and also on the electric resistance of the human body

Converting AC to DC An AC-DC converter consists of a transformer to lower the voltage and a diode, a tiny electronic device that acts as a one way valve to allow electron flow in only one direction

Speed of Electrons in a Circuit When you make a telephone call, the signal is transmitted through the conductors at nearly the speed of light It is not the electrons that move at this speed but the signal

Speed of Electrons in a Circuit (Continued) The electric field lines between the terminals of a battery are directed through a conductor, which joins the terminals. In an AC circuit, the conduction electrons don’t make any net progress in any direction. a

The Source of Electrons in a Circuit The source of electrons in a circuit is the conducting circuit itself. When you plug a lamp into an AC outlet, energy flows from the outlet into the lamp, not electrons.

Source of Electrons in a Circuit (Continued) When you are jolted by an AC electric shock, the energy simply causes free electrons in your body to vibrate in unison. Small vibrations tingle Large vibrations can be fatal.

Direct Current and Alternating Current Direct current – a flow of charge that always flows in one direction Alternating current – electrons in the circuit move first in one direction and then in the opposite direction, alternating back and forth through relatively fixed positions

Light Bulbs in Series

Light Bulbs in Parallel

Analysis of Series Circuit Req = R1 + R2 + R3 = 17 + 12  + 11  = 40 

Voltage Drop for Series Circuits Vbattery = V1 + V2 + V3 + ...

Electric Potential Diagram

Analysis of Series Circuit

Analysis of Series Circuit Ohm's law equation ( V = I • R) Itot = Vbattery / Req = (60 V) / (40 ) = 1.5 amp

Analysis of Series Circuit The 1.5 amp value for current is the current at the battery location. For a series circuit with no branching locations, the current is everywhere the same. Ibattery = I1 = I2 = I3 = 1.5 amp

Analysis of Series Circuit V1 = I1 • R1 V1 = (1.5 A) • (17 ) V1 = 25.5 V V3 = I3 • R3 V3 = (1.5 A) • (11 ) V3 = 16.5 V V2 = I2 • R2 V2 = (1.5 A) • (12 ) V2 = 18 V

Parallel Circuit Current Itotal = I1 + I2 + I3 + ...

Parallel Circuit Current

Series and Parallel Circuits

Parallel Circuit Current Itotal = I1 + I2

Parallel Circuit Resistors 1 / Req = 1 / R1 + 1 / R2 + 1 / R3 + ...

Parallel Circuit Resistors

Parallel Circuit Resistors

Parallel Circuit Resistors

Parallel Circuit Resistors

Voltage Drop for Parallel Circuits

Analysis of Parallel Circuit 1 / Req = 1 / R1 + 1 / R2 + 1 / R3 = (1 / 17 ) + (1 / 12 ) + (1 / 11 ) 1 / Req = 0.23306  -1 Req = 1 / (0.23306 -1) Req = 4.29 

Analysis of Parallel Circuit Req = 4.29  Itot = Vbattery / Req = (60 V) / (4.29) Itot = 14.0 amp

Analysis of Parallel Circuit The voltage drop across each one of the three resistors is the same as the voltage gained in the battery: V battery = V1 = V2 =  V3 = 60 V

Analysis of Parallel Circuit I1 =  V1 / R1 I1 = (60 V) / (17 ) I1 = 3.53 amp I3 = V 3 / R3 I3 = (60 V) / (11 ) I3 = 5.45 amp I2 = V 2 / R2 I2 = (60 V) / (12 ) I2 = 5.00 amp

Series and Parallel Circuits As the number of resistors (light bulbs) increases, what happens to the overall current within the circuit? As the number of resistors (light bulbs) increases, what happens to the overall resistance within the circuit? If one of the resistors is turned off (i.e., a light bulb goes out), what happens to the other resistors (light bulbs) in the circuit? Do they remain on (i.e., lit)?

Practice Problem http://www.wisc-online.com/objects/index_tj.asp?objID=DCE3202