Two Chapter 5

Electric Circuits

• Circuit - An arrangement and connection of electronic components or elements for some purpose like the transmission of radio or television etc.
• Electrons move in the presence of electric fields to regions of lower electric potential.
• As a result of work done on the electrons, they have kinetic energy which can be used to produce heat, light, and motion.
• We do this by connecting sources to loads by means of circuits.
• Load - an electronic component or element.
• Electrons flow from the negative electrode (terminal) to the positive terminal of a source
• While travelling through the load, electrons transfer electric potential energy acquired from the source, to the load.

Circuit Symbols

• To illustrate circuits, circuit diagrams are used.
• These diagrams use the symbols shown in One Chapter 5.
• Elements - components of an electric circuit.
• Schematic diagrams - circuit diagrams

Direction of Current

• Conventional current - The flow of positive charges from positive to negative terminal of the source.
  • Is in the same direction as the electric field.
  • Defined this way because Benjamin Franklin defined positive charge as an excess of electric fluid and negative charge as a deficit of electric fluid.
  • Therefore current should flow from excess (+) to deficit (-).
• Electron Flow Current - The flow of electrons from negative to positive terminals of the source.
  • Is in the opposite direction to the electric field.
  • Is defined this way because today we recognize the existance of 2 fluids, one positive and one negative.
  • The positive fluid in solids is not mobile and therefore current must be caused by the negative fluid (electrons) which must flow from excess (-) to deficit (+).
• In this chapter we will use electron flow current.
• Only a few changes in thinking are required to translate from electrton flow to conventional current.
• Current - The term used to denote the magnitude of the rate of charge flow

• Note that:
  • In liquids and gases, charges can flow in either direction and simultaniously.
  • Negative charge flowing north is electrically equivalent to positive charge flowing south.

Series Circuits

• Series Circuits - electrons have only one path to follow through a circuit.

Parallel Circuit

• Parallel Circuit - the electrons have a choice of several paths through the circuit.

Kirchoff's Laws

• Electrons lose energy when passing through loads
• There are two questions which arise.
  • What determines the amount of electric potential energy lost in a load?
  • When electrons can move through several paths, what determines how many will go in each direction?
• The answers to these questions come from:
  • The Law of Conservation of Energy - energy is neither created nor destroyed but can be changed from one form to another without loss.
    • Electrons gain energy in sources
    • Electrons lose energy in loads
    • Total energy gained in one trip around the circuit equals the total energy lost in one trip around the circuit.
  • Conservation of Charge - Charge is neither created nor destroyed in electric circuits.
• By experiment, Kirchhoff discovered how to apply these conservation laws to circuits. This resulted in two laws.
  • Kirchhoff's Voltage Law - (KVL) - around any complete path through an electric circuit, the sum of the increases in electric potential is equal to the sum of the decreases in electric potential.
  • Kirchhoff's Current Law - (KCL) - At any junction point in an electric circuit, the total electric current into the junction is equal to the total electric current out of the the junction.

Resistance in Electric Circuits

• Resistance - opposition to electron flow.
  • Results in energy loss from electrons
  • This energy is given to atoms and molecules in the resistor.
• To measure resistance we use a circuit like the following

• With the above circuit, the resistance of the resistor is determined by comparing the electric potential difference to the amount of current.
• To compare, Collect data of voltage versus current by varing the voltage from the source and measuring the effect on current to product a table like the one shown.
• Plot a graph of voltage versus current and find the slope.
• The slope is the resistance.

Ohm's Law

• Ohm's Law - The potental difference between two points in a conductor varies directly as the current between the two points

• The units of V in the above equation are volts.
• The units of I in the above equation are Amperes.
• The unit of resistance is the Ohm (W).

1 W = 1 V/A

• The Ohm - is the electric resistance of a conductor that has a current through it of one Ampere when the potential difference across it is 1 volt.

Factors Affecting Resistance

• Resistance depends upon
  • Length
  • Cross-sectional Area
    • R a 1/Area
    • Therefore, doubling the area makes resistance 1/2 as large.
  • Temperature - most materials have increased resistance with increased temperature
    • there is a 3 to 5% increase for every 10^oC increase in temperature.
    • Some materials are the reverse (increased temperature decreases resistance)
    • For most materials, there is a critical temperature below which the material loses all resistance (it becomes a superconductor).
  • Material - Atomic structure has a marked effect on resistance.
• Resistivity - The resistance per unit of a material
  • Unit of material is most often 1 m long and has cross-sectional area of 1 m²
  • Varies with temperature.

• R = resistance in ohms, L = length in meters, = A is cross-sectional area in m², and r = resistivity in W.m²/m.

December 31, 2013