A flow of electric charge Electric Current A flow of electric charge http://www.youtube.com/watch?v=GM5L24DemBA

I=q/t Equation and Unit Current is the amount of charge that passes a given point per unit of time. I=q/t Ampere or amp= coulomb/second

Example 1: A wire in a DC circuit has a constant current of 25 milliamps. How much charge flows out of the wire in one minute? I=q/t 25 x 10 -3 A = q/ 60 sec Q=1.5 c 3

Conductivity Solids-mobile particles are electrons Metals are good conductors Semiconductors are used to make resistors that control current flow. Liquids- mobile particles are ions Electrolytes are liquids that conduct electricity. Gases- usually neutral but may become ionized due to exposure to radiation, electric fields, or collisions with particles. Mobile particles may be ions or e-. Plasma is an ionized gas.

Electricity! Water is actually an insulator – why is this not a good place for an electric eel?

Current in a Metallic Solid! Valence electrons are very easily removed from metals (chemists know this - metals are “givers” of electrons in chemical reactions with nonmetals). When these electrons are excited away from their atoms, they become FREE ELECTRONS, which can then make up current in a metal conductor. Consider a piece of copper wire with no voltage (potential difference) applied. Naturally occurring free electrons drift randomly throughout. 6

Conditions necessary for a Current Source of potential difference or Voltage ex battery, generator Complete or closed conducting path signal-circuit_en.jar

Current in a Metallic Solid! If a voltage (potential difference) is applied, all the free electrons feel the field at the same time! They will then all migrate in one direction. There are many interactions with atoms in the wire, so electrons do not travel very quickly through the wire, but since every electron feels the electrostatic force from the voltage at the same time, current flows at every point in the wire at the same instant. 8

Direction of current flow There are two ways to define current flow. Electron flow- this is the movement of electrons through the circuit. E- flow from the –terminal to the +terminal. Conventional flow- this is the flow of positive charge through the circuit. Flows from the + terminal to the – terminal. We use e- flow!

Conventional Current! Conventional Current Flow! As each electron leaves an atom to become current, it leaves behind a positively charged ion (atom). The atoms in the metal cannot actually move. As the electrons flow to the right, however, they effectively create a flow of positive charge to the left. This imaginary flow of + charges is referred to as Conventional Current. Electron Flow! Conventional Current Flow! 10

Resistance Effectiveness in holding back a current Measured in ohms Ω 1 Ω is the resistance that allows 1 amp of current to flow through a potential difference of 1 volt

Factors that affect resistance of a wire Length- the longer the wire the greater the resistance Cross-sectional area- the larger the area (fat wire) the smaller the resistance

Factors that affect resistance of a wire Resistivity- ρ -How conductive a wire is based on the nature of its material Measured in ohm-meter Table on Reference tables The lower the resistivity the greater the conductivity of the material.

Note that all the resistivites are written with the same order of magnitude. This is so they can easily be compared

Factors that affect resistance of a wire Temperature- increase in temp causes molecules to move faster and more randomly in metals the resistivity increases with temperature In nonmetals and solutions the opposite is true Superconductors- metals super cooled so the resistance is close to zero

R= ρ L A Resistance Equation Resistance is directly proportional to the resistivity, directly proportional to length and inversely proportional to the cross sectional area Wire of least resistance- short, fat, cold

Example: The cross-sectional area of a 35-meter length of copper wire at 20°C is 5.2x10-7 m2. Determine the resistance of the wire. R= ρ L A R= 1.72 x 10 -8 Ωm ( 35m) 5.2 x 10-7 m2 R= 1.2 Ω 17

Effect of Various Electric Currents on the Body Current in amps Effect on the body 0.001 Can be felt 0.005Painful Painful 0.01 Involuntary Muscle Contractions 0.015 Loss of Muscle Control 0.07 Through the Heart probably fatal if current lasts more than a second

The Green Mile http://www.youtube.com/watch?v=0KMnAm7fiUo

Ohm’s Law The current through a circuit varies directly with the applied potential difference (V) and inversely with the resistance. V=IR or R=V/I V V I R I

Ohm’s Law I=2A R=60Ω V=120V I=V/R ohms-law_en.jar What is the current in a 60 Ω resistor attached to a voltage source of 120 V? R=60Ω V=120V I=V/R 120V/60Ω=I I=2A

Power A charge does work as it moves in a circuit The rate at which work is done is power Electrical power is the rate at which electrical energy is converted into another form ex. Mechanical, heat,light Power is equal to the product of current and voltage

Power Equation P=VI= I2R=V2/R Watt=Volt x amp 1kilowatt=1000watts

W=Pt Electrical Energy Joule=watt x sec If power is energy or work per unit time; energy must be power x time W=Pt Joule=watt x sec Power companies bill customers for energy by the kilowatt hour as the joule is too small to be practical.

Required Schematic Symbols! 25

Simple Circuit Source of Potential difference Load Conducting path V = 5v + R = 5Ω - What’s in this circuit?

DC Series & Parallel Circuits! Behavior of current, voltage and resistance in series and parallel circuits.

Series Circuit Characteristics! A basic electrical circuit, with only one path for current to flow. V = 7v + R1 = 5Ω -- R2= 10Ω RL = 20Ω Example: 28

Resistance in a Series Circuit… 1000Ω 100Ω 560Ω

…is Additive! REQ = R1 + R2 + R3 = 1000Ω + 100Ω + 560Ω = 1660Ω !

Series Resistance! So we use this equation to determine the Equivalent (total) Resistance: REQ = R1+ R2 + R3 + … Calculate REQ: V = 7v + R1 = 5Ω -- R2= 10Ω RL = 20Ω Example: 31

Current in a Series Circuit stays… Constant! (There’s only one path for current!)

Series Current! We use Ohm’s Law to determine IT, given an equivalent resistance and a voltage: REQ = 35Ω Calculate IT: V = 7v + R1 = 5Ω - R2= 10Ω RL = 20Ω Example: Otherwise, once we know IT, we can find it anywhere else by: IT = I1 = I2 = I3 = … 33

Series Voltage! We use Ohm’s Law to determine V, given an equivalent resistance and IT: REQ = 35Ω, IT = .2A Calculate V: V = ? + R1 = 5Ω - R2= 10Ω RL = 20Ω Example: 34

Voltage Drops in a Series Circuit… Add up to the battery voltage! .95v + .09v + .58v = 1.62v Are α to the size of the resistors! V = V1 + V2 + V3 + … (V is the battery voltage)

1000Ω 100Ω Notice that the 1000Ω resistor has a voltage drop 10x bigger than the 100Ω resistor!

Series Voltage! Use Ohm’s Law to determine the voltage drops on the resistors and compare to the battery voltage (V = 7v)! V = 7v + R1 = 5Ω - R2= 10Ω RL = 20Ω Example: IT = .2A 37

Parallel Circuit Characteristics! Question: What is a Parallel Circuit? Answer: A basic electrical circuit, with more than one path for current to flow. V + R1 -- R2 Example: 38

Voltage Drops in a Parallel Circuit … V = V1 = V2 = V3 = … = Battery Voltage (V)! That’s because the battery’s full potential is directly connected across each resistor!

Current in a Parallel Circuit… Is driven by the full battery voltage across each resistor and is inversely α the size of the resistor in each branch! Branch currents add up to equal the total current coming out of the battery! (in this case, IT = 20.8 mA) 100Ω 1000Ω IT = I1 + I2 + I3 + …

Kirchoff’s Current Law! The sum of currents entering a node must equal the sum of the currents leaving a node! I1 = 10 A I2 = 4 A I3 = 8 A I4 = ? I2 = 5 A I1 = 4 A I3 = 2 A I4 = ? 41

Resistance in a Parallel Circuit… 70.9 Ω

…is…

…Not Additive! The same resistance is measured across all three resistors! That equivalent resistance (REQ) is always smaller than the smallest branch resistance! Adding more parallel resistors would further decrease REQ!

Parallel Circuit Characteristics! The equation for determining REQ is given in your reference tables as: 1 1 1 1 REQ R1 R2 R3 + = I don’t like this form of the equation because students forget to invert their final answer to get REQ! Invert both sides BEFORE YOU SUBSTITUTE! 45

Equation for Parallel Resistance! REQ = 1 1 1 R1 R2 + … V = 10v + -- R1 = 10Ω R2 = 10Ω

Special Case Equation for TWO Similar Parallel Resistors! REQ = R # of R V = 10v + -- R1 = 10Ω R2 = 10Ω

Summary of Series and Parallel Equations! 48