Current Electricity

11.1 Electric Current

Circuit – continuous conducting path between terminals of a battery (or other source of EMF) Electric Current – flow of charge (electrons) I – current (amperes) Q – charge (coulomb) T – time

11.1 Electric Current Ampere (for Andre’ Ampere) Usually called an amp Open Circuit – break in the circuit, no current flow

11.1 Electric Current Short Circuit – when the load is bypassed Current increase Ground – allows for a continuous path for charge flow

11.1 Electric Current For historical reasons, current is defined as being in the direction that positive charge flows

11.2 Current and Drift Speed

Drift Speed – average distance that an electron moves in a given time period For an electron in a copper wire

11.3 Resistance and Ohm’s Law

George Simon Ohm The actual values depend on the resistance of the conductor Called Ohm’s Law R – resistance measured in Ohms (  )

11.3 Resistance and Ohm’s Law Only true for Ohmic materials Vacuum Tubes, Transistors, Diodes are nonohmic

11.3 Resistance and Ohm’s Law A graph of current vs. potential difference The metallic conductor is ohmic The diode and filament are not

11.3 Resistance and Ohm’s Law Resistor – anything that uses electric energy Resistor – device used to control current The symbol for a resistor is

11.3 Resistance and Ohm’s Law The resistance value of a resistor is indicated by the colored bands on the resistor

11.3 Resistance and Ohm’s Law Misconceptions 1.Cells (batteries) do not put out a constant current. They maintain a constant potential difference. 2.Current passes through a wire and depends on the resistance of the wire. Voltage is across the ends of the wire. 3.Current is not a vector, it is always parallel to the conductor. The direction is from + to -.

11.3 Resistance and Ohm’s Law Misconceptions 4. Current or charge do not increase or decrease. The amount of charge in one end of the wire comes out of the other end.

11.4 Resistivity

Resistance is found to be directly proportional to its length and inversely proportional to its cross sectional area.  is called the resistivity (  m) Longer extension cords must be thicker to keep resistance low

11.4 Resistivity Some common resistivity values MaterialResistivity (Wm) Temperature Coefficient (C o-1 ) Silver1.59x Copper1.68x Gold2.44x Aluminum2.65x Tungsten5.6x Platinum10.6x Nichrome100x

11.4 Resistivity Best Conductor is Silver, but Copper is close and much cheaper Tungsten is used in filaments Nichrome Apparently an Anime character

11.5 Superconductors

An element or compound that conducts electricity without resistance Become insulators above a critical temperature Uses MagLev Trains

11.6 Electrical Energy and Power

The rate of energy flow for an electric circuit That is more commonly written as Combining with Ohm’s Law it can also be written

11.6 Electrical Energy and Power The power company charges by the kilowatt- hour (kWh) Just a cool picture

11.6 Electrical Energy and Power Household circuits – wires will heat up as current increases In a 20A household circuit In a 15A household circuit Circuits are typically designed to run at 80% of the rated power output Different circuits have different gauge wires (diameter)

11.6 Electrical Energy and Power Circuit Breakers and Fuses Break the circuit

11.7 Sources of EMF

EMF – electromotive force – the potential difference between the terminals of a source when no current flows to an external circuit (  )

11.7 Sources of EMF A battery will have an internal resistance (r) So there is a potential drop due to the current that travels through the cell So the actual potential across the terminals of a cell will be This is called the terminal voltage

11.8 Resistors in Series

11.7 Sources of EMF When resistors are place in a single pathway They are said to be in series A schematic would look like this

11.7 Sources of EMF The current in a series circuit is the same throughout the circuit The potential across the source of EMF is equal to the sum of the potential drops across the resistors

11.7 Sources of EMF Since potential can be defined as We can rewrite the equation for potential as

11.9 Resistors in Parallel

When resistors are place in a multiple pathways They are said to be in parallel A schematic would look like this

11.9 Resistors in Parallel The potential difference in a parallel circuit is the same throughout the circuit The current through the source of EMF is equal to the sum of the current through the resistors

11.9 Resistors in Parallel Since current can be defined as We can rewrite the equation for potential as

11.9 Resistors in Parallel Circuits that contain both series and parallel components need to be solved in pieces This circuit contains 20  resistors in series 25  resistors and load series to each other and parallel to the 40  resistor

11.10 Kirchhoff’s Rules

Circuits that are a little more complex We must use Kirchhoff’s rules Gustov Kirchhoff They are applications of the laws of conservation of energy and conservation of charge

11.10 Kirchhoff’s Rules Junction Rule – conservation of charge At any junction, the sum of the currents entering the junction must equal the sum of all the currents leaving the junction

11.10 Kirchhoff’s Rules Loop Rule – the sum of the changes in potential around any closed pathway of a circuit must be zero For loop 1

11.10 Kirchhoff’s Rules Steps 1.Label the current in each separate branch with a different subscript (the direction does not matter, if the direction is wrong, the answer will have a negative value) 2.Identify the unknowns and apply V=IR 3.Apply the junction rule (at a in our case) so that each current is in at least one equation I1I1 I2I2 I3I3

11.10 Kirchhoff’s Rules Steps 4.Choose a loop direction (clockwise or counterclockwise) 5.Apply the loop rule (again enough equations to include all the currents) a. For a resistor apply Ohm’s law – the value is positive if it goes in the direction of the loop b. For a battery, the value is positive if the loop goes from – to + (nub to big end) I1I1 I2I2 I3I3

11.10 Kirchhoff’s Rules Steps We’ll do the two inside loops 6. Combine the equations and solve I1I1 I2I2 I3I3

11.11 RC Circuits

Used windshield wipers timing of traffic lights camera flashes When the switch is closed current flows and potential difference across the capacitor increases

11.11 RC Circuits Eventually the potential difference across the capacitor is equal to the EMF of the battery Current is now zero

11.11 RC Circuits The shape of the curve is given by RC = the time constant Measures how quickly the capacitor becomes charged All circuits have some resistance, so they all take time to charge