Electricity and Circuits Moving Charges

Current The amount of charge that passes a point in space in a given amount of time Units are Amperes 1 Ampere = 1 Coulomb/second No current flows unless there is a voltage difference across the circuit

Resistance A property of a material that hinders the flow of current through it Units are Ohms (Ώ) Insulators Current cannot flow through High resistance Conductors Current flows easily Low resistance

What is a circuit? There must be a voltage source (e.g. battery) Provides the potential difference to drive the charges Charges must move around a complete loop http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l2a.html

Parts of a Circuit Internal circuit External circuit What is inside the voltage source External circuit What is outside the voltage source

Roles of the Battery Provides energy Converts chemical potential energy stored in bonds to electrical potential energy Pumps the charge from the - to + terminal Does work against the electric potential Maintains a potential difference across the external circuit

Anatomy of a Light Bulb The base of the bulb is electrically isolated from the sides Current has to flow through the thin filament which has moderately high resistance The filament heats up until it glows because of the resistance The glass globe contains a vacuum so that the filament doesn’t react with the air and burn out http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l2b.html

Find 4 Ways to Light the Light Bulb High potential Low potential Remember you have to go from the positive to the negative end of the battery!

Which ones will work? http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l2b.html

Which ones will work? http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l2b.html

Electricity and Circuits Electric Cell – a device that is a source of electric current because it has a voltage (potential difference) between _the terminals (the positive and negative ends. The charge is separated chemically to produce an electric potential difference.

Battery Battery – a combination of cells put together to produce a _higher voltage_. The size of the battery corresponds to ___how much energy it holds______ (this is NOT the same as voltage)! 

Building Circuits -3 components A source (this really means a source of electrons) A load (this is what is causes charge to “build up” in a circuit) Examples: A wire (this creates a closed loop between the load and the source so that electrons can “flow”

Current and Circuits Conventional Current The direction positive charges would flow in the circuit Yes, we do know NOW that it is really electrons that move through a wire! Ben Franklin thought it was positive charges that flow and his convention stuck Conventional current moves from the positive terminal of the battery to the negative one. In the real world electrons move from the negative terminal of the battery through the external circuit to the positive terminal

Inside the Wire Without an electric field, electrons move randomly Motion is collisional (bumper cars!) In any given period of time as many electrons cross through a cross sectional area going one way as going the other No net current

Inside the Wire With an electric field Motion is still collisional (bumper cars!) Motion in the direction of the field is favored In any given period of time more electrons go through a cross sectional area going with the field than going the other way Net current created Current is the same everywhere in a series circuit No place where charge builds up in the circuit

Turning on the Switch Electric Field is felt everywhere at close to the speed of light Charges start moving in response to the electric field (move to lower potential) everywhere at once Actual drift velocity of electrons is slow (1 m/s) Light bulb lights immediately because electrons within the filament start to move

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Drawing a Circuit Diagram Battery Long line is positive Switch Usually draw open Wires Drawn with perpendicular lines

Series and Parallel Circuits Series circuits have a single loop Parallel circuits have several possible loops Current is split between the loops Overall resistance is lower because there are more paths for current to flow through Series Parallel http://www.curriculum.edu.au/sciencepd/electricity/circ_circ.htm

Potential Changes in the Circuit Going through a wire makes little change in potential Battery increases potential Potential drops in external circuit Potential drops across each device in the circuit Potential gain in battery = - Potential loss in circuit

Which one has the brighter lamps? Series Parallel http://www.curriculum.edu.au/sciencepd/electricity/circ_circ.htm

Relating the Variables

Electric Power Rate at which energy is used in the circuit