Electricity and Circuits

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 • 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

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

Circuit Symbols http://www.curriculum.edu.au/sciencepd/electricity/circ_symbols.htm

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

Electricity and Circuits