Electricity and CircuitsMoving 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

Remember you have to go from the positive to the negative end of the battery!

High potential

Low potential

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

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 Parallelhttp://www.curriculum.edu.au/sciencepd/electricity/circ_circ.htm

Potential Changes in the Circuit

Battery increases potential Potential drops inexternal circuit

Potential drops across eachdevice in the circuit

Potential gain in battery = - Potential loss in circuit

Going through a wire makes little change in potential

Which one has the brighter lamps?

Series Parallel

http://www.curriculum.edu.au/sciencepd/electricity/circ_circ.htm

Relating the Variables

rrentVoltage/CuResistance

sistanceVoltage/ReCurrent

ResistanceCurrentVoltage

I

VR

R

VI

IRV

Electric Power Rate at which energy is used in the circuit

Watts!of units -has-before likeJust second

Work

second

Coulombs

Coulomb

Work Power

so secondper Coulombs isCurrent

Coulomb.per Work toequal

is which Coulombper PEin change is Voltage

CurrentVoltagePower

VIP