Electric Current Chapter 2 Section 2 Flow of

Electric Current Chapter 2, Section 2

Flow of Electric Charges • Lightning – Large amounts of electrical energy • Can’t use to power electrical devices – Tvs, radios, clocks, etc. • Only lasts for an instant • Devices need charges that flow constantly

What is Electrical Current? • Electric current – Electric current: continuous flow of electric charges through a material • Charges made to flow through wire or similar material • Static electric charges don’t flow continuously – Rate of electric current – amount of charge passing through a wire in a unit of time • Ampere, amp (A) – unit of electric charge • Describes amount of charges flowing past a given point in 1 second – Electric current – similar to tomatoes on conveyor belt » Move in confined path » More tomatoes pass worker if increase flow » More charges move past point if current increases

Current in a Circuit • To produce electric current, charges must flow continuously from one place to another – Electric current doesn’t automatically exist in materials • Requires specific path to follow – Complete electric circuits – charges flow continuously • Break circuit – charges can’t flow – Circuits all around • Toasters, radios, televisions, etc.

Conductors & Insulators • A conductor transfers electric charge well. – Charges flow easily in circuit made of metal wire • An insulator does not transfer electric charge well – Charges do not flow easily through every material • Plastic wires

Conductors • Conductor: material through which charge can flow easily – Good conductors: silver, copper, aluminum, iron • Electrons easily move through conductors – Loosely bound to atoms • As electrons flow – form electric current – Conductors used to carry electric chrage – Electrons are always present in conductors • You don’t have to wait for electricity to be sent to you – Flip switch – electrons pushed and pulled – Continuous flow of electrons as soon as circuit is completed

Insulators • Insulator: material through which charges cannot flow easily – Good insulators: rubber, glass, sand, plastic, wood • Electrons do not move easily through insulators – Bound tightly to atoms • Insulators stop the flow of charges – Rubber coating on appliance cords • Cord carries charges from outlet to appliance – Inner wire is conductor for current – Rubber coating around wire is insulator keeps charges from you

Voltage • Electric charges flow in a similar way that roller coaster cars move on track

Charges need Energy to Flow • Roller coaster cars need energy to give you a ride – No energy when you climb aboard • Motor provides energy to move a chain attached to cars • Chain pulls cars to the top of the hill gaining potential energy – Potential energy: energy object has due to position, height – Higher cars go, more potential energy they have • Cars rush down hill from area of high potential energy to low – Difference in potential energy between top of the hill and bottom allow cars to move

Charges need Energy to Flow • Charges in electric circuit flow because of potential energy difference • Roller coaster – Cars = Charges making up current – Steel track = circuit – Motor = energy source, i. e. battery • Higher motor takes the cars, the faster they move • Higher the difference between charges, terminals, faster charges move

Voltage • Voltage: difference in electrical potential energy between two places in a circuit (potential difference) – Volt, (V) – unit of measure for voltage – Voltage causes a current in an electric circuit • Like a force pushing an electric current

Voltage Sources • Voltage source: device that creates potential difference in an electrical circuit – Batteries and generators • Two terminals – Voltage between terminals cause charges to move around circuit

Resistance • Roller coaster analogy – Rusty wheels on the roller coaster • Slow cars down because wheel don’t turn well – Similar to current in a circuit

Current Depends on Resistance • Current depends on resistance (as well) – Resistance: measure of how difficult it is for charges to flow through material • The greater the resistance the less current there is for a given voltage • Ohm, (Ω – capital letter omega), unit to measure resistance

Factors that Determine Resistance • Four factors determine resistance – Material that wire is made of • Insulators have high resistance – Electrons tightly bound to atoms and difficult to move • Conductors have low resistance – Electrons loosely bound to atoms and can move easily – Length • Less water flows through longer pipes because it bumps into the inside wall more • Relationship between length and resistance – As length increases, electrical resistance increases

Factors that Determine Resistance • Four factors determine resistance – Diameter • Less water can flow through smaller pipe • Relationship between diameter and resistance – As diameter increases, electrical resistance decreases – Temperature of wire • Relationship between resistance and temperature – As temperature increases, electrical resistance increases – As temperature decreases, electrical resistance decreases

Path of Least Resistance • Person told “taking the path of least resistance” – Person is doing something the easiest way possible – If electric charge has a choice to flow through two paths • More electric charge flows through path with lower resistance – Bird perched on uninsulated wire • Bird isn’t hurt • Charges flow through path of least resistance – Bird’s body more resistance than wire

Homework • Read Chapter 2, Section 2, p 44 -51 • Answer ? s 1 -4, p 51