Electric Current

1. Electric Current Chapter 2, Section 2

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

3. 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

4. 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.

5. 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

6. 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

7. Insulators • Insulator: material through which charges cannot flow easily • Good insulators: rubber, glass, sand, plastic, wood • Electrons do not move 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

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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. Homework • Read Chapter 2, Section 2, p44-51 • Answer ?s 1-4, p51