Transcript Electric Current

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 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, p44-51
• Answer ?s 1-4, p51