Electricity – Potential, Power, and Resistance
Dec 17, 2015
Drill #25Quote: A goal is a dream with a deadline.
~Napoleon Hill
Objective: TAKS Warm-upPotential, Power, ResistanceFun w/ Electricity
TAKS question:In animal cells like the one shown above, which organelle
is used for storing water?F NucleusG MitochondrionH RibosomeJ Vacuole
What is an electric field???What is an electric field???
• It is an area of influence around a charged It is an area of influence around a charged object.object.
• Positively charged objects within that field Positively charged objects within that field will experience an electrical force.will experience an electrical force.
+++
+++
+++
+ ++
Electric Field
Test Charge Electric
Field
-- -- -- -
--
--
-- --F F
Electric Potential Energy• Review
– Potential energy = mgh• Based on height
– Charges naturally move from higher to lower
• To move a charge against natural direction in relation to electric field = WORK!
• Same applies to electricity! (related to placement of charge in electric field)
Electric Potential• Dependent solely on location of
charge…– E potential = PE
Q (charge)
+ positive - negative
• To light a bulb, two different circuits are at work!– External (wiring)
– Internal (battery)• Movement of charge from PE to PE
• The Electric Potential Difference =
Voltage = Volts = 1 Joule energy / Coulomb of charge
Circuitry
V final – V initial Work
charge
∆PE
charge
= =
Electric Current
• The rate at which electrical The rate at which electrical charges flow through a wire charges flow through a wire or conductor is called or conductor is called electric currenelectric current.t.
Electrical current = charge passing through a given Electrical current = charge passing through a given area / timearea / time
I = I = ΔΔq/q/ΔΔtt
SI Unit = C/s = Amperes (A)SI Unit = C/s = Amperes (A)
Practice1. A current is said to exist whenever _____.
a. a wire is charged
b. a battery is present
c. electric charges are unbalanced
d. electric charges move in a loop
2. The diagram at the right depicts a conducting wire. Two cross-sectional areas are located 50 cm apart. Every 2.0 seconds, 10 C of charge flow through each of these areas. The current in this wire is ____ A.
D.
Current is the ratio of charge to time. The quantity of charge passing through a cross section in 2 seconds is 10 C. The ratio of charge to time isI = Q / t = ( 10 C) / ( 2 s) = 5 C/s = 5 Ampere
Current• Positive charges = carriers of charge
– In most cases!
• Current = direction that positive charges move in…– Electrons move in opposite direction
Electrical Power• Rate at which energy is added/removed
from a circuit
• Measured in wattage (W)– Do not confuse with W for work!
Wattage = 1 Joule
sec
Power = Work = energy consumed Time Time
Electrical Power• Power = rate at which energy is added/removed
from a circuit by a battery or a load. • Current = rate at which charge moves within a
circuit. • Electric potential difference = potential energy
difference per charge between two points.
Then…….
PracticeDetermine the ...• a. ... current in a 60-watt bulb plugged into a 120-volt
outlet.
• b. ... current in a 120-watt bulb plugged into a 120-volt outlet.
• c. ... power of a saw that draws 12 amps of current when plugged into a 120-volt outlet.
• d. ... power of a toaster that draws 6 amps of current when plugged into a 120-volt outlet.
• e. ... current in a 1000-watt microwave when plugged into a 120-volt outlet.
Practice • a. The current in a 60-Watt bulb plugged into a 120-Volt
outlet is 0.5 A. I = P / V = (60 W) / (120 V) = 0.5 A
• b. The current in a 120-Watt bulb plugged into a 120-Volt outlet is 1.0 A.
• I = P / V = (120 W) / (120 V) = 1.0 A
• c. The power of a saw that draws 12 amps of current when plugged into a 120-Volt outlet is 1440 W.
• P = V • I = (120 V) • (12 A) = 1440 W
• d. The power of a toaster that draws 6 amps of current when plugged into a 120-Volt outlet is 720 W.
• P = V • I = (120 V) • (6 A) = 720 W
• e. The current in a 1000-Watt microwave when plugged into a 120-Volt outlet is 8.3 A.
• ResistancResistance - e - the tendency for the tendency for a material to oppose the flow a material to oppose the flow of electronsof electrons
- All materials have some electrical - All materials have some electrical resistance.resistance.
- Resistance depends on length, - Resistance depends on length, cross-sectional area, material, cross-sectional area, material, and temperatureand temperature
Ex. Making wires thinner, Ex. Making wires thinner, longer, or hotter increases longer, or hotter increases the resistance.the resistance.
Resistance
• Resistors provide a specified amount or resistance to a conductor.
• Used to regulate the amount of current in a conductor.
Resistors
•Ohm’s law - the current in a circuit (I) equals the voltage difference (V) divided by the resistance (R).
R = V/I
•Resistance is measured in ohms .
Ohm’s Law
Sample Problem The resistance of a steam iron is 19.0 Ω. What is the current in the iron when it is connected across a potential difference of 120 V?
R = V/IR = V/I V= 120 VV= 120 V R= 19 R= 19 ΩΩ
I = V/RI = V/R = 120 V / 19 = 120 V / 19 ΩΩ = = 6.32 A6.32 A
Electrical Power revisited….• Electrical Power is the rate at which
electric charge converts electrical potential energy to non-electrical forms of energy.
P = IΔV = I²R = ΔV²/RSI Unit = watt (W)
Sample ProblemAn electric space heater is connected across a 120 V outlet. The heater dissipates 1320 W of power in the form of electromagnetic radiation and heat. Calculate the resistance of the heater.
V= 120 VP = 1320 WP = ΔV²/R
R = ΔV²/P= (120 V)²/ 1320 W = 10.9 Ω