Electricity – Potential, Power, and Resistance

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.

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Electric Field

Test Charge Electric

Field

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-- --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

Electric Power• Electric companies measure energy

consumed in kilowatt-hours (kW •h)

Electric PowerHow much does it cost to operate a 100.0 W light bulb for 24 h if electrical energy costs $0.080 per kW•h?

Total Energy Used = (0.100 kW)( 24 h) =Total Energy Used = (0.100 kW)( 24 h) =

2.4 kW•h

Cost = (2.4 kWCost = (2.4 kW•h)($ 0.080/ kW•h) =•h)($ 0.080/ kW•h) = $0.19