Transcript Electric Charge

Electricity – Potential,
Power, and Resistance
Drill #25
Quote: A goal is a dream with a deadline.
~Napoleon Hill
Objective:
TAKS Warm-up
Potential, Power, Resistance
Fun w/ Electricity
TAKS question:
In animal cells like the one shown above, which organelle
is used for storing water?
F Nucleus
G Mitochondrion
H Ribosome
J Vacuole
What is an electric field???
• It is an area of influence around a charged
object.
• Positively charged objects within that field
will experience an electrical force.
++
+
+ +
+ ++
++ +
Electric Field
F
+
Test Charge
F -- - -- --- - - -
Electric Field
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
Circuitry
• 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 =
V final – V initial = Work = ∆PE
charge charge
Voltage = Volts = 1 Joule energy / Coulomb of charge
Electric Current
• The rate at which electrical
charges flow through a wire or
conductor is called electric
current.
Electrical current = charge passing through a given area / time
I = Δq/Δt
SI Unit = C/s = Amperes (A)
Practice
1. A current is said to exist whenever _____.
a. a wire is charged
b. a battery is present
D.
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.
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 is
I = 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
Power = Work = energy consumed
Time
Time
• Measured in wattage (W)
– Do not confuse with W for work!
Wattage = 1 Joule
sec
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…….
Practice
Determine the ...
• a. ... current in a 60-watt bulb plugged into a 120-volt
outlet.
• b. ... current in a 120-watt bulb plugged into a 120volt 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 120Volt 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.
Resistance
• Resistance - the tendency for a
material to oppose the flow of
electrons
- All materials have some electrical
resistance.
- Resistance depends on length, crosssectional area, material, and
temperature
Ex. Making wires thinner, longer, or
hotter increases the resistance.
Resistors
• Resistors provide a specified
amount or resistance to a conductor.
• Used to regulate the amount of
current in a conductor.
Ohm’s Law
• 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 ().
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/I
V= 120 V
R= 19 Ω
I = V/R= 120 V / 19 Ω = 6.32 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/R
SI Unit = watt (W)
Sample Problem
An 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.
P = ΔV²/R P = 1320 W
V= 120 V
R = ΔV²/P = (120 V)²/ 1320 W = 10.9 Ω
Electric Power
• Electric companies measure energy
consumed in kilowatt-hours (kW •h)
Electric Power
How much does it cost to operate a
100.0 W light bulb for 24 h if electrical
energy costs $0.080 per kW•h?
Total Energy Used = (0.100 kW)( 24 h) =
2.4 kW•h
Cost = (2.4 kW•h)($ 0.080/ kW•h) = $0.19