Transcript Ohms Law, Power, and Energy

What you need:
• Notebook
• Calculator
• Writing utensil
Ohm’s Law,
Power,
&
Energy
Currents and Circuits
• Current - the flow of charged
particles
• Circuit - a closed continuous path
through which current flows
- requires a voltage source to
maintain the potential
difference
Ohm’s Law
• Resistance = Voltage/ Current
•R=V/I
• Units
– Resistance (R) = ohms (Ω)
– Voltage (V) = volt (V)
– Current (I) = ampere (A)
Ohm’s Law
• Increasing voltage increases
resistance
• Increasing current decreases
the resistance
Resistors
• Any device that changes
electrical energy into another
form
Example #1:
• A 30.0 V battery is connected to
a 10.0 Ω resistor. What is the
current in the circuit?
• I = V/R
• I = 30.0 V / 10.0 Ω
• = 3.00 A
Power
• Measures the rate at which
energy is transferred
• Power = Current x Voltage
• P = IV
• The unit of power is the Watt
Example #2:
• A 6.0 V battery delivers a 0.50 A
current to an electrical motor.
What power is consumed by the
motor?
• P = IV
• P = (0.50 A)(6.0 V)
• P = 3.0 W
Electrical Energy
• Energy = Power x time
• E = Pt
• If you are using Watts, use
seconds for time. In this
case, energy is measured in
Joules.
Continue Example #2:
• Using the information from the
Example #2, how much electric
energy is delivered if the motor
runs for 5.0 minutes?
• E = Pt
• E = (3.0 W)(300 s)
• E = 9.0 x 102 J
The Kilowatt-Hour
• The joule (a “watt-second”) is a
relatively small amount of
energy; too small for
commercial sales use.
• For that reason, electric
companies measure their energy
sales in “kilowatt-hours”.
The Kilowatt-Hour
• A kilowatt-hour is equal to 1000
watts delivered continuously for
3600 seconds (1 hour).
1 kWh = (1000 J/s)(3600 s) =
6
3.6x10
J
Example #3:
• A plasma tv draws 2 A when
operated on 120 V.
– How much power does the tv use?
– If the tv is operated for an average
of 7 hr/day, what energy in kWh does
it consume per month (30 days)?
– At 11¢ per kWh, what is the cost of
operating the tv per month?