Ohm's Law _ Electrical Power

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					Ohm’s Law &
Electrical Power
Resistance
• Resistance - the tendency for a
  material to oppose the flow of
  electrons
  - All materials have some electrical
    resistance.
  - Resistance depends on length, cross-
    sectional 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
• 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

				
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