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*Question:* A 150W light bulb is connected to a 120V line.
What is the current drawn from the line? What is the resistance of the light
bulb whilst it is burning? How much energy is consumed if the light is
kept on for 6 hours? What is the cost of this energy at 8 cents/kWh?

*Answer:* Since power is equal to , it follows that

From Ohm's law, the resistance of the light bulb is

The energy consumed is the product of the power (the energy consumed per
unit time) and the time period for which the light is on, so

Since,
, it
follows that

The cost of the electricity is product of the number of kilowatthours used
and the cost per kilowatthour, so

** Next:** Magnetism
** Up:** Electric Current
** Previous:** Example 7.3: Kirchhoff's rules
Richard Fitzpatrick
2007-07-14