POWER AND POWER CONSUMPTION!

A MATTER OF TIME AND ENERGY

POWER: the rate of change in energy; also the rate at which work is done or energy is transformed

JOULE (J): the unit for measuring energy

WATT (W): one joule (J) of energy transformed in one second

ELECTRICAL POWER: the rate of change in electrical energy Electric loads change electrical energy into

other types of energy The amount of electrical energy changed

or transformed on a load per second is the power rating of the load

Ex. a 60W light bulb uses 60J of electrical energy every second and produces 60J of heat/light energy

CALCULATING ELECTRICAL POWER

It is more common to calculate electrical power using voltage and current rather than energy and time.

Electrical power (P) is the product of voltage (V) and current (I) P=VI (or P=V x I)

Practice Question: If a 6.0V battery provides a current of 2.0A, what is the battery’s power output?

POWER RATING

POWER RATING: a measurement of how much electrical energy an electrical device consumes for every second it’s in use REMEMBER: power is the rate of change of

energy 1.0W is the transfer of 1.0J of energy/second

Ex.: a 1500W hair dryer uses 1500J of electrical energy each second

CALCULATING ENERGY CONSUMPTION We can figure out the amount of electrical energy

that a device consumes by looking at its power rating and the amount of time it’s used for.

POWER is the energy transferred (E) per time interval (t)

P=E/t (or P =E divided by t) This formula can be rearranged to find the

ELECTRICAL ENERGY (E) E (in joules) = P (watts) x t (sec.)

Practice Question: How much electrical energy is consumed by a 1200W hair dryer if it is used for 5.0min.?

A LARGER UNIT OF ENERGY

Small items can use a lot of energy. A hair dryer being used for 5 minutes consumes 360 000 J of energy! Can you imagine how big this number would be for larger items? A larger unit of energy that you can measure in is kilowatts and the unit of time is in hours. 1 kW = 1000 W 1 h = 3600 sec.

KILOWATT-HOUR: the product of power in kilowatts and time in hours

kilowatt-hour = 1.0 kilowatt x 1.0 hour 1.0kWh = 1.0 kW x 1.0 h

PAYING FOR ELECTRICITY

Ex.: A family uses 1500 kW x h of electrical energy in December. If they are charged 7 cents for every kWh, how much will they be charged in total?