Measuring Energy Input and Output Power Is the

Measuring Energy Input and Output

Power Is the rate at which a device converts energy Unit of power is Watt (W) =which is equal to one joule per second The faster the device converts energy, the greater its power rating P = I x V P = Power (W) I = Current (A) V = Volts (V) I = P V V = P I

Example: A hair dryer has a power rating of 1000 W. It is plugged into a 120 -V outlet. What is the current flowing through the hair dryer? Steps to Solving the Problem Solution 1. Identify known quantities 2. Identify the unknown quantity 3. Use the correct formula 4. Solve the problem Information and Power (P) = 1000 W Voltage (V) = 120 V Current (I) = x I = P / V I = 1000 W / 120 V I = 8. 33 V

Energy Is the amount of energy a device uses over a period of time. Measured in joules (watts per second) E = P x t P = E T t = E P E = Energy (J) P = Power (W) t = time (s)

Example: A microwave has a power rating of 800 W. If you cook a roast in this oven for 30 min at high, how many joules of electrical energy are converted into heat by the microwave? Steps to solving the Problem Information and Solution 1. Identify known quantities Power (P) = 800 W time (t) = 30 min 2. Identify the unknown Energy (E) = x quantity 3. Use the correct formula 4. Solve the problem E = P x t E = 800 W x 30 min E = 800 W x 1800 s E = 1 440 000 J = 1. 4 MJ

Kilowatt hours It does not take an electrical device long to consume a large number of joules, so a kilowatt hour is often used The energy calculation is the same, except that hours are substituted for seconds, and kilowatts (k. W) are substituted for watts 800 W used in 30 minutes E = 0. 8 k. W x 0. 5 h = 0. 4 k. W h

Law of Conservation of Energy cannot be created or destroyed, Energy does not just appear or disappear, only transformed from one form to another • However we usually find that the output energy of a device is smaller than the input energy The missing energy is lost or dissipated as heat. Do you Remember the Conservation of Energy Song? ? ? If no one remembers we will listen to it… Someone has to start singing it…

Efficiency The ratio of useful energy that a device produces verses the amount of energy that went into the electrical device The more input energy that a device converts into useable output energy, the more efficient the device is. Percent efficiency = joules of useful output x 100 joules of input energy