advertisement

Section 3.3 Measuring Energy Input and Output 1 p. 332 - 338 We use energy in every aspect of our life. Driving, ipods, heating our homes, cooking food, watching tv. How can we choose which type of energy to use? (ex: we could use electricity, gasoline, propane, hydrogen) We need to measure the types and amounts of energy going into and coming out of the devices. Power Power is the rate at which a device converts energy. Unit of power is the watt (W). 1 watt is equal to 1 joule per second 1W = 1J/s Ex: 44 W = 44 J/s The faster a device converts energy, the greater its power rating. Is an appliance with high watts or low watts better? Why? Is an appliance with high watts or low watts better? Why? Depends on the way you look at it. High watts, is more powerful, stronger, might not have to use it as long However, low watts is more energy efficient. Might save you money in the long run. Power formula For an electrical device, the power is the current multiplied by the voltage. P = I x V (watts = amps x volts) P I V Power Calculation 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? Known: Power (P) = 1000 W, Voltage (V) = 120 V Unknown: Current (I) = ? Formula: I = P/V Solve: I = 1000 W/ 120 V I = 8.33A Energy You can use the power rating of a device to determine the amount of energy the device uses (power is the rate at which a device converts energy). Energy formula Energy consumption of an electrical device is its input power multiplied by the time the device is used. E P t E = P x t (joules = watts x seconds) Energy Calculation Example A microwave oven has a power rating of 800W. If you cook a roast in the oven for 30 min at high, how many joules of electrical energy are converted into heat by the microwave? *Remember that W = J/s • Known: P = 800 W (J/s), t = 30 min 30 min = ? s 30min x 60 s/min = 1800s • Unknown: E = ? • Formula: E = P x t • Solve: E = 800 W x 1800s E = 1 440 000 J = 1.4 MJ Write this at the bottom of the page… Kilowatt Hours It doesn’t take common electrical devices long to consume a large number of joules. Because of this, the kilowatt hour is often used. Energy calculation is the same, except you use hours instead of seconds, and kilowatts instead of watts. 1 kW = 1000 W