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Understanding Efficiency Human Circuit Learner Outcomes: Apply the concepts of conservation of energy and efficiency to the analysis of energy devices. Compare energy inputs and outputs of a device, and calculate its efficiency. Describe techniques for reducing energy waste in common household devices. Power: Rate at which a device converts energy. Tokyo's Toshima Incineration Plant Burns 300 tons of garbage a day, turning it into electricity, hot water and a kind of recyclable sand. The unit of power is: The Watt (W) – Named for James Watt A watt is equal to one joule per second. The faster a device converts energy, the greater its power rating. Power Ratings: Most small appliances: 1500 W or less Stove: 7000 W Calculator: 0.4 mW Review… Ohm's Law… One the most important and fundamental laws of electronics is that of ohms law. This law defines the relationships between current, voltage and resistance. Ohm's Law… States that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship- it’s a powerful predictor… Mathematically: Power is equal to the current multiplied by the voltage. Power (P); current (I); voltage (V) Energy: Measured in Joules (watts x seconds) You can use the power rating of a device to figure out the amount of energy the device uses. Remember that power is the rate at which a device converts energy. You can find the amount of energy by multiplying this rate by the length of time the device operates. Kilowatt Hours People use A LOT of joules of energy in their homes/businesses, so kilowatt hours are often used as a unit for energy. Calculation is the same, except hours are substituted for seconds, and kilowatts (kW) are substituted for watts. Electricity meters measure the energy used in kilowatt hours. Remember what Mr. Edwards taught us?/Watch video below: Law of Conservation of Energy Energy cannot be created or destroyed. It does not just appear or disappear… it must be transformed from one form to another. Energy Dissipation However… A lot of the time we find that the energy output of a device or system is smaller than the input energy. Sometimes, a lot smaller??? Heat… Most often the missing energy is lost or dissipated (scattered in all directions) as heat. Hot plate/beaker with water. The hot plate radiates some heat to into the surrounding air instead of into the water, or into nearby objects, including you. Heat is “lost”. Mechanical Devices All mechanical systems dissipate some energy, so their useable energy is always less than their input energy. Irrigation System/Pumping System You can hear it running, so some energy is dissipated as sound. If the motor has been running for a while, if you touch it, it will be hot. Current flowing through wires always produces some heat, and the friction between moving parts generates some heat as well. All of the “missing” input energy has been transformed into energy you cannot use. Understanding Efficiency The efficiency of a device is the ration of useful energy that comes out of a device compared to the total energy that went in. Efficiency is usually calculated as a percent: So… The more input energy that a device converts to useable output energy, the more efficient the device is. Easy Bake Oven? Remember that they use the “wasted” energy of a light bulb to bake a cake or cookies! For each device, try calculating the percent of its input energy that it converts to useful output energy. Which device is the most efficient? Which is the least efficient. What do you think causes the difference between the most and least efficient? Comparing Efficiencies By comparing efficiencies of devices we can judge both their energy cost and their environmental impact. Which is more efficient? Hint- Which one do you see used the most in our school? Check and Reflect: Page 338, #s 1-9 We will correct in class on Wednesday. Reducing the Energy Wasted by Devices Limits to Efficiency An electric heater can be 100% efficient in converting electricity to heat, but devices that convert electricity to other forms of energy can never be 100% efficient. Moving parts… Generate a certain amount of thermal energy that is not useful output. Friction is one cause of energy lost in engines. Increasing Efficiency? How to increase efficiency depends on its purpose… Many devices are made to convert electrical energy to mechanical energy, where the worst energy waste “offender” is friction. The easiest way to increase efficiency in these devices: Decrease friction as much as possible- use improved bearings and lubricants. In devices where heat is produced, heat loss is a major concern: Heat escapes- add more insulation (for example, around a stove), or refrigerator- reduces the amount of heat that transfers into the fridge. You need less energy to keep the fridge cold. Efficiency has increased over time with better technology: Check & Reflect: Check & Reflect, page 342, #1-7 We will correct in class on Wednesday.