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AGC/United Learning • 1560 Sherman Ave., Suite 100 • Evanston, IL 60201 • 800-323-9084

THE MORE INPUT ENERGY CONVERTED TO USEFUL ENERGY, THE MORE EFFICIENT THE ENERGY CONVERSION. WE TALK ABOUT ENERGY EFFICIENCY AS A RATIO. IT’S THE USEFUL EN- ERGY OUTPUT DIVIDED BY THE ENERGY INPUT. AND WE EX- PRESS IT AS A PERCENTAGE. LAUREEN: GOT IT! SARAH: NOW, SUPPOSE A CAR USES ONE-TENTH OF A LITER OF GASOLINE, WHICH CONTAINS 3.35 MEGAJOULES OF CHEMI- CAL POTENTIAL ENERGY. BRENDON: UH-HUH. SARAH: IT CONVERTS THAT FUEL TO 0.97 MEGAJOULES OF KI- NETIC ENERGY. CAN YOU FIGURE OUT ITS EFFICIENCY? BRENDON: WELL, THE CAR’S EFFICIENCY EQUALS THE USE- FUL ENERGY OUTPUT DIVIDED BY ENERGY INPUT. LAUREEN: SO EFFICIENCY IS EQUAL TO USEFUL ENERGY OUT- PUT, 0.97 MEGAJOULES, DIVIDED BY ENERGY INPUT, 3.35 MEGAJOULES, WHICH IS — BRENDON: 0.290, TIMES 100%. LAUREEN: THAT’S 29.0%. SARAH: BUT WE REALLY ONLY HAVE TWO SIGNIFICANT DIG- ITS. BRENDON: SO WE ROUND IT OFF TO 29%. SARAH: GOOD WORK! LAUREEN: USEFUL! SARAH: SO WE DESCRIBE ENERGY EFFICIENCY AS A RATIO. IT’S THE USEFUL ENERGY OUTPUT DIVIDED BY THE ENERGY INPUT. AND WE EXPRESS IT AS A PERCENTAGE. INPUT. WASTE. AND OUTPUT. DANA: ONE OF THE MOST SPECTACULAR ENERGY CONVER- SIONS IN NATURE TAKES PLACE DURING A LIGHTNING STRIKE. A SINGLE BOLT OF LIGHTNING CAN DEVELOP 20 MEGAJOULES OF ENERGY. MUCH OF THIS ELECTRICAL ENERGY IS TRANS- FERRED TO THE AIR AS THERMAL ENERGY. IT ACTUALLY RAISES THE TEMPERATURE OF THE AIR IN THE LIGHTNING CHANNEL TO ABOUT 30,000 DEGREES CELSIUS. THIS FORCES THE AIR MOLECULES APART QUICKLY, SO THAT THE AIR EX- PANDS MUCH LIKE GASES IN AN EXPLOSION. THIS MOVEMENT CREATES SOUND WAVES, WHICH WE HEAR AS THUNDER. SO

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