Temperature & Heat Level : Conceptual Physics Teacher : Kim 1. Temperature - Temperature is the quantity that tells how hot or cold something is called - When temperature increases nearly all matters ( ) - When temperature decreases nearly all matters ( ) - A common thermometer measures temperature by showing the expansion and contraction of a liquid- usually mercury in a glass tube using a scale - We express temperature by a number that corresponds to a degree mark on some chosen scale - The scale used in everyday life are Celsius scale(ºc) and Fahrenheit scale(ºf) Review questions 1. Ice melts at 0ºC (= the melting point of water is 0ºC) (True False) 2. The lowest temperature an object can obtain is 0ºC ( True False) 3. There are some substances that has a melting points below 0ºC ( True False) 4. Water boils at 100ºF ( True False) 5. When an object reaches 0ºC, the kinetic energy of the molecules is zero ( True False) Kelvin Scale - The Kelvin scale used in scientific research - The Kelvin scale is based on the concept of absolute zero. - Absolute zero is theoretically the lowest temperature that an object can reach. - At this temperature, the kinetic energy of a molecule will be zero 1
Temperature Conversions - The formula to convert between temperature scales are K = ºC + 273 or ºC = K 273 ºF = 1.8 ºC + 32 or ºC = 0.556 ( ºF 32) Q1) Convert 25 ºC to Fahrenheit and Kelvin ºF = ( ) K = ( ) Q2) Complete the table water melting point(m.p) boiling point(b.p) Celsius(ºC) Fahrenheit (ºF) Kelvin (K) Q3) Complete the table by selecting from the box below -259.14ºC 100ºC 1,538ºC 0ºC -259.87ºC 2862ºC water / ice boiling point(b.p) melting point(m.p) Unit : ºC hydrogen boiling point(b.p) melting point(m.p) Iron boiling point(b.p) melting point(m.p) *~ Hydrogen naturally exists in its gaseous state. To make hydrogen into liquid, we need to lower the temperature below its boiling point. To make hydrogen into solid, we need to lower the temperature below its melting point. *~ Iron naturally exists in its solid state. To make iron into liquid, we need to raise the temperature above its melting point. To make iron into gas, we need to raise the temperature above its boiling point. 2
2. Heat - Heat and temperature are two quantities that can be easily confused. - Temperature is a measure of the average kinetic energy of a substance. - Heat is energy(or thermal energy) being transferred from one substance to another by a temperature difference. - The direction of heat flow is from high to low temperature Example) If you touch a hot stove, heat will flow to your finger from the stove. If you touch ice, the heat will flow from your finger to the ice - The unit of heat is Joules(J) or calorie(cal), where 1cal = 4.1868 J 3. Specific Heat Capacity The specific capacity of any substance is defined as the quantity of heat required to raise the temperature of unit mass of the substance by 1 degree If a substance has a high specific heat, it will heat up slowly. But once heated up, it will cool down slowly If a substance has a low specific heat, it will heat up quickly. But once heated up, it will cool down quickly We can think specific heat as the ability to store heat. Q6) Compare heating a pot of water and iron of equal mass for 15 seconds. Which would heat up faster? Which would cool down faster? Which has greater specific heat capacity? Q7) When you go to a beach, you will notice a large difference between the temperature of the sand and the water. Even though, both sand and water are under the same Sun, why is the sand hotter than water? Which has greater specific heat capacity? 3
** Specific Heat of Water** If you have 1 gram of water and add 1 calorie of heat, the temperature will increase by 1ºC => C=1cal/g ºC Since, sometimes it can be more practical using kg than grams and Joules than calories depending on what we are measuring, we should be aware of different conversion to represent specific heat of water C=1cal/g ºC C=1000cal/kg ºC C=4.19J/g ºC C=4190J/kg ºC The specific heat of water, 1cal/g ºC or 4.19J/g ºC is a high value compared to other substances, which means the temperature of water changes more slowly than the temperature of many other substances. That is, it is very difficult to change water s temperature, and if a substance is difficult to heat up, it will also be difficult to cool down. Ex) water:1cal/g ºC glass:0.201cal/g ºC copper:0.09231cal/g ºC lead:0.03051cal/g ºC Heat transfer and Measurement of Heat - Heat is energy and like other forms of energy, thermal energy is neither created nor destroyed, it only changes form. - When an object is heated, it must get that energy from somewhere, and when an object cools, the heat must go somewhere else. - Heat is transferred from a hotter object to a colder object; heat is energy transferred from one substance to another by a temperature difference *To measure how much heat will transfer Heat transfer(q) = mass specific heat(c) change in temperature Q = mc T = mc(tf Ti ) 4
Heat transfer(q) = mass specific heat(c) change in temperature Q = mc T = mc(tf Ti ) Problems on Specific Heat 1. Substances that heat up quickly will cool down quickly or slowly. True or False? 2. If a substance heats up slowly, then does it have high specific heat or low specific heat? 3. Which has higher specific, water or metals? 4. In the old days, on a cold winter night, it was common to bring a hot object to bed with you. Which would be better, a 10kg-iron brick or a 10kg-jug of hot water at the same temperature? 5. Calculate the amount of calories of heat needed to raise temperature of 500g of water by 50 ºC? (specific heat of water is 1cal/g ºC) a) 15,000cal b) 20,000cal c) 25,000cal d) 30,000cal 6. Calculate the amount of calories given off by 500g of water cooling from 50ºC to 20ºC? (specific heat of water is 1cal/g ºC) a) 15,000cal b) 20,000cal c) 25,000cal d) 30,000cal 7. A 30g piece of iron is heated to 100ºC and then dropped into cool water where the iron s temperature drops to 30ºC. How many calories does it lose to the water? (the specific heat capacity of iron is 0.11cal/gºC) a) 124cal b) 168cal c) 198cal d) 231cal 5
Heat transfer(q) = mass specific heat(c) change in temperature Q = mc T= mc(tf Ti ) 8. Suppose the same 30g piece of iron from question 7. is dropped into another container of water and gives off 165 calories in cooling. Calculate the iron s temperature change. a) 50ºC b) 40ºC c) 30ºC d) 20ºC 9. How much mass of water will give up 240 calories when its temperature drops from 80ºC to 68ºC? a) 20g b) 30g c) 40g d) 50g 10. When a 50g piece of aluminum at 100ºC is placed in water, it loses 735 calories of heat while cooling to 30ºC. Calculate the specific heat capacity of the aluminum. a) 0.48cal/gºC b) 0.36cal/gºC c) 0.21cal/gºC d) 0.15cal/gºC 11. What would be the final temperature when 100g of 25 ºC water is mixed with 75g of 40 ºC water? (*hint : 100g gains heat, 75g loses heat until reaches the same temperature. Amount of heat gained = amount of heat lost) a) 26.4ºC b) 31.4ºC c) 38.3ºC d) 45.1ºC 12. What is the specific heat capacity of a 50g piece of 100ºC metal that will change 400g of 20ºC water to 22ºC? a) 0.205cal/gºC b) 0.378cal/gºC c) 0.453cal/gºC d) 0.643cal/gºC 6