Heat and Temperature Physical Science Chapter 14

2 Temperature Hot and cold can be used to describe temperature. Heat is related to temperature, but they are not the same thing. Physical Science chapter 14

3 Tiny moving particles All matter is made up of tiny particles that are in constant motion. The particles have kinetic energy. The faster they move, the more they have. Temperature is a measure of the average kinetic energy of the particles in a sample of matter. Physical Science chapter 14

4 Temperature Which particles are moving faster, those in a hot cup of coffee or those in a bowl of ice cream? The coffee – higher temperature means more kinetic energy, which means that the particles are moving faster Physical Science chapter 14

Thermometers Glass – use expansion of liquids  Liquids expand when heated and move up the tube In Thermostats – use expansion of metals  Metals expand at different rates when heated  Makes coil wind or unwind and moves pointer Physical Science chapter 145

Temperature Scales Fahrenheit –  Water freezes at 32 °F and boils at 212 °F  Used in US for weather Celsius –  Water freezes at 0 °C and boils at 100 °C  Used in US for science and for nearly everything in most of the world Kelvin –  Water freezes at K and boils at K  Degree symbol not used  Each degree is the same size as a Celsius degree  Used for science Physical Science chapter 146

Converting between temperature scales Physical Science chapter 147

Examples Practice problem 2 on page 477: complete the table. Physical Science chapter 148

9 Heat The energy that flows from something with a higher temperature to something with a lower temperature.  Always flows from warmer to cooler Heat is measured in joules Physical Science chapter 14

Feelings associated with temperature difference Object at lower temperature  Takes energy away from your skin when you touch it  Feels cold Object at higher temperature  Adds energy to your skin when you touch it  Feels warm Physical Science chapter 1410

11 Heat vs. work Heat is energy transferred between objects at different temperatures. Work is energy transferred when a force acts over a distance. Physical Science chapter 14

Discuss Describe the relationship between temperature and energy Physical Science chapter 1412

13 Conduction The transfer of energy by direct contact of particles. When particles collide, the faster moving one gives some of its energy to the slower moving one.

14 Conduction Can transfer energy through a given material or from one material to another.  Example: holding a metal spoon with one end in boiling water.

15 Conduction Can take place in solids, liquids, or gases. Solids usually conduct heat better  particles are closer together

16 Conductors and Insulators Good heat conductors – conduct heat easily  Metals Insulators - poor heat conductors – don’t conduct heat easily  plastic  wood  glass  fiberglass

17 Convection The transfer of energy by the movement of matter The particles move from one place to another, carrying the energy with them. When a fluid is heated, the particles move faster. Since they can move, they do – and they spread out. Fluids expand when heated.

18 Heating water When the water at the bottom gets hot, it expands, and becomes less dense. The cooler, more dense water above it sinks and pushes the warm water up. As the water rises, it becomes cooler and more dense, and moves towards the bottom again.

19 Convection currents This movement creates convection currents that transfer energy from warmer to cooler parts of the fluid.

20 Radiation The transfer of energy in the form of invisible rays. Does not require matter to be present. Radiant energy – energy that travels by radiation

Discuss Compare and contrast conduction, convection, and radiation. Physical Science chapter 1421

22 Specific Heat Also called heat capacity Different materials require different amounts of energy to produce the same temperature change. The specific heat (c) of a material is the amount of energy it takes to raise the temperature of 1 kg of the material 1 kelvin. Units are J/kg∙K Physical Science chapter 14

23 Specific heat Water has a high specific heat, so it takes a lot of energy to raise its temperature. That’s why the temperature of a lake or unheated swimming pool is always cooler than the temperature of the air around it. Physical Science chapter 14

24 Using Specific heat We can’t measure change in thermal energy directly. However, we can measure the change in temperature and use the specific heat to calculate the change in thermal energy. Physical Science chapter 14

25 Change in Thermal Energy Physical Science chapter 14

26 Delta The Greek letter  (delta) means “change in”  T means change in temperature Always take final temperature minus initial temperature. When  T is positive, the object has increased in temperature and taken in heat. When  T is negative, the object has decreased in temperature and given off heat. Physical Science chapter 14

27 Example Calculate the thermal energy change when 230 g of water warms from 12 °C to 90 °C. Physical Science chapter 14

28 You try A 3.1 kg block of aluminum cools from 35°C to 20 °C. What is the change in its thermal energy? Physical Science chapter 14

You try How much energy must a refrigerator absorb from 225 g of water to decrease the temperature of the water from 35 °C to 5 °C? Physical Science chapter 1429

Adding Heat to an object Raises temperature OR Changes state (solid-liquid-gas) Not both at the same time Physical Science chapter 1430

31 Forced air heating systems Fuel is burned to heat air. A fan forces the warm air into a room. Convection currents carry the warm air throughout the room. Cool air returns to the furnace to heat again.

32 Hot water heating systems A fuel is burned to heat water. The hot water travels through pipes to the radiator. The cooled water returns to the furnace to heat again.

33 Steam heating systems A fuel is burned to boil water. The steam travels through pipes to the radiator The cooled water returns to the furnace to heat again.

34 Active solar heating Collectors on the roof or the south side of the building. Energy is absorbed by liquid in pipes in collectors.  Heated liquid runs through house to heat exchanger.  Cooled liquid is pumped back to collectors to heat again

35 Refrigerators Heat naturally flows from warmer to colder. Refrigerators move the warm air from inside to the even warmer air outside.  Work must be done for this to happen This work is powered by electricity.

36 Refrigerators A liquid is pumped through the refrigerator coils. As it evaporates, it absorbs heat from inside the fridge. It is then compressed, causing it to lose heat to the room. Work is done in pumping and compressing the liquid.

37 Air conditioners Work like refrigerators, only they are designed to cool larger areas.

First law of Thermodynamics The total energy used in any process is conserved. Energy might be transferred as work, heat, or both. Physical Science chapter 1438

Second law of Thermodynamics Heat always moves from an object at a higher temperature to an object at a lower temperature.  Unless work is done, like in a refrigerator Physical Science chapter 1439

Entropy The amount of disorder Any system will naturally move towards higher entropy (and lower energy) Physical Science chapter 1440

Usable energy always decreases The total amount of energy stays the same whenever energy is transferred or transformed. Often, it is transformed into unusable forms – like heat coming from friction. Physical Science chapter 1441

42 Heat engines Devices that convert chemical energy into mechanical energy by combustion (rapid burning)

43 Internal combustion engine Fuel burns inside the engine, in the cylinders. Each cylinder has two valves that open and close. A piston is moved up and down in the cylinders. The piston moves the crankshaft, which moves the car’s wheels. The wheels exert a force on the road. The equal and opposite force of the road on the tires accelerates the car forward

44 Four-stroke cycle See figure 4 on page 493 Each movement of the piston up or down is called a stroke

45 Intake stroke For a fuel injected engine, air enters the cylinder through the open intake valve.

46 Compression stroke The intake valve closes. The piston moves up, compressing the air into a smaller space. Fine droplets of fuel are injected into the compressed air.

47 Power stroke When the air-fuel mixture is very compressed, a spark plug produces a hot spark that ignites the mixture. As it burns, the hot gases produced expand, forcing the piston back down. Energy is transferred from the piston to the crankshaft, powering the motion of the car.

48 Exhaust stroke The piston moves up again, compressing the leftover waste products. The exhaust valve opens, releasing the exhaust

49 Carburetor In engines with a carburetor instead of a fuel injector, the gasoline is mixed with air in the carburetor and the fuel-air mixture enters the cylinder on the intake stroke

50 Diesel engines No spark plugs The fuel air mixture is compressed so much that it ignites without a spark.