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Physics 11 – Unit 8: Work, Energy, Power: Practices

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Physics 11 – Thermal Energy and Heat

v  Thermal Energy and Heat:

Ø  Thermal energy and heat are not exactly the same, and temperature is different from both of them.

Ø  Thermal energy is the total kinetic energy and potential energy of the atoms or molecules of a substance. It depends on the mass, temperature, nature, and state of the substance.

Ø  Hear is the transfer of energy from a hot body to a colder one.

Ø  Temperature is a measure of the average kinetic energy of the atoms or molecules of a substance, which increases if the motion of the particles increases.

e.g. Consider, 100 g of water at 50 oC and 500 g of water at 50 oC. The samples have the same temperature, but bigger 500 g sample contains more thermal energy. If these samples were mixed, no heat would transfer between them because they are at the same temperature.

e.g. Consider 500 g of water at 50 oC and 500 g of water at 90 oC. The warmer sample has more thermal energy because the motion – in other words, the average kinetic energy – of the molecules is greater at a higher temperature. If the two samples were mixed, heat would transfer form the 90 oC sample to the 50 oC sample.

v  Methods of Heat Transfer:

Ø  Conduction

The process of heat transferring through a material by the collision of atoms is called conduction.

Conduction occurs best in metals, which have electrons that move much more freely than in other substances. Conduction occurs much less in solids such as concrete, brick, and glass, and only slightly in liquids and gases.

Ø  Convection

The process of transferring heat by a circulating path of fluid particles is called convection. The circulating path is called a convection current. The particles of the fluid actually move, carrying energy with them.

Both conduction and convection involve particles. However, heat can also transfer through a vacuum, a space with no particle.

Ø  Radiation

Radiation is the process in which energy is transferred by means of electromagnetic waves.

Examples of these waves are visible light, microwaves, radio waves, radar, X rays, and infrared rays. Infrared rays are also called heat radiation because the dominant form of radiation emitted form objects at everyday temperatures in infrared radiation.

Heat emitted from an object in the form of infrared rays can be detected by an infrared photograph called a thermograph.

v  Calculating heat Transfer:

The transfer of heat from one body to another causes either a temperature change or a change of state, or both.

Different substances require different amounts of energy to increase the temperature of a given mass of the substance. This occurs because different substances have different capacities to hold heat.

e.g. Water holds heat better than steel. Therefore, water is said to have a higher specific heat capacity than steel.

In above example, the word specific indicates that we are considering an equal mass of each substance.

Ø  Specific heat capacity (c) and Heat gained or Lost:

The specific heat capacity is a measure of the amount of energy needed to raise the temperature of 1.0 kg of a substance by 1.0 oC. It is measure in joules per kilogram degree Celsius, J / (kg × oC). /

The quantity of heat gained or lost by a body, Q, is directly proportional to the mass, m, of the body, its specific heat capacity, c, and the change in the body’s temperature, Dt. The equation relating these factors is

e.g. How much heat is needed to raise the temperature of 2.2 kg of water from 20 oC to the boiling point? The specific heat capacity of water is 4.18 ´ 103 J / (kg × oC). (0.74 MJ)

Practice:

1.  Calculate the amount of heat needed to raise the temperature of the following:

(a)  8.4 kg of water by 6.0 oC (2.1 ´ 105 J)

(b)  2.1 kg of alcohol by 32 oC (1.7 ´ 105 J)

2.  Determine the heat lost when

(a)  3.7 kg water cools from 31 oC (1.1 ´ 105 J)

(b)  1 540 g piece of silver cools from 78 oC to 14oC (8.3 ´ 103 J)

3.  An electric immersion heater delivers 0.050 MJ of energy to 5.0 kg of a liquid, changing its temperature from 32 oC to 42oC. Find the specific heat capacity of the liquid. (1.0 ´103 J / (kg × oC))

4.  Determine how much brass can be heated form 20 oC to 32 oC using 1.0 MJ of energy. (2.2 ´ 102 kg)

5.  A 2.5 kg pane of glass, initially at 41 oC, loses 4.2 ´ 104 J heat. What is the new temperature of the glass? (21 oC)

v  Principle of Heat Exchange:

When heat is transferred form one body to another, the amount of heat lost by the hot body equals the amount of heat gained by the cold body.

e.g. A 200 g piece of iron (the specific heat capacity of iron = 4.5 ´ 102 J / (kg × oC)) at 350 oC is submerged in 300 g of water at 10 oC to be cooled quickly. Determine the final temperature of the iron and the water. (33 oC)

Practice:

1.  A 120 g mug at 21 oC is filled with 210 g of coffee at 91 oC. Assuming all of the heat lost by the coffee is transferred to the mug, what is the final temperature of the coffee? The specific heat capacity of the mug is 7.8 ´ 102 J / (kg × oC). (84 oC)

2.  A 0.500 kg sample of water is at 15.0 oC in a calorimeter. A 0.0400 kg block of zinc at 115 oC is placed in the water. Fine the final temperature of the system. The specific heat of zinc is 388 J / (kg × oC). (15.7 oC)

3.  A 1.0 ´ 102 g aluminum block at 100.0 oC is placed in 1.00 ´ 102 g of water at 10.0 oC. The final temperature of the mixture is 25 oC. What is the specific heat of the aluminum? (9.2 ´ 102 J / (kg × oC)

v  Change of State:

Ø  Melting Point:

The temperature when a substance is melting is called melting point. When a substance is melting, added thermal energy increases the potential energy of particles, breaking the bonds holding them together. The added thermal energy does not increase the temperature.

The amount of energy needed to melt one kilogram of a substance is called the heat of fusion of that substance.

e.g. The heat of fusion of ice is 3.34 ´ 105 J / kg. If 1 kg of ice at its melting point, 0 oC or 273 oK, absorbs 3.34 ´ 105 J, the ice becomes 1 kg of water at the same temperature.

Ø  Heat of Fusion:

The heat, Q, required to melt a solid of mass m is given by

e.g. If 5.00 ´ 103 J is added to ice at 0 oC, how much ice is melted? (0.0150 kg)

e.g. How much heat must be transferred to 100.0 g of ice at 0.0 oC until the ice melts and the temperature of the resulting rises to 20.0 oC? (41800 J)

Ø  Boiling Point:

As the temperature continually increases, some particles in the liquid acquire enough energy to break free form other particles. At a specific temperature, know as the boiling point, further addition of energy causes another change of state, It does not raise the temperature at the time; it converts particles in the liquid state to particles in the vapour or gaseous state.

The amount of thermal energy needed to vaporize one kilogram of a liquid is called the heat of vaporization.

e.g. For water, the heat of vaporization is 2.26 ´ 106 J / kg.

Ø  Heat of Vaporization:

The heat, Q, required to vaporize a mass, m, of liquid is given by

Practice:

1.  How much heat is absorbed by 1.00 ´ 102 g ice at -20.0 oC to become water at 0.0 oC? (3.75 ´ 104 J)

2.  A 2.00 ´ 102 g sample of water at 60.0 oC is heated to steam at 140 oC. How much heat is absorbed? (5.02 ´ 105 J)

3.  How much heat is needed to change 3.00 ´ 102 g of ice at -30.0 oC to steam at 130.0 oC? (9.40 ´ 105 J)

4.  A 175 g lump of molten lead at its melting point, 327 oC, is dropped into 55 g of water at 20.0 oC.

(a)  What is the temperature of the water when the lead becomes solid? (16 oC to 36oC)

(b)  When the lead and water are in thermal equilibrium, what is the temperature? (62 oC)