Lecture 6: Heat and Specific Heats, Study notes of Nutrition

Download Lecture 6: Heat and Specific Heats and more Study notes Nutrition in PDF only on Docsity!

LECTURE 6

HEAT

Lecture Instructor: Kazumi Tolich

Lecture 6

2 ! Reading chapter 16-4 to 16-5. ! Heat and mechanical work ! Specific heats ! Calorimetry

Heat and mechanical work

! As the weights fall, they turn a paddle wheel, which does work on the water. ! Joule found that it takes about 4186 J of work to increase the temperature of 1kg of water by 1°C. ! Heat was measured in calories before his work, and this experiment showed the mechanical equivalent of heat. 4 Insulating walls 1 cal = 4. 186 J

Units for heat

5 ! In nutrition, the Calorie (C) is used instead of calorie. ! Another common unit for measuring heat is the British thermal unit (Btu). A Btu is the energy required to heat 1 lb of water from 63 °F to 64 °F.

1 kcal = 1 C

1 Btu = 0. 252 kcal = 1055 J

Demo: 2

7 ! Cork popper ! Demonstration of friction producing heat

Example: 1

! During a workout, a person repeatedly lifts a barbell with a weight w = 53 N through a height of h = 0.46 m. How many “reps” of this lift are required to burn off 120 C? 8

Heat capacity

! The heat required ( Q ) for a given increase in temperature (Δ T ) of an object is given by the heat capacity ( C ) of a substance. The heat capacity is a positive constant for a given object. ! Q > 0 if Δ T > 0. Heat is added to a system. ! Q < 0 if Δ T < 0. Heat is removed from a system. ! We can think of heat capacity as thermal inertia. 10 C =

Q

Δ T

Q = C Δ T

Specific heat

! Different substances have different capacities for storing thermal energy. ! The specific heat (c) of a substance is defined as ! m is the mass of an object. 11 c =

Q

m Δ T at P at

Example: 2

13 ! Suppose Q = 63.0 J of heat is added to a piece of aluminum with a mass of m = 128 g at a temperature of T 0 = 25.0 °C. What is the final temperature of the aluminum?

Specific heat capacity of water

! Water has a much higher specific heat capacity than most common materials. ! The climate in many places are influenced by ocean currents. ! Islands and peninsulas do not have extreme temperatures (hot and cold) that are observed in the interior of a continent. 14

Demo: 3

16 ! Balloon heat capacity ! Demonstration of heat capacity

Clicker question: 1

17

Demo: 4

19 ! Calorimetry and specific heat ! Measuring specific heat for aluminum, steel, and lead Q m

• Q w = 0 m m c m T − T

( m)

• m w c w T − T

( w)

= 0 c m = m w c w T − T

( w)

m m T m

( −^ T )

Example: 3

20 ! A blacksmith drops an iron horseshoe with a mass of m h = 0.50 kg into a bucket of water with a mass of m w = 25 kg. If the initial temperature of the horse shoe is T hi = 450 °C, and the initial temperature of the water is T wi = 23 °C, what is the equilibrium temperature of the system, T? Assume no heat is exchanged with the surroundings.