Joules and Calories

Lesson #21
E.Q:How is the specific heat and the energy related in a phase change?

Specific Heat

• What is specific heat?

• Take notes on the video

Phase changes:

it takes energy to changes phases from a solid to a liquid and from a liquid to a gas. The substance releases energy when changing phase from gas to liquid or from liquid to solid. How much energy per unit mass depends on the substance in question. When you get out of the shower you often feel cold. This is because the water on you is evaporating, and heat is flowing from you to the water droplets in order for them to change phase from water to gas. You are losing heat and thus feel cold.

A calorie is a unit of energy. The food Calorie, with 'cal', is actually 1,000 calories (a kcal). Thus, for example, a snicker bar labeled with 200 Cal is actually 200,000 cal.

Food calories are determined by burning the food and measuring the heat released.

Equation

q= heat

m = mass

C = Specific Heat

ΔT = Change in Temperature





Please list the SI units for each
variable

Multiple Choice

I just said this today: What units are used to measure HEAT?

• grams

• Joules

• ml

• °C

Multiple Choice

ΔT = Change in Temperature. How would you calculate the change in temperature?

• Final temperature minus starting temperature

• Starting temperature minus final temperature

Multiple Choice

250g of water (specific heat of 4.184 J/g^oC) is heated from 20^oC to 46^oC.

What is the change in temperature?

• 26^oC

• -26^oC

• 250g

• 66^oC

Open Ended

Calculate the amount of heat needed to raise the temperature of 15g of copper from 25^oC to 60^oC if copper has a specific heat of 0.386 J/g^oC

Multiple Choice

What is the formula for specific heat?

• 
  

  $$q\ =m\ c\ \Delta T$$

• $$m\ =\ q\ c\ \Delta T$$

• $$\Delta T\ =\ m\ c\ q$$

• $$c\ =\ m\ q\ \Delta T$$

Multiple Choice

The higher the specific heat the

• more energy needed to raise the temp

• less energy needed to raise the temp

• it doesn't matter

​​This is why at the beach the sand will get so hot, while the water can stay very cool. Sand heats up quickly because it has a much lower Specific heat then the water.

Fill in the Blank

How much energy must be absorbed by an unknown object with a specific heat of 1.27 cal/g^oC if this object has a mass of 12.0 g and it heats from 7.25^oC to 11.21^oC?

ROUND YOUR ANSWER TO 3 SIG FIGS!

.

Fill in the Blank

What must be the mass of an unknown piece of metal with a specific heat of 0.586J/g^oC if after absorbing 57 Joules of energy it heats up by 12^oC?

ROUND YOUR ANSWER TO 3 SIG FIGS!

Fill in the Blank

What must be initial temperature of a 15.5 gram block of metal if it has a specific heat of 0.705 cal/g^oC and after absorbing 325 cal of heat it is now 45.7 ^oC?

ROUND YOUR ANSWER TO 3 SIG FIGS!