Thermodynamics: The Physics of Heat
Specific Heat Lab Analysis
- Some substances heat more quickly than others. 75ml of oil heats more quickly than 75ml of water, for example.
- If you have twice the mass of a substance, it takes twice as long to heat when the heat given is constant.
- This means that HEAT and TEMPERATURE must be different!!
- Ex: If you have 75g of water in beaker A and 150g of water in beaker B and both are heated with the same flame for 3 minutes, which will have a higher temperature at the end? – The 75g of water will have a higher temperature.
- This is true even though the SAME AMOUNT OF HEAT went from the Bunsen burner into each beaker, which heated the water. Same flame, same time, therefore same amount of heat.
- Ex: Which substance can store more heat, 100g of water or 100g of oil? – 100g of WATER
- This is because for a given amount of heat added, the temperature of water changes less than oil for the same amount of heat.
- We say that water has a higher SPECIFIC HEAT (symbol “c”) than the oil.
- Specific Heat is: the amount of heat required to raise the temperature of 1g of a substance by 1°C.
- But in order to establish actual numbers for specific heat, we need a reference point. We use water: water is defined to have a specific heat of 1 unit of heat for every gram, for every calorie. This means that it takes a particular amount of to raise 1 gram of water by 1 degree Celsius, and we define this amount of heat as one unit, and we give this unit a name and call it a calorie (symbol “cal”).
- A calorie is defined as the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius.
- So water has a specific heat of 1 cal/g°C.
- Oil has a lower specific heat than water, meaning it takes less heat (less calories) to change 1 gram by 1°C.
- Ex: Which has more heat, 100g of water of 200g of water?
- YOU CAN’T TELL until you know the temperatures of the water!
- Ex: Which has more heat, 100g of 30°C water or 200g of 30°C water?
- The 200g of water has twice as much heat!
- This means the 200g of 30°C water could melt twice as much ice as 100g of 30°C water could.
- This gives rise to the idea of THERMAL CAPACITY, which is simply the specific heat times the mass. T.C. = specific heat x mass
- If we want to objectify our definition of how much heat a substance can contain, we need to know what it depends upon.
- How much heat something has depends upon:
- The amount of the substance (mass)
- The temperature of the substance
- The specific heat of the substance
- Written using an equation it goes like this:
- Q = cmΔt
- Q = amount of heat measured in calories
- c = specific heat of the substance in question
- m = mass of the stubstance
- Δt = change in temperature of the substance
- In English this equation reads:
- For a material with specific heat of c and a mass m, a change in temperature Δt requires a definite amount of heat Q.