Energy Our working definition of energy is:   Energy is a substance like quantity that could be transferred from one object to another. This "substance" cannot be created nor destroyed. Energy is responsible for all change. Energy can be transferred from one object to another. The way energy is stored can be compared to the way money is "stored" in a bank. In a bank, you can keep money in a savings account, checking account or certificate of deposit. Likewise, objects can store energy in various "accounts" such as the kinetic account or the potential account. How does energy manifest itself when it is stored in these accounts? Thermal account (Eth): The Eth account is the account that affects the speed and the amount of vibration of the particles in a substance. It depends on the mass and velocity of the particles. The temperature of a substance is a measure of the energy in the thermal “account”. Potential account: Interaction account (Ei): This is a measure of the energy due to the arrangement of the particles. Energy is required to move particles of matter farther apart, therefore solids have minimal interaction energy, liquids have more and gases have the most.  Chemical account (Ech): This is a measure of the energy stored due to the bonding of particles to each other. Carbohydrate rich foods and fuels have lots of chemical energy whereas substances like water and carbon dioxide have low amounts of chemical energy.We can represent the energy stored in a system by using a bar graph.

 

A cold liquid                                A warm liquid                                 A hot liquid

We can define a system as being either open or closed. For example, people can move in or out of a classroom because it is an open system. People cannot move in or out of a room that is locked from both sides because it is a closed system. A can of cold soda eventually warms up because it is an open system. Energy can enter from the outside. Cold soda in a thermos stays cold because it is a closed system where energy cannot either leave or enter. We represent the movement of energy into or out of a system with an energy flow diagram. Energy can move in or out of a system three ways. It can be lost or gained by heating (Q), working (W) or radiating (R).

A beaker of water sits on a hot plate and gets warmer.

A hot filament on an electric stove.

The piston of a motor expands

Now we can put together energy bar graphs and energy flow diagrams. Lets take the example of a cold liquid being heated on a hot plate.

Below is a hot solid is put in water and cools off.

Conventions Using our model, we express how much energy there is in each account by stating it has so many bars of energy in the kinetic account, so many in the interaction account and so many in the chemical account. In the interaction energy account Solids have one bar of energy Liquids have two bars of energy Gases have four bars of energy Not all molecules in a population have the same kinetic energy. Some particles have lower than average kinetic energies, some have higher than average kinetic energies, but most have a kinetic energy close to the average. The average velocity of a population of molecules is dependent on the temperature. As temperatures increase, the average speed increases.

Units of Energy Temperature is a measure of the AVERAGE kinetic energy of the particles in a material. To express the total energy, we can use two units for energy. The older unit is the calorie and the SI unit is the Joule. Physical Changes in Matter When a substance is heated or cooled one of two things can occur. Either there will be a change in temperature or a change in state. Both changes cannot occur simultaneously. Here is what a typical heating graph for water being heated from a solid through to a vapor. A cooling curve looks like the graph below in reverse.

In section 1, there is a change in temperature but no change in state. We can represent this change in temperature as a change in the energy in the thermal account (Eth). Since there is no change in state in this section, there is no change in interaction energy (Ei). The same is true for section 3 and 5. In section 2 , there is no change in temperature and therefore, no change in thermal energy (Eth). There is a change in state from solid to liquid so the interaction energy (Ei) changes from one bar to two bars. For section 4, the interaction energy changes from two to four bars but there is no change in thermal energy.

Quantifying Energy Transfer Changes in Temperature Temperature is a measure of the average kinetic energy. The total energy is also dependent on how much of a substance there is. A drop of boiling hot water has less energy than a teapot full of boiling water. How can we calculate the energy transferred to 250 g of cold water at 20 C when it is heated to 90 C? Specific heat capacity is a conversion factor we can use to calculate the energy needed. Every substance has its own unique heat capacity. The heat capacity of water is 4.18 J / (1g)(1C). We can calculate the energy required by essentially converting temperature change to energy change. We work towards an answer that has the right units (Joules).  (90 – 20)C |250 g| 4.18 J     =    73,150 J                      |          |(1g)(1C)

Changes in State Since there is no temperature change in a change of state, we need a different conversion factor. For melting or freezing, the conversion factor is the heat of fusion (Hf). For boiling or condensation, the conversion factor is the heat of vaporization (Hv). Every substance has its own unique heat of fusion and vaporization. For water Hf = 334 J/g and Hv = 2,260 J/g. When 250 g of ice is melted, we can convert the mass melted to energy by using the heat of fusion, which is the conversion factor. The energy required to melt the ice is : 250 g | 334 J =   83,500 J            | 1 g For boiling, we use the heat of vaporization as the conversion factor. The energy required to boil the water is : 250 g | 2,260 J =  565,000J            | 1 g

   
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