November 20th, 2013
The parts from Monday that I want you to know:
From the lecture:
Chemical bonds require energy to break: it takes energy to separate any two things that are attached to each other. Think velcro: until you put in a bit of energy to separate the pieces, they will remain stuck together.
Energy is not created or destroyed. If you have to put in energy to break chemical bonds, you get energy back when you make chemical bonds. The energy of chemical bonds is potential energy: it is energy of composition.
Almost all chemical reactions break some bonds and make other bonds.
Some chemical reactions require that you put in energy to keep them going. Those are reactions where the energy it takes to break the bonds is greater than the energy you get back from making bonds. These reactions absorb heat, so they make things feel cold, like a chemical cold pack. We call these reactions endothermic.
Other reactions require just a little energy - heat, vibration, something - to get started, and then they give off energy, usually as heat. Those are reactions where the energy it takes to break bonds is not as much as the energy you get back from making bonds. These reactions give off heat: burning gasoline is an example, where you put in a little energy - a lit match - and get back a lot of energy. We call these reactions exothermic. ("Ex-" meaning "outside", like in "exterior", or "external".)
Nuclei (plural of "nucleus") are also sources of potential energy. If you add up the mass of the protons, neutrons, and electrons that make up an atom, the sum of the masses of the individual particles is less than the actual measured mass of the atom. Like with breaking chemical bonds, breaking a nucleus required energy, so putting a nucleus together gives back energy. That energy is expressed in a lower mass for the nucleus than for the protons & neutrons that make it up. This is weird: it is like saying that, if you weighed all the parts of a car and then assembled them into a car, that car would weigh less than its parts did.
So, all the individual parts of an atom weigh more than the whole atom does, and the missing mass is the energy you get from putting the nucleus together from smaller pieces. Again, this is weird, very weird, and yet it is true.
Light nuclei - lighter than iron - give off energy when they are slammed together to form larger nuclei. "Fusion" is the name for making a larger thing from two or more smaller things. In nuclear fusion, you might slam hydrogen-1 and hydrogen-3 nuclei together to form a helium-4 nucleus and a whole lot of energy.
Heavy nuclei - heavier than iron - give off energy when they break into two smaller nuclei. "Fission" is the name for breaking something into smaller pieces. In nuclear fission, you fire a neutron at, say, a uranium-235 nucleus to give a couple smaller nuclei, one to three neutrons, and a whole lot of energy. The neutrons from this can then slam into other uranium-235 nuclei and split them, giving off more neutrons and a lot more energy.
- Chemical and physical changes "almost always" come with energy changes. (It takes energy to change matter.)
- Energy is neither created nor destroyed, it is only transferred or converted from one form to another.
- Total energy is kinetic energy plus potential energy.
- Kinetic energy is the energy of movement. (Note: movement of anything - molecules, pool balls, cars, etc.)
- Potential energy is the energy of position or composition. For an example of the energy of position, think gravity: water in a water tower has potential energy because it can run down from where it is. For an example of the energy of composition, think about a spring: because of the way it is made, it can store energy for release later.
- Thermal energy is a kind of kinetic energy: it is the energy of moving atoms or molecules.
- Nothing can every be 100% energy-efficient: some energy is always wasted.
- Energy spreads out. For example, think about heat. Heat a pot of water, and leave it out overnight. In the morning, the water and pot are cold. The thermal energy did not just disappear. The heat of the pot & water went into heating up the air in the room, until the pot, water, and air all ended up at the same temperature.
From the lecture:
Chemical bonds require energy to break: it takes energy to separate any two things that are attached to each other. Think velcro: until you put in a bit of energy to separate the pieces, they will remain stuck together.
Energy is not created or destroyed. If you have to put in energy to break chemical bonds, you get energy back when you make chemical bonds. The energy of chemical bonds is potential energy: it is energy of composition.
Almost all chemical reactions break some bonds and make other bonds.
Some chemical reactions require that you put in energy to keep them going. Those are reactions where the energy it takes to break the bonds is greater than the energy you get back from making bonds. These reactions absorb heat, so they make things feel cold, like a chemical cold pack. We call these reactions endothermic.
Other reactions require just a little energy - heat, vibration, something - to get started, and then they give off energy, usually as heat. Those are reactions where the energy it takes to break bonds is not as much as the energy you get back from making bonds. These reactions give off heat: burning gasoline is an example, where you put in a little energy - a lit match - and get back a lot of energy. We call these reactions exothermic. ("Ex-" meaning "outside", like in "exterior", or "external".)
Nuclei (plural of "nucleus") are also sources of potential energy. If you add up the mass of the protons, neutrons, and electrons that make up an atom, the sum of the masses of the individual particles is less than the actual measured mass of the atom. Like with breaking chemical bonds, breaking a nucleus required energy, so putting a nucleus together gives back energy. That energy is expressed in a lower mass for the nucleus than for the protons & neutrons that make it up. This is weird: it is like saying that, if you weighed all the parts of a car and then assembled them into a car, that car would weigh less than its parts did.
So, all the individual parts of an atom weigh more than the whole atom does, and the missing mass is the energy you get from putting the nucleus together from smaller pieces. Again, this is weird, very weird, and yet it is true.
Light nuclei - lighter than iron - give off energy when they are slammed together to form larger nuclei. "Fusion" is the name for making a larger thing from two or more smaller things. In nuclear fusion, you might slam hydrogen-1 and hydrogen-3 nuclei together to form a helium-4 nucleus and a whole lot of energy.
Heavy nuclei - heavier than iron - give off energy when they break into two smaller nuclei. "Fission" is the name for breaking something into smaller pieces. In nuclear fission, you fire a neutron at, say, a uranium-235 nucleus to give a couple smaller nuclei, one to three neutrons, and a whole lot of energy. The neutrons from this can then slam into other uranium-235 nuclei and split them, giving off more neutrons and a lot more energy.