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184_notes:charge_and_matter [2018/05/17 15:25] – [Charges in Matter] curdemma | 184_notes:charge_and_matter [2021/01/25 00:05] – bartonmo | ||
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Sections 14.1-14.7 in Matter and Interactions (4th edition) | Sections 14.1-14.7 in Matter and Interactions (4th edition) | ||
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===== Charges and Matter ===== | ===== Charges and Matter ===== | ||
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==== Charges in Matter ==== | ==== Charges in Matter ==== | ||
- | All matter is made up of atoms, which are in turn made up of a dense positively charged nucleus (made up of protons and neutrons) and a sparse, negatively charged electron cloud. This is a different perspective than the most common model of electrons - [[https:// | + | All matter is made up of atoms, which are in turn made up of a dense positively charged nucleus (made up of protons and neutrons) and a sparse, negatively charged electron cloud. This is a different perspective than the most common model of electrons - [[https:// |
[{{ 184_notes: | [{{ 184_notes: | ||
- | Most matter is //**neutral**//, which means that the net charge (or sum of all the charges) of most atoms is zero. Since the charge of a proton is $+1.602 \cdot 10^{-19} \text{ C}$ and the charge of an electron is $-1.602 \cdot 10^{-19} \text{ C}$ (and the charge of a neutron is $0 \text{ C}$), this tells us that the number of protons in a neutral atom has to equal the number of electrons. //__Notice that if an object is neutral, it does not mean that the object has zero charge. It means that the amount of positive charge in the atom is equal to the amount of negative charge in the atom, so the **net** charge is zero.__// | + | **Most matter is neutral, which means that the net charge (or sum of all the charges) of most atoms is zero.** Since the charge of a proton is $+1.602 \cdot 10^{-19} \text{ C}$ and the charge of an electron is $-1.602 \cdot 10^{-19} \text{ C}$ (and the charge of a neutron is $0 \text{ C}$), this tells us that the number of protons in a neutral atom has to equal the number of electrons. //__Notice that if an object is neutral, it does not mean that the object has zero charge. It means that the amount of positive charge in the atom is equal to the amount of negative charge in the atom, so the **net** charge is zero.__// |
- | If an object is //**charged**//, this means that the net charge of the object is no longer zero. If the object has a negative net charge, this means that the object has an excess of electrons. If an object has a positive net charge, this means that the object is missing electrons. In theory you could also get a negative net charge by removing protons (or a positive charge by adding protons); however, protons are extremely difficult to remove (or add) because they are held together in the nucleus by the [[https:// | + | **If an object is // |
We can use this model of the atom (dense positive nucleus with an electron cloud) then to talk about how we get charged objects (charging/ | We can use this model of the atom (dense positive nucleus with an electron cloud) then to talk about how we get charged objects (charging/ | ||
==== Polarization ==== | ==== Polarization ==== | ||
- | {{ 184_notes: | + | [{{ 184_notes: |
- | If we start with a neutral atom by itself, we know that there will be a positive nucleus with a negative, circular electron cloud around it. The electron cloud is circular because it is equally likely that the electrons are anywhere around the nucleus (shown in Figure 2a/ | + | If we start with a neutral atom by itself, we know that there will be a positive nucleus with a negative, circular electron cloud around it. The electron cloud is circular |
- | **What would change about our atom if we put a charge next to a neutral atom?** | + | //What would change about our atom if we put a charge next to a neutral atom?// |
The electrons cannot leave their nucleus (unless [[https:// | The electrons cannot leave their nucleus (unless [[https:// | ||
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==== Types of Matter ==== | ==== Types of Matter ==== | ||
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Now that we know what happens to a single atom, it makes sense to talk about what happens when you collect atoms into matter or objects. For our purposes, we will be considering two types of matter that behave differently when near charge: insulators and conductors. There are other types of matter like [[https:// | Now that we know what happens to a single atom, it makes sense to talk about what happens when you collect atoms into matter or objects. For our purposes, we will be considering two types of matter that behave differently when near charge: insulators and conductors. There are other types of matter like [[https:// | ||
=== Insulators === | === Insulators === | ||
- | An insulator is an object or material where the electrons are tightly bound to the nucleus. This means that the electrons in an insulator can only move very small amounts and must stay close to their nuclei. | + | An insulator is an object or material where the electrons are tightly bound to the nucleus. This means that the electrons in an insulator can only move very small amounts and must stay close to their nuclei. **Charges cannot move freely through an insulator.** Common insulators include: plastic, glass, rubber, paper, wood, etc. |
- | {{184_notes: | + | [{{ 184_notes: |
- | {{ 184_notes: | + | |
If you put a negative charge next to (but not touching) a neutral insulator, the electron clouds of the atoms on the surface will be pushed away, causing polarization to occur in the insulator. This means that the surface of the insulator now has a positive charge on the surface, with a negative line of charge behind it. The positive charges on the surface will try to attract the negative charge while the negative charges will try to repel it. Because the positives charges are closer, the attraction will be slightly stronger than the repulsion, causing a small attraction between the insulator (even though it is overall neutral) and the negative charge. (The first few layers of atoms on the surface of the insulator are shown to the left with a negative charge.) | If you put a negative charge next to (but not touching) a neutral insulator, the electron clouds of the atoms on the surface will be pushed away, causing polarization to occur in the insulator. This means that the surface of the insulator now has a positive charge on the surface, with a negative line of charge behind it. The positive charges on the surface will try to attract the negative charge while the negative charges will try to repel it. Because the positives charges are closer, the attraction will be slightly stronger than the repulsion, causing a small attraction between the insulator (even though it is overall neutral) and the negative charge. (The first few layers of atoms on the surface of the insulator are shown to the left with a negative charge.) | ||
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//**A conductor is an object or material where charged particles can move easily through the material.**// | //**A conductor is an object or material where charged particles can move easily through the material.**// | ||
- | {{ 184_notes: | + | [{{ 184_notes: |
When you put a negative charge next to a conductor, like a piece of metal, the negative charge repels the free electrons (the electron sea). Since these electrons are free to move through the metal, they move as far from the negative charge as they can, which makes the opposite surface of the conductor negatively charged (shown in the figure to the right). At the same time, the surface closest to the negative charge is now lacking negative charges because all of the free electrons have been repelled. This leaves the right surface of the metal with a net positive charge. Because the negative charges are on far side of the metal, the attraction from the positive surface is much stronger than the repulsion from the negative surface, making the attraction between the conductor and negative charge much stronger than it was in the insulator. | When you put a negative charge next to a conductor, like a piece of metal, the negative charge repels the free electrons (the electron sea). Since these electrons are free to move through the metal, they move as far from the negative charge as they can, which makes the opposite surface of the conductor negatively charged (shown in the figure to the right). At the same time, the surface closest to the negative charge is now lacking negative charges because all of the free electrons have been repelled. This leaves the right surface of the metal with a net positive charge. Because the negative charges are on far side of the metal, the attraction from the positive surface is much stronger than the repulsion from the negative surface, making the attraction between the conductor and negative charge much stronger than it was in the insulator. | ||
- | {{184_notes: | + | [{{ 184_notes: |
When you put a positive charge next to a conductor (shown in the figure to the left), the electrons in the electron sea are attracted to the surface of the metal closest to the positive charge. This leaves the opposite surface with a positive charge because those atoms now look like they are missing an electron. Since the positive charges are much further away from the positive charge than the negative charges, the attraction from the conductor is much stronger than the repulsion. This means that the positive charge is strongly attracted to the metal even though the metal is overall neutral. | When you put a positive charge next to a conductor (shown in the figure to the left), the electrons in the electron sea are attracted to the surface of the metal closest to the positive charge. This leaves the opposite surface with a positive charge because those atoms now look like they are missing an electron. Since the positive charges are much further away from the positive charge than the negative charges, the attraction from the conductor is much stronger than the repulsion. This means that the positive charge is strongly attracted to the metal even though the metal is overall neutral. | ||
- | **Again, the electrons are what moves in both cases.** | + | //Again, the electrons are what moves in both cases.// |
You might have experienced this effect when you were working with the tape challenge in the first class. Your body is mostly composed of salt water, which is a very good conductor. No matter what kind of charge was on your tape, you may have observed that it was always attracted to your hand, sometimes more than it was to the other piece of tape. This microscopic model of conductors would explain why the tape was always attracted to your hand. | You might have experienced this effect when you were working with the tape challenge in the first class. Your body is mostly composed of salt water, which is a very good conductor. No matter what kind of charge was on your tape, you may have observed that it was always attracted to your hand, sometimes more than it was to the other piece of tape. This microscopic model of conductors would explain why the tape was always attracted to your hand. |