184_notes:charge_and_matter

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184_notes:charge_and_matter [2021/01/24 23:46] – bartonmo184_notes:charge_and_matter [2021/01/25 00:06] (current) – bartonmo
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 [{{ 184_notes:atom_w_cloud.png?200|Electron cloud around positive nucleus }}] [{{ 184_notes:atom_w_cloud.png?200|Electron cloud around positive nucleus }}]
  
-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://en.wikipedia.org/wiki/Strong_interaction|strong interaction]]. Since electrons are relatively easy to remove compared to a proton, almost all charged objects that you will encounter will be due to electrons being added or removed.+**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://en.wikipedia.org/wiki/Strong_interaction|strong interaction]]. Since electrons are relatively easy to remove compared to a proton, almost all charged objects that you will encounter will be due to electrons being added or removed.
  
 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/discharging) and how atoms respond to charges nearby (polarization).   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/discharging) and how atoms respond to charges nearby (polarization).  
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 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 (spherical in 3D) because it is equally likely that the electrons are anywhere around the nucleus (shown in Figure 2a/3a).  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 (spherical in 3D) because it is equally likely that the electrons are anywhere around the nucleus (shown in Figure 2a/3a). 
  
-**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://en.wikipedia.org/wiki/Ionization|the interaction is very strong]]), but they are attracted to the positive charge. With a positive charge nearby, it is now more likely that the electrons will be on the left side of nucleus compared to the right (shown in Figure 2b), shifting the electron cloud toward the positive charge. Often, we will simplify this drawing to be just an oval that indicates which side of the atom is more positive and which side is more negative (shown in Figure 2c).   The electrons cannot leave their nucleus (unless [[https://en.wikipedia.org/wiki/Ionization|the interaction is very strong]]), but they are attracted to the positive charge. With a positive charge nearby, it is now more likely that the electrons will be on the left side of nucleus compared to the right (shown in Figure 2b), shifting the electron cloud toward the positive charge. Often, we will simplify this drawing to be just an oval that indicates which side of the atom is more positive and which side is more negative (shown in Figure 2c).  
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 === 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. //**Charges cannot move freely through an insulator.**// Common insulators include: plastic, glass, rubber, paper, wood, etc. +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:insulatorpos.png?150|Charge distribution in an insulator from a positive charge}}] [{{  184_notes:insulatorpos.png?150|Charge distribution in an insulator from a positive charge}}]
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 === Conductors === === Conductors ===
  
-//**A conductor is an object or material where charged particles can move easily through the material.**// In some conductors (like salt water), there are charged ions ($\text{Na}^+$ and $\text{Cl}^-$) that can travel relatively freely through the material (water). In other conductors, like metals, the inner electrons of every atom are tightly bound to the nucleus, but the outer electrons (or [[https://en.wikipedia.org/wiki/Valence_electron|valence electrons]]) of the atom are much easier to remove. When you have lot of metal atoms together, generally one electron from each atom can leave the atom and join a "sea" of electrons that are free to move through the metal. These electrons are not completely free - it is very difficult to remove these electrons from the metal - but they are relatively free to move within the piece of metal. This is how we model the metal as a conductor - a mobile electron sea. Common conductors include: salt water, copper, iron, aluminum, gold, etc.  +**A conductor is an object or material where charged particles can move easily through the material.** In some conductors (like salt water), there are charged ions ($\text{Na}^+$ and $\text{Cl}^-$) that can travel relatively freely through the material (water). In other conductors, like metals, the inner electrons of every atom are tightly bound to the nucleus, but the outer electrons (or [[https://en.wikipedia.org/wiki/Valence_electron|valence electrons]]) of the atom are much easier to remove. When you have lot of metal atoms together, generally one electron from each atom can leave the atom and join a "sea" of electrons that are free to move through the metal. These electrons are not completely free - it is very difficult to remove these electrons from the metal - but they are relatively free to move within the piece of metal. This is how we model the metal as a conductor - a mobile electron sea. Common conductors include: salt water, copper, iron, aluminum, gold, etc.  
  
 [{{  184_notes:conductorneg.png?150|Charge distribution in a conductor from a negative charge}}] [{{  184_notes:conductorneg.png?150|Charge distribution in a conductor from a negative charge}}]
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 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.
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