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| 184_notes:q_in_wires [2021/02/18 20:14] – bartonmo | 184_notes:q_in_wires [2021/02/23 20:22] – [Hypothesis 1 - Electric field comes from the battery alone] bartonmo | ||
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| We have already talked about how to model a battery as a separation of charges. However, if we connect the two ends of the battery with a conducting wire, what happens to the charge distribution in the wire? Using [[184_notes: | We have already talked about how to model a battery as a separation of charges. However, if we connect the two ends of the battery with a conducting wire, what happens to the charge distribution in the wire? Using [[184_notes: | ||
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| If this were true, the electric field at Point 1 in the wire would point to left given that it is near the positive plate. Likewise, the electric field at Point 2 would also point to the left, given that it is near the negative plate. At Point 3, we would expect the electric field to be point to the right because it is between the positive and negative plates, though it would be smaller because it is further away from the plates. | If this were true, the electric field at Point 1 in the wire would point to left given that it is near the positive plate. Likewise, the electric field at Point 2 would also point to the left, given that it is near the negative plate. At Point 3, we would expect the electric field to be point to the right because it is between the positive and negative plates, though it would be smaller because it is further away from the plates. | ||
| - | However this is problematic for a few of reasons: | + | **However this is problematic for a few of reasons:** |
| - If the electric field is smaller in some parts of the wire than others, this would mean that the electrons move faster in parts of the wire (where the electric field is bigger - close to the battery) and slower in other parts of the wire (where the electric field is smaller - in the middle of the wire). We can measure the electron current at each part of the wire in such a circuit and show that this is not in fact true. //The electron current close to the battery is exactly the same as the electron current in the middle of the wire// | - If the electric field is smaller in some parts of the wire than others, this would mean that the electrons move faster in parts of the wire (where the electric field is bigger - close to the battery) and slower in other parts of the wire (where the electric field is smaller - in the middle of the wire). We can measure the electron current at each part of the wire in such a circuit and show that this is not in fact true. //The electron current close to the battery is exactly the same as the electron current in the middle of the wire// | ||
| - The electric field from the battery plates would always point either left or right (away the positive plate and towards the negative plate); thus, this electric field could never push the electrons down or up the vertical parts of the wires, let alone explain how the electrons are able to bend around the corner of the wire. | - The electric field from the battery plates would always point either left or right (away the positive plate and towards the negative plate); thus, this electric field could never push the electrons down or up the vertical parts of the wires, let alone explain how the electrons are able to bend around the corner of the wire. | ||
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| - If the force that pushes the electrons comes from the surface of the wires, then the proximity to the battery doesn' | - If the force that pushes the electrons comes from the surface of the wires, then the proximity to the battery doesn' | ||
| - | Thus, surface charges as the model for how charges move through wires is far more consistent with the observations we have in real circuits in everyday life. If you have a very large battery (also called a high voltage power supply) it is actually possible to observe the surface charges on a wire. Here's a video demonstrating | + | Thus, surface charges as the model for how charges move through wires is far more consistent with the observations we have in real circuits in everyday life. If you have a very large battery (also called a high voltage power supply) it is actually possible to observe the surface charges on a wire. |
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| + | In this video, you can see the mechanical effects of surface charge when a piece of foil is held by the positive end, negative end, and neutral center of the wire. On the ends of the wire, the foil is initially attracted, polarized, and then repelled. Since the surface charge at the center of the wire is 0 nothing happens to the foil. A metal pen can also be used to see what kind of charge the foil has picked up based on if it is attracted to or repelled by the pen. | ||
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| ==== Examples ==== | ==== Examples ==== | ||
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