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| 184_notes:examples:week3_balloon_wall [2018/01/24 22:05] – [Solution] tallpaul | 184_notes:examples:week3_balloon_wall [2021/01/26 21:21] (current) – [Solution] bartonmo | ||
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| - | =====Balloon Stuck to a Wall===== | + | [[184_notes: |
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| + | =====Example: | ||
| When you rub part of a rubber balloon against wool (or your hair), electrons will leave the wool, which is slightly conductive, and go onto the balloon. The rubber on the balloon is much less conductive (rubber is more of an insulator than wool), and the electrons will not readily leave the balloon. As a result, the balloon becomes negatively charged. Imagine you bring the negatively charged balloon up to a wall, and it sticks (This is possible! A quick internet search will yield many explanations and demonstrations. You can also try it yourself). Why would the balloon stay in one place on the wall? Draw a free body diagram for the balloon to help your explanation. | When you rub part of a rubber balloon against wool (or your hair), electrons will leave the wool, which is slightly conductive, and go onto the balloon. The rubber on the balloon is much less conductive (rubber is more of an insulator than wool), and the electrons will not readily leave the balloon. As a result, the balloon becomes negatively charged. Imagine you bring the negatively charged balloon up to a wall, and it sticks (This is possible! A quick internet search will yield many explanations and demonstrations. You can also try it yourself). Why would the balloon stay in one place on the wall? Draw a free body diagram for the balloon to help your explanation. | ||
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| * The wall is perfectly vertical (parallel to the gravitational force). | * The wall is perfectly vertical (parallel to the gravitational force). | ||
| </ | </ | ||
| - | {{ 184_notes: | + | [{{ 184_notes: |
| ===Goal=== | ===Goal=== | ||
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| We know the balloon is motionless, so air resistance is not a factor here, as it often is with balloons. The only other force we could have is the electric force between the balloon and the wall. Since the net force on our balloon is zero, the free body diagram looks something the following representation: | We know the balloon is motionless, so air resistance is not a factor here, as it often is with balloons. The only other force we could have is the electric force between the balloon and the wall. Since the net force on our balloon is zero, the free body diagram looks something the following representation: | ||
| - | {{ 184_notes: | + | [{{ 184_notes: |
| In order to describe how we might get this diagonal electric force, we'll make a few more assumptions. | In order to describe how we might get this diagonal electric force, we'll make a few more assumptions. | ||
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| We can use the following representation, | We can use the following representation, | ||
| - | {{ 184_notes: | + | [{{ 184_notes: |
| We know that the balloon is negatively charged from rubbing it on wool/hair. When we bring the charged balloon close to the wall, the atoms in the wall near to the balloon become polarized with the electron clouds being pushed away from the negative balloon. See the notes on [[184_notes: | We know that the balloon is negatively charged from rubbing it on wool/hair. When we bring the charged balloon close to the wall, the atoms in the wall near to the balloon become polarized with the electron clouds being pushed away from the negative balloon. See the notes on [[184_notes: | ||
| - | Notice that the electric force needs to point both to the left and upward in order for the net force to be zero. If you were to try this experiment out yourself, you may notice the balloon rolling slightly up and down (oscillating) before settling to a motionless state. As the balloon rolls, the charge distribution on the balloon moves in the same way that the balloon does, which changes the direction of the electric force on the balloon from the wall. When the balloon settles, we know it has come to a place where the direction and magnitude of the electric force results in a net force of zero. A force diagram on the motionless balloon is shown below. | + | Notice that the electric force needs to point both to the left and upward in order for the net force to be zero. If you were to try this experiment out yourself, you may notice the balloon rolling slightly up and down (oscillating) before settling to a motionless state. As the balloon rolls, the charge distribution on the balloon moves in the same way that the balloon does, which changes the direction of the electric force on the balloon from the wall. When the balloon settles, we know it has come to a place where the direction and magnitude of the electric force results in a net force of zero. |
| - | {{ 184_notes: | + | |
| //Another note on our assumption about friction//: We do not include friction in the force diagram. We assume the electric force has enough of an upwards component that friction contributes nothing. However, depending on assumptions that you make, you may have friction in your force diagram. In reality, there is probably both friction and a slightly upwards electric force component at play here. | //Another note on our assumption about friction//: We do not include friction in the force diagram. We assume the electric force has enough of an upwards component that friction contributes nothing. However, depending on assumptions that you make, you may have friction in your force diagram. In reality, there is probably both friction and a slightly upwards electric force component at play here. | ||