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| 184_notes:r_energy [2018/02/08 20:37] – dmcpadden | 184_notes:r_energy [2021/06/14 23:41] (current) – schram45 | ||
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| Sections 18.3, 18.8-18.10, and 19.4 in Matter and Interactions (4th edition) | Sections 18.3, 18.8-18.10, and 19.4 in Matter and Interactions (4th edition) | ||
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| ===== Energy around the Circuit ===== | ===== Energy around the Circuit ===== | ||
| - | One of the consequences of adding a resistor in the circuit (with higher electron speed and a higher electric field) is that a large energy transfer occurs across the resistor. In thin wire resistors (sometimes referred to as filaments), this effect is particularly visible. The amount of energy transferred to a filament is sufficient to heat the thin wire to the point where it produces heat and light. This is actually how [[https:// | + | One of the consequences of adding a resistor in the circuit (with higher electron speed and a higher electric field) is that a large **energy transfer** occurs across the resistor. In thin wire resistors (sometimes referred to as filaments), this effect is particularly visible. The amount of energy transferred to a filament is sufficient to heat the thin wire to the point where it produces heat and light. This is actually how [[https:// |
| {{youtube> | {{youtube> | ||
| - | Let's continue to look at the simple circuit that we were using above (a mechanical battery, wires, and a thin filament). To analyze the energy in our circuit, we can refer back to the [[184_notes: | + | Let's continue to look at the simple circuit that we were using in the video above (a mechanical battery, wires, and a thin filament). To analyze the energy in our circuit, we can refer back to [[184_notes: |
| $$\Delta E_{sys}=0$$ | $$\Delta E_{sys}=0$$ | ||
| If we breakdown what is in our system, this means that | If we breakdown what is in our system, this means that | ||
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| Another way to talk about energy in circuits is to look at how much energy (aka, heat or light) is used up per second by a lightbulb or more generally by a resistor (in contrast to voltage which is energy per charge). When you are talking about the change in energy per change in time, this is called **power**: | Another way to talk about energy in circuits is to look at how much energy (aka, heat or light) is used up per second by a lightbulb or more generally by a resistor (in contrast to voltage which is energy per charge). When you are talking about the change in energy per change in time, this is called **power**: | ||
| $$P=\frac{\Delta U}{\Delta t}=\frac{dU}{dt}$$ | $$P=\frac{\Delta U}{\Delta t}=\frac{dU}{dt}$$ | ||
| - | Power is a scalar quantity that has units of watts or joules per second ($W=\frac{J}{s}$). For reference, a typical lightbulb in your house is a 60 W lightbulb. On the other hand, a large power plant that produces electricity for a city generally produces $1-5$ MW $=1-5 \cdot 10^6$W. In circuits, it is fairly easy to calculate the power if you know the potential difference across a circuit element and the current that passes through that element. To get power, you multiply current times the potential difference since current has units of amps or coulombs per second, and electric potential has units of volts. $$\frac{C}{s}*V=\frac{J}{s}$$ since a volt*coulomb is a joule, we get units of energy per second, which is what we want. In other words, | + | Power is a scalar quantity that has **units of watts or joules per second** ($W=\frac{J}{s}$). For reference, a typical lightbulb in your house is a 60 W lightbulb. On the other hand, a large power plant that produces electricity for a city generally produces $1-5$ MW $=1-5 \cdot 10^6$W. In circuits, it is fairly easy to calculate the power if you know the potential difference across a circuit element and the current that passes through that element. To get power, you multiply current times the potential difference since current has units of amps or coulombs per second, and electric potential has units of volts. $$\frac{C}{s}*V=\frac{J}{s}$$ since a volt*coulomb is a joule, we get units of energy per second, which is what we want. In other words, |
| $$P=I\Delta V$$ | $$P=I\Delta V$$ | ||
| Note we are using conventional current here, not the electron current. | Note we are using conventional current here, not the electron current. | ||
| ==== Examples ==== | ==== Examples ==== | ||
| - | [[: | + | * [[: |
| + | * Example Video: Changing the Dimensions of a Wire | ||
| + | {{youtube> | ||