26. Capacitors & Dielectrics

Energy Stored by Capacitor

# Energy Released by Flashbulb

Patrick Ford

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Alright, guys, let's take a look at this one carefully. We have a flashlight were given the capacitance and the voltage of that flashlight. And then what happens is the flashlight goes off, the flash goes off and this bulb loses a some amount of charge, and we're supposed to figure out how much energy is released when that happens. So in other words, we have energy stored inside of this flashlight right here, some initial energy. And then what happens is it releases some of that charge. 80% of it. We're supposed to figure out how much energy is released by that. So in other words, we're figuring out what happens when when you have some initial energy and some final energy and you're finding the difference between them. So Delta U is equal to u F minus you. I Now we have this potential energy that we can relate. Using our equations, we know that you is equal to one half c v. Squared equals one half Q V and one half Q squared oversee. Now the question becomes which one of these things where we're gonna use which which form we're told that this charged or sorry. This potential energy has to do with some loss of charge. So in other words, one of the equations is gonna have to use our Q. We know that the charge que final is gonna be Well, let's see, it's gonna be 80% loss of charge. That means that cute final is going to be 20% of Q initial because it loses 80%. So we're gonna have to definitely relate this to some charge. Now the other question becomes, what other variable do we know? We know the capacitance and the voltage. So now we're kind of stuck between Let's say we don't have to use 11 half C V squared. We're going to use one of these two now What happens is we know that from Q equals C V. If you have a capacitance that capacities in a parallel plate capacitor is fixed, it can't change. So, for instance, if this flashlight has a capacity of 1000 military IDs, then it's always that value, and it has to do with the charge and the voltage. What happens is as the charge gets dropped, Then the voltage also decreases. But this capacitance stays the same. So that means that we can't use Q V because because I'm not right It right here. So can't use this one because V is not constant. So we can't use that one half Q V Instead, we're gonna have to relate this back to the charge which is lost and then the capacity to which is a fixed value. So this is the actual form of the potential energy we're gonna use. So we know that this you final minus you initial is gonna be one half. Now we've got two Q final squared, divided by the capacitance, minus one half que initial squared, divided by the capacitance. Okay. And that's basically what we're gonna use. So we have one half now the charge. How do we find the charge when we find it using Q equal CV? So the initial charge is going to be the capacitance, which is 1000 militaries, which, by the way, 1000 military it's just equals one Ferid times the voltage, which is 500. Now we know that that's equal to 500 cool OEMs of initial charge. How do we find the final charge? Remember the final charges 20% of the initial charge, which means that 20% of 500 is just equal to 100 cool homes. So now that we have our initials and finals now you can just plug these values inside of these equations right here and figure out what the change in energy is the amount of energy that's released. So you got one half of five. Sorry. That's 100 cool homes that's gonna be squared and then divided by the capacitance, which is one minus one half of the final charge or sorry, initial charge 500 squared, divided by the capacitance. So you go ahead and work this out. Thea's want of energy that's released is going to be negative on 120,000, and that's gonna be in jewels. Or you could have written. This is 1.2 times 10 to the fifth in jewels. This makes sense because we got a negative number. Some of the words all of this charge gets released, So that means that the amount of energy that amount of potential energy is going to decrease. All that stuff gets converted toe light. All right, so let me know if you guys have any questions with this

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