# Question Video: Determining the Charge Flowing through a Point in a Filament Physics • 9th Grade

If a filament of a light bulb experiences a current of 0.3 A, the electric charge that will flow through a point P in the filament in 25 s will be ＿.

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### Video Transcript

If a filament of a light bulb experiences a current of 0.3 amperes, the electric charge that will flow through a point P in the filament in 25 seconds will be blank. (A) Five coulombs, (B) 12 coulombs, (C) 8.3 coulombs, (D) 7.5 coulombs.

Here, we are asked to consider a filament of a light bulb with a current of 0.3 amperes through it. We are asked to figure out how much charge will flow through a point P in the filament during a time period of 25 seconds.

In order to answer this question, it’ll help to recall that electric current is defined as the rate of flow of charge over time. The equation defining the electric current through a point is current 𝐼 is equal to the charge 𝑄 that flows through that point divided by the time 𝑡 that it takes for that charge to flow. In this problem, we know that the current 𝐼 is equal to 0.3 amperes and the time 𝑡 is 25 seconds.

We are trying to calculate an amount of charge, which means we need to rearrange this equation to make charge 𝑄 the subject. We can do this by multiplying both sides of the equation by 𝑡. Then, after canceling the 𝑡’s on the right-hand side, we get an equation that says charge 𝑄 is equal to current 𝐼 multiplied by time 𝑡. We can now substitute in our values for the current 𝐼 and the time 𝑡. We find that 𝑄 is equal to 0.3 amperes multiplied by 25 seconds.

Before we evaluate this expression, let’s take a look at the units we have on the right-hand side. We have a current in units of amperes and a time in units of seconds. We can remember though that units of amperes are equivalent to units of coulombs per second. If we replace the units of amperes by units of coulombs per second in this expression, we can see that the seconds and per second cancel each other out. This leaves us with units of coulombs. And we can recall that the coulomb is the unit for electric charge, which means we know we have the right units here.

Now that we know our units are correct, we can solve this equation. The charge 𝑄 is equal to 0.3 multiplied by 25, with units of coulombs. This is equal to 7.5 coulombs. This matches the value given in option (D). Therefore, the correct answer is option (D), 7.5 coulombs.