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# Simple AS physics question - Electric Charge Watch

1. When a small torch is switched on, the current drawn from the cell is 0.25 A.

Q1) Calculate the charge passing the point in the bulb filament when the torch is switched on for 15 minutes.

So 0.25 * 900 = 225 C. Easy.

Q2) (This is what I'm stuck on)

How many electrons drift past the point in this time? The charge on one electron is 1.6 * 10^-19.

I don't understand how to do this question at all. The book didn't really explain this.

Thank you!
2. (Original post by yodawg321)
When a small torch is switched on, the current drawn from the cell is 0.25 A.

Q1) Calculate the charge passing the point in the bulb filament when the torch is switched on for 15 minutes.

So 0.25 * 900 = 225 C. Easy.

Q2) (This is what I'm stuck on)

How many electrons drift past the point in this time? The charge on one electron is 1.6 * 10^-19.

I don't understand how to do this question at all. The book didn't really explain this.

Thank you!
So basically, you've worked out that 225 C of charge has passed through the filament in 15 mins, and they've told you that each electron carries 1.6*10^-19 C. so therefore if you divide 225 / 1.6*10^-19 you get the number of electrons that are required to carry that charge i.e. to pass through the filament in 15mins. = 1.41*10^21

Hope this helps
3. (Original post by ZakRob)
So basically, you've worked out that 225 C of charge has passed through the filament in 15 mins, and they've told you that each electron carries 1.6*10^-19 C. so therefore if you divide 225 / 1.6*10^-19 you get the number of electrons that are required to carry that charge i.e. to pass through the filament in 15mins. = 1.41*10^21

Hope this helps

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