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# Internal Resistance watch

1. Hi sf, please could someone help me with this physics problem:

An ultra high tension power pack provides a p.d. of 6kV and has an internal resistance of 50megaohms.

What will the current be if an ammeter of resistance 5.0ohms is connected directly across the power supply's terminal with no other componetns in the circuit?

I know the answer is 6 microamps but I tried to do it using the resistors in parallel equation. If you put the ammeter across the power supply then wouldn't the internal resistance now be in parallel with the ammeter which would give a current of 1200A if you use this equation.
2. (Original post by Janey_)
Hi sf, please could someone help me with this physics problem:

An ultra high tension power pack provides a p.d. of 6kV and has an internal resistance of 50megaohms.

What will the current be if an ammeter of resistance 5.0ohms is connected directly across the power supply's terminal with no other componetns in the circuit?

I know the answer is 6 microamps but I tried to do it using the resistors in parallel equation. If you put the ammeter across the power supply then wouldn't the internal resistance now be in parallel with the ammeter which would give a current of 1200A if you use this equation.
Well it's series because *all* of the current flowing in that circuit has to pass through both the internal resistance of the PSU and the external ammeter - there isn't an alternative path for some of the current to take.
3. (Original post by Joinedup)
Well it's series because *all* of the current flowing in that circuit has to pass through both the internal resistance of the PSU and the external ammeter - there isn't an alternative path for some of the current to take.
Thanks, that makes sense. Out of interest what would be a use of an ultra high tension machine if it can only draw 120microamps maximum?
4. (Original post by Janey_)
Thanks, that makes sense. Out of interest what would be a use of an ultra high tension machine if it can only draw 120microamps maximum?
Quantitative electrostatic experiments by a responsible student... it says here http://www.philipharris.co.uk/produc...upply-B8H28305

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