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# Units Question - Ohm's Law watch

1. In Ohm's Law, I know that the S.I units are Volts, Amps and Ohms.

If you have Volts and Milliamps, what unit would the resistance be in? Would it be kilo-ohms or just ohms?
2. (Original post by Ishan_2000)
In Ohm's Law, I know that the S.I units are Volts, Amps and Ohms.

If you have Volts and Milliamps, what unit would the resistance be in? Would it be kilo-ohms or just ohms?
You should always work in whole amps, writing eg 23mA as 23x10(^-3)A. This means you do not have any problems trying to work out what prefixes to use. If ohms come out as 42000 or 42x10(^3)A then you can convert it at the end to 42kilo-ohms.
3. (Original post by Science_Girl)
You should always work in whole amps, writing eg 23mA as 23x10(^-3)A. This means you do not have any problems trying to work out what prefixes to use. If ohms come out as 42000 or 42x10(^3)A then you can convert it at the end to 42kilo-ohms.
Ok, yes, but what let's say you used Volts and milliamps, then what unit would the Resistance be in? Would it still be ohms?
4. (Original post by Ishan_2000)
Ok, yes, but what let's say you used Volts and milliamps, then what unit would the Resistance be in? Would it still be ohms?
Not too difficult to work out:

Since R=V/I

R(units) = Volts/(Amps x 10^-3) = Ohms x 10^3 = KiloOhms
5. (Original post by natninja)
Not too difficult to work out:

Since R=V/I

R(units) = Volts/(Amps x 10^-3) = Ohms x 10^3 = KiloOhms
Thanks, I thought so.
6. (Original post by Ishan_2000)
Thanks, I thought so.
But... as someone always said - convert to SI to do your calculations and then if necessary do the orders of magnitude at the end.

Also a useful check if you don't know the correct units for something or you think an answer is wrong is just put the units into the formula and if the answer seems the right sort of order of magnitude and has the right units, at GCSE and A-level either the answer is right or you made a calculator error
7. (Original post by natninja)
But... as someone always said - convert to SI to do your calculations and then if necessary do the orders of magnitude at the end.

Also a useful check if you don't know the correct units for something or you think an answer is wrong is just put the units into the formula and if the answer seems the right sort of order of magnitude and has the right units, at GCSE and A-level either the answer is right or you made a calculator error
Yeah, ok, thanks.
8. (Original post by Ishan_2000)
Ok, yes, but what let's say you used Volts and milliamps, then what unit would the Resistance be in? Would it still be ohms?
The resistance would still be in Ohms.

The only thing that would change would be how many Ohms dependent on the scale of both the voltage and current.

For instance:

If the voltage was say 240V and the current 100A, then:

If the voltage was 240V and the current 10mA (10mA = 0.01A = 10x10-3A):

9. (Original post by uberteknik)
The resistance would still be in Ohms.

The only thing that would change would be how many Ohms dependent on the scale of both the voltage and current.

For instance:

If the voltage was say 240V and the current 100A, then:

If the voltage was 240V and the current 10mA (10mA = 0.01A = 10x10-3A):

Yeah, I understand it now from previous posts. Thanks anyway

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