Voltage drop - series resistors

[gazdkw82]

I have a question in a mock exam paper for the 202 exam.

It asks "Four resistors in series have the voltage drops across them of 37V,55V,19V and 9V. Calculate the supply voltage.

I'm struggling with how to approach this. I know how to work out VD but that requires more information (current).

I'm not sure how I obtain total resistor voltage with just the individual VD of each resistor. Is it as simple as just adding them up? wouldn't that just give total VD within the resistors?

 

[telectrix]

yep. add the volt drops up. the total is the supply voltage.

 

[gazdkw82]

ok easy enough. However I don't really understand how the total VD within the resisters = the supply voltage

 

[hightower]

Think of voltage as POTENTIAL. At the beginning of the supply it has 100% potential. The resistor means it has to do some work and use up some of the potential. So it goes in to the first resistor with 100% potential, but because the resistor uses up some energy it leaves with say 70%. It goes through another resistor which asks double the work that the first did, it went in with 70%, and left with 10%. It then uses up the last 10% on a last resistor. The amount it uses on each is in proportion to the resistor size.

At the negative terminal of the supply (think battery for simplicity) is always zero voltage. Think of it as you going for a run and not coming home until you can't walk any longer. So if there was only two resistors it wouldn't be 30% in the first, 60% in the second, and it makes it home with 10% left. It always gets home absolutely spent, so with two resistors it would be 33.3% across the first and 66.7% across the second. Adding up the potential dropped (voltage drop) at each resistor will give you the total, or supply voltage.

 

[gazdkw82]

AAHH..... I think i'm confusing myself with the term Voltage drop. If the question was to have said " four resistors in series with values of 37V, 55V, 19V and 9V, calculate the total Voltage resistance" it would have been easy.

I the question was some how calculating VD within R1,R2,R3 and R4....but that's impossible :-s

 

[telectrix]

if you drew it out, 1 end to L and the other end to N, ( or + and - if d.c.), it would become obvious.

 

[hightower]

I'm not even sure what you mean (and I don't want you to explain for fear you might end up more messed up over it), but Voltage, Volts, Voltage Drop, Potential Difference - they are all the same thing. If a supply (battery) is 12v that means it will drop 12v across it (positive to negative). Two resistors in series of the same resistance, each one will drop 6v across it.

 

[gazdkw82]

yeh yeh makes complete sense.

What I mean is, everytime iv had a question regarding resistors its been, 4 resistors in series/parallel, what is the total voltage etc....

Because they used the term Voltage drop I over thought it and over complicated it