Understanding Resistor Volt Drops

[berenmacbowma]

I've come across a booklet on understanding electricity, and it's been very interesting, but I've hit a rut! The packet says that the sum of the voltage drops of resistors equals the source voltage. How is that possible? Were the voltage to continuously drop, wouldn't no voltage exist at all once passing through the last resistor?

 

[grzz]

The 'voltage drop' across a resistor is the work done per unit charge by the voltage source as this source is pushing the charge against the resistance of the resistor.

So the voltage of the source will be the total work per unit charge as the source is pushing this unit charge across the various resistances of the circuit.

 

[berenmacbowma]

so the "voltage drop", i think that term is what's confusing me, doesn't actually lower the voltage of the current?

 

[Studiot]

The packet says that the sum of the voltage drops of resistors equals the source voltage

Yes that's correct.

The easiest way to understand this is to think of links in a chain.

Every link is say 10cm long.

If you lay the chain out the lengths of all the links (say 5) add up to the overall length of the chain (10+10+10+10+10 = 50cm in the example case)

You can see that if the links have different lengths, the overall length of the chain is still the sum of the individual lengths.

So it is with voltage. Voltage is always measure between two points (eg the ends of a resistor).
We usually call this the voltage drop.

So if you chain a number of components the overall voltage is the sum of the individual voltage drops.

 

[berenmacbowma]

okay! but if each resistor is consuming voltage, how does the current continue after the last resistor with no energy?

 

[Studiot]

but if each resistor is consuming voltage, how does the current continue after the last resistor with no energy?

It is better to think of resistors as dividing the available voltage so that none is left over.

If you take a chain of say 3 resistors and put say 12 volts across the chain the 3 resistors will divide that 10 volts so that there is say {4volts, 4volts, 4volts} across each resistor.

Now say we add a fourth resistor but keep total voltage (remember your booklet called it the source voltage?)

Again the resistors divide the 12 volts but this time there is {3, 3, 3, 3} across each resistor.

The next step is to learn how we calculate what voltage drop is allocated to each resistor.

Notice I have not mentioned current. Learning about current is the step after that.

 

[tiny-tim]

It is better to think of resistors as dividing the available voltage …

here's a diagram of a voltage divider …