Elementry Question about resistors

[dude]

I know two electrolytic filter caps in series of the same rating and value, double the rating and half the capacitance.

What about two resistors? In series the value increases as each resistor is added but does the voltage rating change too.

Example: two 5 watt, 500ohm resistors in series is 1K value, is the rating 10 watts?

thanks,

al 

[phsyconoodler]

With resistors the resistance doubles but the wattage rating remains the same.

[dude]

That's what I figured, thanks

[jeff]

Could you please explain. I've heard this before but don't get why.

If you use a 1K 10W resistor and, for the sake of round numbers, you're dropping 100V @ .1A that's 10W(100VX.1A).
If you use two 500ohm 5W resistors in it's place aren't you now only dropping 50V @ .1A through each resistor(50VX.1A=5W)?
And therefore two 500ohm 5W resistors would do the same job as one 1K 10W resistor.

[phsyconoodler]

resistance doubles but the wattage handling remains the same.
  No matter what the wattage is,the resistance doubles.
2-500 ohm resistors in series adds up to 1000 ohms resistance.If they were 1/2 watt,the wattage handling remains at 1/2 watt.
  Now if you want to up the wattage handling,take two 1/2 watt resistors and hook them up in parallel.The wattage handling doubles but the resistance is halved.
  Use your meter to confirm this.Use your brain to confirm the wattage or CURRENT the resistor can handle.

[jojokeo]

Basic series circuit = the current through each of the components is the same, and the voltage across the components is the sum of the voltages across each component.
Basic parallel circuit = the voltage across each of the components is the same, and the total current is the sum of the currents through each component.

To help w/ understanding:
Resistors in series - the current still goes through each one - one at a time. All you've done is increase the resistance but not the ability for each to handle more power. They each must handle the same/full current on their own.

Resistors in parallel - they each SHARE half the total current. They would each be able to handle their own 5 watts therefore the total power handling is added together.

With your values:
Series - Think of a resistor twice as long but the same thickness.
Parallel - Think of a resistor twice as fat but half the length.

[jjasilli]

OK, but note that the points made are valid only under the specific given circumstances -- that both resistors are of the same value.  The wattage rating requirement will change if the values are different, say 1R & 10R.  Let's say 1A of current is flowing through those resistors in series.  Per Ohm's law there will be a voltage drop of 11.  The 1R drops 1V; the 10R drops 10 volts.  Watts = Volts x Amps.  For the 1R: Watts = 1 X 1 = 1W; for the 10R:  Watts = 10 x 1 = 10W. 

[jeff]

Resistors in series - the current still goes through each one - one at a time. All you've done is increase the resistance but not the ability for each to handle more power. They each must handle the same/full current on their own.

Use your brain to confirm the wattage or CURRENT the resistor can handle.

My brain is telling me the CURRENT is the same but the VOLTAGE isn't. and wattage = current X voltage


I agree that two series resistors will have the same CURRENT as one resistor twice the value, BUT... each series resistor will only have half the VOLTAGE across it compared to the voltage across one resistor twice as big.
And isn't wattage the current times the voltage? (P=IV)
So if the voltage is halved at the same current then the wattage is halved.(I[V/2]=P/2)
 

Assume you're drawing .1A
If you used a 1K resistor
V=IR
V=.1A X 1000ohm
V= 100V
and you'd need a 10W resistor
P=IV
P=.1A X 100V
P= 10W

if you use a 500 ohm resistor @ .1A

V=IR
V=.1A X 500ohm
V= 50V
and you'd need a 5W resistor
P= IV
P= .1A X 50V
P= 5W

Either two 500 ohm 5W resistors in series are dropping 100V(each dropping 50V)
Or one 1K 10W resistor is dropping 100V. Either way the total resistance, voltage and current is the same.

So isn't two 500ohm 5W resistors in series equal to one 1K 10W resistor and not a 1K 5W resistor? In other words the wattage rating doubles for equal resistors in series?

Sorry I'm not trying to be argumentative I'm only trying to understand better. I've always heard that two resistors in series are the same wattage as one, and I've always believed it, but I can't mathematically understand why.
What am I missing?

[RicharD]

>So isn't two 500ohm 5W resistors in series equal to one 1K 10W resistor and not a 1K 5W resistor? In other words the wattage rating doubles for equal resistors in series?

Nope.  Two 500 ohm 5W resistors in series is = to one 1k 5W resistors.  Two 2k 5W resistors in parallel is = to one 1k 10W resistor.

>I'm only trying to understand better.

Think of electricity like water.  Voltage is pressure, ampacity is the rate of flow, and wattage is volume of water.  A resistor is a water valve where 0 ohms is a completely open valve and 10M is effectively a shut valve.  Sticking with a 2 resistor example, in a series circuit you have a single pipe and 2 valves, each one restricting the flow.  Pressure or voltage will be greater at the source, less between the valves, and even less on the load end.    It's a single pipe so regardless of pressure,the water is flowing at the same speed throughout the pipe.  Your circuitry is sucking the water.  The volume of water it demands is the wattage.  If it's trying to suck a greater amount than the pipe and valves are rated for, it will try to collapse.  Now in parallel, you have 2 pipes.  If we consider the  2 resistors to be equal, then flow through the 2 pipes is equal.  You can now safely double your consumption.     

[jeff]

So are four 25W 8ohm speakers in series/parallel a 50W cab, not a 100W cab.
The top two speakers are parallel for 4 ohms at 50W same with the bottom two. Both those 4 ohm 50W sets are in series so that would only be 8 ohms@50W?

I have a Marshall cab wired like this with four 35W speakers and the cab is marked 140W.

[jeff]

OK, but note that the points made are valid only under the specific given circumstances -- that both resistors are of the same value.  The wattage rating requirement will change if the values are different, say 1R & 10R.  Let's say 1A of current is flowing through those resistors in series.  Per Ohm's law there will be a voltage drop of 11.  The 1R drops 1V; the 10R drops 10 volts.  Watts = Volts x Amps.  For the 1R: Watts = 1 X 1 = 1W; for the 10R:  Watts = 10 x 1 = 10W. 

Why will the wattage rating requierment change using two different value resistors but not change using two of the same resistors?

If a 1W 1R plus a 10W 10R equals a 11W 11R, why do you need a 11W 5.5R + 11W 5.5W to equal a 11W 11R resistor?

What would you use if you used a 5R and a 6R? according to this you'd need a 5R 5w resistor and a 6R 6W resistor. So why do you need two 5.5R 11W resistors to make a 11R 11W resistor and not two 5.5R 5.5W resistors
Is this wrong? Should you use a 10R 11W and a 1R 11W.

EDITED(Sorry I originally had a bad pipe analogy here, I took off but I didn't know buttery was posting his respocne,  so his next post has to do with this analogy that is no longer here)

[RicharD]

/Sorta.... First off, remember resistor are variable water valves if they're pots.  10W would be steel, 5W would be plastic, & 1/2 watt = toy spigot.  The tube itself has to have sufficient strength to not burst under pressure.  This relates to the voltage rating of the insulation.  The larger the pipe, the greater the flow capability so for higher current, we need bigger wire.  Now, we're accustomed to opening a faucet and water is pushed out.  With electricity, the water is sucked off the reservoir.  

Watts is consumption.  50V * 1A = 50W  |  100V *.05A = 50W  |  As your voltage increases, your current goes down for the same amount of work.

In a series string, the circuit is only as robust as it's weakest component.  In a parallel circuit, you have 2 or more paths so the work is shared.  Assuming the supply voltage is set and there are 2 paths that are identical, the current is divided equally.  

4 - 8 ohm 25W speakers in series = 32 ohms @ 25W
4 - 8 ohm 25W speakers in parallel = 2 ohms @100W
4 - 8 ohm 25W speakers in parallel series pairs = 8 ohms @50W

edit:
Quit posting under the influence.

[jeff]

Sorry I took my pipe analogy down to think about it and didn't know you were posting so it looks like you're answering a question that isn't there. I will put that back up.

Any thoughts on the post before yours? (two posts up)

[DummyLoad]

apply Kirchhoff Current Law & Kirchhoff Voltage Law.

in a series ckt: current = sum of the V drops / Sum of the resistance.

in a || ckt: current = the product / sum of the resistance branches * applied voltage.

Example: two 5 watt, 500ohm resistors in series is 1K value, is the rating 10 watts?

see diagram. yes, you can halve the resistance and halve the wattage to equal the total resistance and total wattage if you make them up in a series ckt.

in other words, take your two 500R 5W resistors, strap them in series and mold them into one long one, you'll have a 1K 10W resistor.

[RicharD]

edit: delete

[DummyLoad]

4 - 8 ohm 25W speakers in series = 32 ohms @ 25W  =100W
4 - 8 ohm 25W speakers in parallel = 2 ohms @100W
4 - 8 ohm 25W speakers in parallel series pairs =8 ohms @50W =100W

[RicharD]

I'm screwing everybody up.  Listen to DL.

[dude]

I can't help but  over here at the ranch.

Six to one; half a dozen to the other, WT?

So this is what I get out of all this:

In a speaker cab:

Add the rating of the speakers no matter how you wire them and that's the max wattage. This is what I was taught so Dummyload is correct.

Concerning resistors:

Two or more resistors in series: add the resistance and the "lowest rating" is the voltage of the chain...correct? 
Ex: two 5w 500 ohm = 1K  with a rating of 5watts

In parallel with equal value resistance:

Half the value but double the rating
Ex. Two 5w 2k ohm = 1K with a rating of 10watts

All correct?

[jeff]

 
What's your power tube dissipation? What's the wattage rating on your 1ohm resistors to measure the bias?
The current through the tube is the same current through the resistor.

[DummyLoad]

Two or more resistors in series: add the resistance and the "lowest rating" is the voltage of the chain...correct?
Ex: two 5w 500 ohm = 1K  with a rating of 5watts

no. 10W

--DL

[jeff]

Edited: removed to avoid confusion

[DummyLoad]

I'm utterly confused.

Example: two 5 watt, 500ohm resistors in series is 1K value, is the rating 10 watts?

yes.

no more confusion.