Voltage/Power Confusion

Discussion in 'Homework Help' started by still0fthenight, Aug 1, 2016.

1. still0fthenight Thread Starter New Member

Aug 1, 2016
6
0
Hello, can anyone critique what I am about to say? This is my understanding of what I have been reading in an Introductory EE textbook using conventional flow. I want to see if there are any flaws in my understanding.

Voltage can be thought of as the amount of energy measured in Joules to move 1 Coloumb of charge. So, an 8V battery would use 8 Joules of energy to move 1 Coloumb of charge where 1 Coloumb is 6.25x10^18 electrons.

Watts are the amount of Joules that are used up every second, so Watts is similar to Volts except that Watts adds a "time dimension". So 16mW would be less than 1 Joule of energy per second that is being used.
--------------------------------------------
Now if I am correct about what I said about, I am confused about the following matter:
In my book, there is a simple circuit diagram that lists an 8V battery with 2 resistors hooked up. So I thought that the circuit is using 8 Joules to move 1 Coloumb. But then it says the Power at the source is 16mA. This is confusing me because how could the circuit be using 8 Joules of energy when there is only 16mA of energy being used every second?!

2. Kermit2 AAC Fanatic!

Feb 5, 2010
3,780
934
The amount of charge any particular voltage can "move", depends entirely on the resistance present in the circuit.

Take your voltage (8) and your current (16 mA) and figure out the resistance.

3. still0fthenight Thread Starter New Member

Aug 1, 2016
6
0
Sorry I meant to say that the Power at the source was 16mW, not 16mA. Here is the circuit:

Last edited: Aug 1, 2016
4. Kermit2 AAC Fanatic!

Feb 5, 2010
3,780
934
Watts not only has a time parameter, it has voltage AND current terms.
So it is not as similar to volts as you state in the first post.

5. still0fthenight Thread Starter New Member

Aug 1, 2016
6
0
Power Watts is measured in Joules per second and Voltage is measured in Joules per Coloumb. What was confusing me was how a meter would read 4mW across R1 and 12 mW across R2 when the source is supplying only 16/1000 W of power per second. As soon as I wrote this I think I cleared up my own confusion (hopefully) lol. If the Power Source is supplying 16/1000 W of power per second then the measurements of 4/1000 W across R1 and 12/1000 W across R2 make sense because they are each less than 16/1000 W.

I had to write everything out in big fractions because I think it is extremely easy to get confused and make sense of the various units and math even at the most basic level.

6. crutschow Expert

Mar 14, 2008
13,000
3,229
Volts is simply the "pressure" to move the current. It has no intrinsic energy component.
It's only when it is moving carriers to form a current that it generates energy.
If 1V moves 1 coulomb of charge through a resistance then 1 Joule of energy is dissipated in that resistance.

7. still0fthenight Thread Starter New Member

Aug 1, 2016
6
0
But my book says that 1 Volt = 1 Joule per Coulomb.

So I keep thinking that it is an energy needed to move 1 Coulomb anywhere throughout the circuit.

Now I am even more confused because if Vs = 8V = 8Joules/Coulomb, and Ps = .016 Joules/sec then I would think that 8 Joules should be used around the circuit, not .016

I will just have to stare at the book for another lifetime until I figure it out lol.

Last edited: Aug 1, 2016
8. Kermit2 AAC Fanatic!

Feb 5, 2010
3,780
934
I'll bet that resistance is stated as a factor somewhere in that book you mentioned.

9. still0fthenight Thread Starter New Member

Aug 1, 2016
6
0
Would it be correct to say that only 16mW of power is used by the battery in order to have 8V of force to move the charges?

10. WBahn Moderator

Mar 31, 2012
17,722
4,788
You are trying to mix concepts that aren't intrinsically linked the way you are trying to force them to be. Voltage (which is always measured between two points) is the amount of energy that will be converted from electrical potential energy to some other form per unit of electrical charge that moves between those two points. It doesn't matter how fast or how slow that happens. Electrical power is the rate at which electrical potential energy is converted to some other form.

Perhaps a simple analogy will help you out. Let's say that your local recycling center will pay you \$0.10 per bottle that you turn in. So the recycler is like the battery and the rating (\$0.10/bottle) is the potential money that can be converted from the recycler's bank account to your bank account per bottle that flows from you to the recycler. Notice that having a potential does not necessarily create any money for you -- if you don't have any bottles flowing from you to the recycler, then you get no money. If you give the recycler 1000 bottles, then you get \$100 -- and it doesn't matter whether you give them 1000 bottles in 10 seconds, or 1000 bottles in 10,000 seconds. But now consider how fast money is flowing from the recycler to you. If you give them 1000 bottles in ten seconds, then they are giving you money at the rate of \$10/second, but if you give them that same 1000 bottles over 10,000 seconds, then they are giving you money at the rate of \$0.01/second. The rate at which you are getting paid is like power. It is the product of the potential (\$0.10/bottle) and the bottle flow rate (100 bottles/second in the first example).

11. crutschow Expert

Mar 14, 2008
13,000
3,229
Let's try this:
The 8V is moving charge at a 2mA rate or 2mC/sec through the resistors.
To move 1 coulomb of total charge thus requires 1C / 2mC = 500 seconds.
In that 500 seconds the total energy expended in the resistors will be 16mW *500sec = 8 watt-sec = 8 Joules

As expected this is the same answer as multiplying 8V * 1 coulomb.

Make sense now?

12. WBahn Moderator

Mar 31, 2012
17,722
4,788
8 joules/coulomb simple is not comparable to 0.016 joules per second. This is like trying to compare 8 miles per hour to 0.016 miles. They are two different things. The fact that you are blithely ignoring this is evident when you say, "I would think that 8 Joules should be used around the circuit, not 0.016." You are implying that that 0.016 is 0.016 joules. It ISN'T! It is 0.016 joules per second. Fundamentally a different quantity.