Is there an implicit "unit of time" involved in electricity measurement? (e.g. miles per hour, kb per second, amps per ???)

Question

I'm very new to electronics and I am going through what must be a common difficulty in grasping voltage, current, and resistance. I'll restrict my question to current as I suspect understanding that piece may shed light on voltage and resistance.

I've read a few questions here:

• Help with understanding Current, Voltage and Resistance

• Understanding voltage and current

And they helped a bit but I'm still struggling. One specific part that's difficult for me to resolve mentally is that I am reading about the basic units of measurement, but I'm not entirely sure what is being measured. For example, a pound is measuring the force of gravity pulling on a collection of atoms. A gallon is the amount of liquid that can occupy a fixed amount of space. Electricity... I get lost on the details of what's being observed.

Many units of measurement are a fixed quantity of something that does not change (unless acted upon). For example:

• 1 Gallon of milk
• 16 ounces of beef
• 30 cubic liters of air

That doesn't seem to make sense with something like current that is measuring electrons constantly in motion. Alternatively we perform measurements of something as it changes over time:

• 35 miles per hour
• 128 kilobits per second
• 5,000 gallons per minute

When it comes to current, we just say "amps", not "amps per something". Well, I get that "amps" measure the flow of electrons, but what exactly does that "flow" mean? Is it the number of electrons (or the number of something else) passing through a location on a circuit in a second (or some other unit) of time? When I touch the leads of my multimeter to a wire, what exactly is it "looking at"?

I've read that volts are a measure of potential energy related to joules and coulombs (http://www.allaboutcircuits.com/vol_1/chpt_2/1.html) (more confusion but that's fine) and I believe that coulombs are measured per second. Does the per-second carry over to amps as well?

The only other thing I can think of is that amps might be more like pressure where you're measuring pounds per square inch.

I know electricity is electricity and no analogy is perfect. I'm trying to understand electricity for what it is, I'm just not sure how these measurements are actually made. Perhaps I'm overthinking, but any deeper insight would be great.

(If this has already been explained to death I apologize, I may not know the best search term to use.)

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Man, as someone new to this site I'm so blown away that so many people took so much time to help me understand this. Like a lot of things I think it's going to take time and a lot more reading / experience to "sink in" but all of the answers were so helpful. I'm marking the "amps include time" answer as the one that helped me the most because it answered the core of my question "amps per what?". I'm picturing "amps" kind of like "knots" in the sense that the quantities are part of the definition of the word as opposed to being explicitly stated as they would be in another unit like "miles per hour". Not a perfect analogy but at least it helps me understand where all the hard numbers went.

Answer 1

Amps includes time...

Amps = Coulombs per second

That says more simply that...

Current = amount of charge per time interval

It's a flow rate metric. Like water... liters (volume --> amount) per minute (time)

In more depth

In practical terms, the ampere is a measure of the amount of electric charge passing a point in an electric circuit per unit time with 6.241 × 10¹⁸ electrons, or one coulomb per second constituting one ampere.

--Wikipedia Article

Probing

When I touch the leads of my multimeter to a wire, what exactly is it "looking at"?

If you are in the voltage measurement mode, you are effectively measuring the "pressure" between the two leads -- the degree to which charges in one lead seek to reach the other (but can't). The reason the charge gradient can't be neutralized depends on the circuit. In a capacitor, for example, a barrier of some kind prevents it. The existence of a voltage between two points requires that such a gradient exists.

If you are in a current measurement mode, the leads are installed in the current path (in series with) and the meter is measuring how much charge flows through them in unit time (it actually does this indirectly by applying Ohm's law).

Further reading

Bodanis, David (2005), Electric Universe, New York: Three Rivers Press, ISBN 978-0‐307‐33598‐2

Answer 2

The most fundamental unit of charge is the electron, but it's impractically small to work with. A coulomb is a larger unit of charge representing the charge of about 6,241,509,324,000,000,000 electrons. An ampere is a shorthand unit representing a flow rate of one coulomb (i.e. 6,241,509,324,000,000,000 electrons) per second, which is to say that if a wire has one ampere of direct current flowing through it, there will be about 6,241,509,324,000,000,000 more electrons entering one end and leaving the other, than vice versa.

Answer 3

Rather than answer your question directly (others have done that quite well), I'd like to introduce a mental model and analytical tool that should help you understand those answers. That tool is dimensional analysis.

The fundamental concept is that a unit a symbol that can be manipulated algebraically. I think an example is best. We know that the volume of a rectangular cuboid is its width, times its height, times its depth. Let's say we measure it to be 1 meter high, 2 meters wide, and 3 meters deep. Then:

$$ \text{volume} = 1m \cdot 2m \cdot 3m$$

If you pretend that \$m\$ is just a symbol, like the proverbial \$x\$ in algebra, then you know that:

$$ 1m \cdot 2m \cdot 3m = 6m^3$$

That is, the volume of this cuboid is six cubic meters. But we can measure volume in units other than cubic meters. In fact, any three units of length, multiplied together, is a unit of volume. Area is two units of length multiplied together, so if I multiply area by length, I get volume. So let's say I want to measure volume in some wacko unit I just made up, the acre-inch.

What's the volume of our cuboid in acre-inches? I can start with \$6m^3\$, which is \$6 \cdot m \cdot m \cdot m\$. I can then multiply it by some fractions where the numerator and denominator are equal, but in different units. These are fractions equal to 1, but multiplying by 1 doesn't change the number. It does, however, let me change the units. By the rules of algebra, any term in the numerator can cancel the same term in the denominator. So somehow, I need to get three \$m\$ in the denominator, and end up with one \$in\$ and one \$ac\$ in the numerator.

$$ \require{cancel} \frac{6 \cancel{m m} \cancel{m}}{1} \frac{1ac}{4046.86\cancel{m^2}} \frac{1in}{2.54\cancel{cm}} \frac{100\cancel{cm}}{1\cancel{m}} \approx 0.058ac \cdot in$$

Six cubic meters is equal to 0.058 acre-inches. Why would I want to measure volume in acre-inches? I have no clue, but I can. Point is, units can be manipulated algebraically.

This yields new insight into what units mean. Pick any unit, like the watt, and wikipedia will tell you something like:

$$ W = \frac{J}{s} = \frac{N\cdot m}{s} = \frac{kg\cdot m^2}{s^3} = V \cdot A $$

The elegance of SI units is that all the units are related by a factor of 1, so we don't have to write it. So what this says is one watt is equal to one joule per second. Or, one newton-meter per second. Or, one kilogram-square-meter per second-cubed. Or, one watt is one volt-amp. These are all the same thing.

See how the units relate to the electrical equations you already know, like power \$P\$ is the product of voltage \$E\$ and current \$I\$:

$$ P = I E $$

Knowing that current can be measured in amperes, and voltage is volts, then power must be measured in volt-amps. And hey, according to Wikipedia, that's a watt:

$$ W = V\cdot A$$

therefore:

$$ \frac{W}{V\cdot A} = 1 $$

Say you measure the voltage to be \$10V\$ and current to be \$10mA\$. Then:

$$ P = \frac{10\cancel{mA}}{1} \frac{10\cancel{V}}{1} \frac{\cancel{A}}{1000\cancel{mA}} \frac{W}{\cancel{V}\cdot \cancel{A}} = 0.1W $$

Here are a few more examples of dimensional analysis:

• https://electronics.stackexchange.com/questions/68745/how-many-d-cell-batteries-would-you-need-to-replace-a-6v-4-5ah-battery/68749#68749
• Current consumption in milliseconds
• How to get high current from 9 volt batteries

Answer 4

When it comes to voltage, we just say "amps", not "amps per something".

You have a misunderstanding.

Amperes measure current.

Volts measure potential difference. Voltage is another word for potential difference, when you are measuring it with the units of volts.

As others have answered, amps measure the flow of electrons, and an amp is equivalent to 1 cuolomb of charge passing per second.

When the current in a wire is changing, it is not uncommon to measure the rate of change in "amps per second" or A/s.

I've read that volts are a measure of potential energy related to joules and coulombs

Volts can be rewritten as watts per amp, or joules per cuolomb. Let's look at the second form, joules per cuolomb.

It means that if the potential at some point of space is held constant at 1 V, it will take 1 joule of energy to push 1 C of charge to that location.

Or it would take 1 J/s to move 1 C/s to that location; 1 Watt per amp of current flowing in to that location.

Answer 5

A mechanical analogy may help sort things out.

In one mechanical analogy, force is analogous to voltage while velocity is analogous to (electric) current.

As you may know, the product of force and velocity is (mechanical) power and analogously, the product of voltage and current is (electric) power.

While force is energy per meter, voltage is energy per Coulomb (Coulomb is the unit of electric charge).

While velocity is meters per second, current is Coulombs per second.

We call force and voltage the across variables while velocity and current are the through variables.

In either case, the product of the across and through variable is energy per second which is power.

Answer 6

Let's give a shot with a common analogy for circuits.

A circuit is like a river. Water in a river always flows "downhill" because water at the top of the hill wants to go downhill. Water will always seek a lower point.

If water always goes downhill then how is this a circuit?

Well, you can think of a "loop" river, flowing downhill -- but at one end, there is a water wheel of some sort that brings the water at the bottom back up to the top. This wheel takes low-level water, with no motivation to flow anywhere, and "pushes" it into the high level, with much motivation to flow downhill.

If we think of "height" as "potential energy", the water wheel takes water of low potential energy and puts it in a position of high potential energy -- essentially "injecting" gravitational potential energy into the water. this newly energized water wastes no time in spending that energy to go downhill once again.

This "proclivity to move downhill" is called potential which in our case is analogous to Voltage.

The current of the river is...well..Current. How would you measure the current of a river?

I would say ... "take a stopwatch and time how many liters of water pass through a certain mark in the river in one second". That sounds like a reasonable way to quantify a current. Liters per second.

In a circuit, your water is charge. Instead of timing how many liters of water pass through a point on a wire after a second, you could measure how many units of charge pass through a specific point on a wire per second.

Just like saying "cubic decimeters" is a mouthful and we give it a convenient unit -- "liter", we give "charge per second" a convenient name as well -- "amps".

We do this a lot -- "miles per gallon" turns into "mileage", "kilograms times meters per second per second" turns into "newton", "joules per second" turns into "watts".

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If gravity is not doing it for you, consider water in pipes and water pressure.

I have pressurized water in one end and unpressurized water at the other. The water will move from the pressurized side to the unpressurized side. Pressure is a measure of the force of all the water molecules wanting to get away from eachother. Water molecules have a comfortable distance and pressurization the act of pushing those water molecules closer and closer past that comfortable point.

You might remember that electrons repel each other. When you have a "high voltage", you really have "high electron pressure" -- stuffing electrons waaay too close together for their own comfort.

Note that this analogy is actually very literal -- voltage really can be seen as literally electron pressure!

Just like putting too much air into a balloon ... things that are stuffed too close together will want to "get away", and there is a real force.

Now, back to our water pipes -- water will want to rush from the pressurized end to the unpressurized end.

Think carefully about the pipe. When we let the water rush...what is actually rushing? Is it the water molecules? Imagine a single water molecule in the pressurized end, and letting the pressure "go". That molecule won't rush to the other end. It'll just stay in place while the pressure equalizes.

So what is actually moving?

The pressure moves.

Let's say you have a little display at every inch on the pipe that measured the pressure at that exact point. At first, all the ones on the left are high; all the ones on the right are low.

When you let the pressure go...you see these displays start changing. The "highness" starts moving to the right.

Let's say at one display says "50" for pressure, and then the next display to the right says "20". One second later, the first display now says "40" and the second says "30". You can see this as 10 "pressure" units moving to the right at a rate of 10 pressure units per second. This is current - 10.

Now I'm playing a little loose with dimensions and sort of hand-waving away some of the the differences between Potential and charge, but the basic principle is the same.