# The measurement of electricity

I'm a student trying to understand electricity. As I learned from school, electricity is the flow of electrons, but I'm confused about the measurement of electricity.

• As I learned, voltage is the "pressure" of the electricity, but it's a bit abstract while I try to understand the particle world and atomic world. Is voltage the speed of the electrons moving, or the number of electrons moving in the flow? Electrons are particles, so how many electrons are moving in the flow if the flow is $5\textrm{ V}$ or $12\textrm{ V}$?

• What caused the different voltage in the atomic world between two points?

• What about amps – how do they relate to voltage?
• Curious correction, @Bernhard. Is "the flow is 5 volt" somehow more correct than the plural "volts"? – Nick Stauner Jul 14 '14 at 10:33
• @NickStauner My preference. I also normally say 5 meter, 5 liter or 5 kilogram, rather than the plural, which is not commonly used. – Bernhard Jul 14 '14 at 10:36
• @Bernhard You're the first person I've ever seen who doesn't pluralize his units. From wikipedia : "the plural forms of units follow the grammar of the language concerned: in English, the normal rules of English grammar are used, e.g. "henries" is the plural of "henry".[49][32]:31 However, the units lux, hertz, and siemens have irregular plurals in that they remain the same in both their singular and plural form." Is a matter of personal preference really worth an edit anyways ? – ticster Jul 14 '14 at 10:40
• @ticster I actually also thought it was much more common. But, I would also write it as $5 \textrm{V}$ rather than 5 volts. Also, it is not really an edit, as it came up in the review queue. – Bernhard Jul 14 '14 at 10:47

electricity is the flow of electrons

Electricity is about a dozen different things, one of which is the flow of charge-carriers, which in metals is a flow of electrons. It's actually a slow drift of free electrons which are quickly jiggling in random directions.

how many electrons are moving in the flow if the flow is 5 V or 12 V

Voltage isn't a measure of flow. Current is.

If 1 coulomb of charge flows past a point in one second, we say the flow (current) is 1 Amp. Electrons have a tiny charge. It takes about 16021765700000000000 electrons to make up 1 coulomb.

If you think of volts as pressure, remember that your bicycle tire has pressure in it, even when it's leaning against your garage wall and nothing is moving (the atoms/molecules in the air in the tire may be jiggling thermally a bit but the air is not flowing anywhere) So a 20000 pascal pressure tire may have a flow of 0 nitrogen molecules per second. Similarly, a piece of plastic across a 9V battery may have no electrons flowing through it.

Electric current is what does the work.

No, Work is energy, so you have to involve volts and time as well as current.

You are breathing in and out right now. It probably doesn't seem like much work. If your party-mad friend only permitted you to exhale in order to inflate party ballons you'd find that the same flow of air involved much more work.

Electricity is the movement of electrons in a conductor.

Not always. Take for example static electricity.

Voltage is required to move the electrons along.

Free electrons in metal are already moving around pretty fast. An electric field (which you can measure in volts per metre) causes electrons in metals to slowly drift in one direction.

• The pressure (voltage) does not affect how many electrons are passing along the wire from negative terminal to positive terminal ? So why pressure affect the power consumption in the formula : Power Consumption (P) = voltage (U) x current (I) ? The formula indicate that if i supply more voltage between two point, so the more power i have earned. – DucFabulous Jul 15 '14 at 6:51
• @user4835: The point is that voltage alone is not enough, you must have a conducting path (i.e. charge carriers). In ohmic conductors, the current depends on both the voltage and on the characteristics of the conductor - it's resistance. Therefore the power used (not earned) depends, as you say, on both voltage and current. – RedGrittyBrick Jul 15 '14 at 8:24

Throughout this I will be comparing an electric circuit to a ball falling down an incline. and giving the corresponding analogies.

1) The voltage difference is the difference in potential between 2 points. In the case of the ball, the gravitational potential is proportional to the difference in height between the beginning and end point of the incline. In other words, it represents how much energy a single kilogram would gain by going down the incline. Similarly, the voltage difference represents the amount of energy gained per second for every Coulomb of electric charge that goes through that potential difference.

2) The electrical equivalent of the "inclined slope" is simply electrical charge. Just like gravity is the force that pulls the ball down the slope, so does the Lorentz force pull electrons towards positive charges and away from negative ones.

3) Amps are the number of Coulomb passing through a point in the circuit every second. Let us understand the relationship between amps and volts by considering the energy output of the circuit. On the inclined slope, the potential difference doesn't tell me how much energy has been gained by the slope. It is merely a characterization of the slope, and the amount of energy gained by going down the slope will depend on the amount of mass that goes down it (it will simply be the product of the two). Likewise, voltage differences only characterize the circuit. To get an idea of how much electrical potential energy is being converted to work at some point in the circuit, we need to multiply the voltage difference around that point by the number of Coulombs going through it every second. This is known as the current, and is measured in amps. This will give us the power generated at that point in the circuit. To get the total energy produced, you simply multiply by time.

To summarize the analogy :

Volt $\Rightarrow$ Gravitational potential difference between the beginning and end point of the slope.

Amp $\Rightarrow$ Number of kilograms going down that slope every second.

• Your answer is still abstract to me. I don't want to convert electron to a ball or water or anything else with unit like kilogram, water pressure, it make me feeling more abstract. Is that all electrons are the same? Or each electron have different energy or different characteristic by itself ? Please look at atom level, where the electron is a particle. So imaging if i have a 5 volt battery and 9 volt battery. What is the different between them microscopically that make the voltage different ? – DucFabulous Jul 14 '14 at 13:34

An electric circuit is formed when a conductive path is created to allow free electrons to continuously move. This continuous movement of free electrons through the conductors of a circuit is called a current

The Volt (V)

is a unit for measuring both electric potential difference and electromotive force. The voltage supplied by most automobile storage batteries is 12 volts. In many countries, including most of those of Europe, electricity is supplied to homes at 220 or 240 volts. In the United States and Canada, homes are typically supplied with electricity at around 120 volts for ordinary use and 240 volts for such appliances as electric ranges and electric water heaters. High-tension power lines have voltages of hundreds of thousands of volts. These lines are used for the transmission of electric energy over long distances.

The Ampere (A)

is a unit for measuring electric current—the flow of electric charges. The ampere (or amp) is a base unit of the SI (metric system) and is defined in terms of the magnetic force produced between two parallel wires carrying an electric current. Houses are typically wired to provide a total of 60 or more amperes. The amount of electric charge transferred by a current of one ampere in one second is one coulomb.

These two are connected with Ohm's Law

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance we get $I = \frac{V}{R}$ or rather $V= IR$. Where V is your voltage, I is your current(measured in amps) , and R is the constant of proportionality and is called electrical resistance

In the model $I = \frac{V}{R}$, We have the current (I: amount allowed through the resistor) is equal to the voltage (V: the difference in pressure between different sides of the resistor) divided by the resistance value (R: how much the resistor restricts the flow). Nonetheless the connection is Ohm's law.

Things to consider, in a highly simplified model

1. Electric current is what does the work.

2. Electricity is the movement of electrons in a conductor.

3. Voltage is required to move the electrons along.

Sources 1 2

Electricity is not flow of electrons, it is the flow of charge which can be positive or negative. When books tell us that electricity is flow of electrons, they are merely talking about conductors or alloys where only electrons can flow as protons are too heavy to flow.

Voltage or the potential difference is generally electric pressure or electric potential. For example in a battery one end is positive and other is negative. this means that both the ends would have different electric pressures or potentials thus creating a potential difference so voltage is basically the difference of electric pressure between positive and negative ends

Current is directly proportional to potential difference so amperes do relate to voltage Current is the rate of flow of charge or speed of the flow of charge and electricity is the flow of charge. but voltage makes electricity flow

Voltage is the electric potential. As already pointed out by ticster, it is analogous to the gravitational potential, which can be intuited as the height of a hill on earth (higher points having higher gravitational potential).

We all know that balls on a smooth hill tend to move toward the bottom. In the case of a ball on a hill, you might also ask, how does it know which way is down? The answer to that is the slope (in this classical mechanics picture), even as you get infinitely close to the ball, the slope of the hill still exists on the ground, so the ball knows which way to go because it goes in the direction of the downward slope arbitrarily close to it (neglecting it's momentum).

In electricity, there is not as intuitive of a picture. Instead the electrons know which way to move because of messenger photons that "tell the electron" the same sort of information as the slope of a hill tells the ball. The voltage at some point in space is the amount of energy it takes for a positively charged object to sit at that location (per unit charge). The unified rule is that objects tend to move towards states of lower, more evenly distributed energy, so electrons tend to "flow" towards higher potentials (since they are negatively charged).

If you really want to think of voltage as a pressure, you can think about the pressure of photons smacking the electron from different directions (though be careful as that picture is misleading, these photons are off-shell or virtual particles, they can carry negative momentum for instance).

In terms of what causes voltage, that's basically related to how much charge is near that location. Again, the driving principle is things want to spread out and be even. You can think about electrons shooting off photons in all different directions all the time, this gives nice intuition for why electrons repel and why the force falls off as $\frac{1}{r^2}$, but it doesn't explain why they attract protons (again this is an issue with not understanding the nature of virtual particles).

Finally, amps are just the rate of charge flow through an area. It's not directly related to voltage except in that charged particles which are free to move about will try to find the lowest energy state, and if there is a potential difference between two points in space, all the free charge will try to align itself such that it minimizes the energy of the system. That motion of charged particles is what defines a current density, which in turn defines many currents (depending on which cross-sectional area you are asking about the current through).