What exactly is voltage?

Question

Bit of a strange question, but what is it? My physics teacher said it was kind of like a "push" that pushes electrons around the circuit. Can I have a more complex explanation? Any help is much appreciated.

Answer 1

Your teacher was right.

Current is electric charges (usually electrons) moving. They don't do that by themselves for no reason, no more so than a shopping cart moves across the floor of a store by itself. In physics, we call the force that pushes charges the electromotive force, or "EMF". It is almost always expressed in units of volts, so we usually take little shortcut and say "voltage" most of the time. Technically EMF is the physical quantity and volts is one unit it can be quantified in.

EMF can be generated several ways:

1. Electromagnetic. When a conductor (like a wire) is moved sideways thru a magnetic field, there will be a voltage generated along the length of the wire. Electric generators like in power plants and the alternator in your car work on this principle.

2. Electrochemical. A chemical reaction can cause a voltage difference. Batteries work on this principle.

3. Photovoltaic. Crash photons into a semiconductor diode at the right place and you get a voltage. This is how solar cells work.

4. Electrostatic. Rub two of the right kind of materials together and one sheds electrons onto the other. Two material that exhibit this phenomenon well are a plastic comb and a cat. This is what happens when you shuffle across the right kind of carpet and then get a zap when you touch a metal object. Rubbing a balloon against your shirt does this, which then allows the balloon to "stick" to something else. In that case the EMF can't make the electrons move, but it still pulls on them, which then in turn pull on the baloon they are stuck on.

   This effect can be scaled up to make vary high voltages and is the basis for how Van de Graaff generators work.

5. Thermo-electric. A temperature gradient along most conductors causes a voltage. This is called the Siebeck effect. Unfortunately you can't harness that because to use this voltage there is eventually a closed loop. Any voltage gained by a temperature rise in part of the loop is then offset by a temperature decrease in another part of the loop. The trick is to use two different materials that exhibit a different voltage as a result of the same temperature gradient (different Siebeck coefficient). Use one material going out to a heat source and a different coming back, and you do get a net voltage you can use at the same temperature.

   The total voltage you get from one out and back, even with a high temperature difference is pretty small. By putting many of these out and back combinations together, you can get a useful voltage. A single out and back is called a thermocouple, and can be used to sense temperature. Many together is a thermocouple generator. Yes, those actually exist. There have been spacecraft powered on this principle with the heat source coming from the decay of a radio-isotope.

6. Thermionic. If you heat something high enough (100s of °C), then the electrons on its surface move so fast that sometimes they fly off. If they have a place to land that is colder (so they won't fly off again from there), you have a thermionic generator. This may sound far fetched, but there have also been spacecraft powered from this principle with the heat source again being radio-isotope decay.

   Electron tubes use this principle in part. Instead of heating something so that electrons fly off on their own, you can heat it to almost that point so that they fly off when a little extra voltage is applied. This is the basis of the vacuum tube diode and important to most vacuum tubes. This is why these tubes had heaters and you could see them glow. It takes glowing temperatures to get to where the thermionic effect is significant.

7. Piezo-electric. Certain materials (quartz crystal for example) generate a voltage when you squeeze them. Some microphones work on this principle. The varying pressure waves in the air we call sound squish and squash a quartz crystal alternately, which causes it to make tiny voltage waves as a result. We can amplify them to eventually make signals you can record, drive loudspeakers with so you can hear them, etc.

   This principle is also used in many barbecue grill igniters. A spring mechanism whacks a quartz crystal pretty hard so that it makes enough of a voltage to cause a spark.

Answer 2

Using a fluid analogy, Voltage is pressure, Current is Flow rate.

Answer 3

"Voltage" is a derived quantity. It is hard to understand its Physical meaning without understanding the quantities it is derived from.

It all starts with the force between two point charges. Let the charges of the points \$ P_1 \$ and \$ P_2 \$ be \$ q_1 \$ and \$ q_2 \$. Let the distance between them be \$ r \$. The fundamental theorem says that, the force between these two charges are proportional with the amount of charges, and inversely proportional with square of the distance between the charges. That is:

\$ F = k\dfrac{q_1 q_2 }{r^2} \$

Let the location and the charge of \$ P_1 \$ be fixed. Now the force depends on the location and charge of \$ P_2 \$. So we define a vector field called "Electrostatic Field". Direction of the vector field is the same with direction of the field of the force between \$ P_1 \$ and \$ P_2 \$ when \$ q_2 \$ is positive unit charge. And magnitude of the field is the force per charge \$ q_1 \$ when \$ q_2 \$ is unit positive charge. That is:

\$ \bar{E} = \lim \limits_{q_1 \to 0} \dfrac{\bar{F}}{q_1} \quad \mbox{(} q_2 \mbox{ is unit positive charge)} \$

We make \$ q_1 \$ approach to zero in order to neglect some other electromagnetic effects; don't let it confuse you so much. It is something like "an aura that is able to generate some force per unite electrical charge". Its direction is the same with the direction of the force it generates, and its magnitude is proportional to the magnitude of the force.

Now we come to see that these quantities we defined are very similar to some other Physical quantities we know. For example, the force above is very similar to the force between the Earth and a space object, like the Moon. And the \$ \bar{E} \$ field is very similar to the gravitational field of the Earth.

Then the idea of defining electrical potential arises which is similar to the potential of a space object with respect to the Earth. Potential of a point in the space around Earth is energy per unit mass to bring an object (which has unit mass) from infinity to that point. When we define it in Electrostatics, the potential of the point \$ P_2 \$ becomes:

\$ V_2 = - \int \limits_{\infty}^{P_2} \bar{E} d\bar{\ell} \$

Then, the potential difference between two independent points (\$ P_2 \$ and \$ P_3 \$) in the space within the \$ \bar{E} \$ field (caused by \$ q_1 \$) is:

\$ V_2 - V_3 = \left(-\int \limits_{\infty}^{P_2} \bar{E} d\bar{\ell}\right) - \left(-\int \limits_{\infty}^{P_3} \bar{E} d\bar{\ell}\right) = \int \limits_{P_3}^{P_2} \bar{E} d\bar{\ell}\$

Note that electric field is curl-free, which means it can always be represented as gradient of a scalar field (\$ \bar{E} = - \bar{\nabla} V \$). These line integrals are independent of path.

So, this is the definition of the potential field. A point will always have a potential even if there is no charge on it. Think it as of "the energy needed to bring a unit charge to there from infinity". Potential difference between two points is similar; it is the energy needed to carry a unit charge from one point to another. Or think it on a more concrete example like for celestial bodies. Potential difference between 100km height and 200km height above Earth's surface is nothing but differences of potential energies between two 1kg objects at the given heights.

When we come to real world, potential of a point is some of all individual potentials caused by the charges around (theory of superposition applies).

Answer 4

A voltage appears whenever there is an imbalance of electrical charge (i.e. electrons). Since like charges repel and opposite charges attract, any collection of electrically charged particles creates some kind of force on each other. If there is an imbalance of negative to positive, a kind of "pressure" or "push" is formed. In conducting materials, electrons are free to flow through the material, as opposed to being fixed in atoms, and will therefore flow to the point of least "pressure".

Some complicating considerations:

• Electricity and chemistry are closely connected. In a battery, for example, a chemical imbalance creates an electrical imbalance (voltage) across the terminals, by forcing charged particles to one side. Chemistry also affects electrical conditions in other ways.
• Current (I) is the flow of electrons, however, electrons (since they are negative) flow in the opposite direction of the "current". The current is then the conceptual flow of positive charge, even though the actual flow is negative, but in the other direction. This demonstrates that a negative "push" is the exact same as a positive "pull".

Answer 5

A definition I've heard is:

Voltage is the potential (for charge) to do work.

In other words, voltage is the energy given to a unit of charge, i.e., \$ V = {dE \over dQ} \$, where \$ E \$ is energy and \$ Q \$ is charge.

Answer 6

The quickie, first approximation, rule-of-thumb answer: voltage is electrical pressure.

But expanding on that: Voltage is not like pressure, not exactly. Instead, it's a math/physics concept called "potentials." Voltage is more like altitude in a gravity field, where each electron or proton is like a boulder. Altitude isn't pressure or weight or force. If a boulder is at the top of a hill, the boulder at a high-potential location. This means the boulder is storing potential energy (PE), and will release this energy as kinetic energy (KE) if it's allowed to move downhill (move to a low-potential location.) Lifted to the same voltage (altitude,) larger boulders would have higher PE.

More precise: voltage is electric Potential. It isn't force (it's not like the boulder's down-force or weight, nor is it like the amount of force upon an electric charge in an electrical field.) Also voltage is not potential energy, since if we take away the boulder, then the gravity, altitude, and potential still exists. Potentials are part of the field itself. Patterns of voltage can hang in empty space.

Voltage is a way of describing/visualizing/measuring electric fields.

To describe e-fields, we can draw flux-lines between opposite electric charges. Or instead, we can draw the pattern of voltage, the iso-potential surfaces, drawing them perpendicular to the flux lines. Wherever we find some electric lines-of-force, we'll also find voltage.

What is voltage not? What are typical misconceptions? Here's a big one: "voltage is a kind of potential energy." Nope, wrong. Instead voltage is the math-concept "Potentials," which aren't energy, nor are they "potential to do something." Here's another miscon: "voltage is the potential energy per unit charge." Nope, wrong. That's just the physics definition of the Volt unit, linking it with Joule and Coulomb units. Actually it goes the other way: the amount of energy (amount of work done in moving a charge across a certain voltage-difference) is found by multiplying charge by change in voltage! Electrical energy is determined by voltage! But Voltage itself needs no moving charge nor potential-energy stored, since voltage is a way to describe a field in empty space. The test-charges used to describe voltage are imaginary infinitesimal charges. Another miscon: "voltage appears on the surface of wires." Wrong, voltage actually extends into the space around wires. Half-way between your 9V battery terminals you'll find a 4.5V potential, hanging alone in empty space! But typical voltmeters won't detect the space-voltage, since that requires a voltmeter with infinite Z(inp), or at lest a few hundred gigohms. Normal 10Meg DMM voltmeters draw significant current, will short out any pure e-fields, so they must be touched to conductor surfaces in order to measure voltage.

What is voltage? It's a stack of invisible membranes which fill the space between charged capacitor plates. Voltage is the pattern of concentric onion-layers which surround any charged object, with the onion-layers running perpendicular to the flux-lines of the electric field. So, 'stacks of voltage-layers' is one way of describing an electric field. The other more familiar way is to use 'lines of force.'

Answer 7

Actually we can't.

Electrostatic force is proportional to the potential gradient, but not directly to potential. Force on a one coulomb of charge is proportional to the potential gradient:

\$ F= Q \times {d[V]\over dl } \$

Actually, 1 V means if you have 1 joule of electrical energy, it will be transferred into mechanical energy on a +1 coulomb charge [so it will accelerate, or increase its 1/2mV^2 by 1 J]. It's actually analogous to energy.

Answer 8

Adding to what Gunnish said:

Voltage at point A is literally a measurement of the work you would expend if you were to push a positive charge from 0V (usually either defined as infinitely far from A, or ground) to A.

Voltage is important in electronics because if we start with a positive charge at point A, it is able to DO that same amount of work getting to 0V (ex. turning on an LED in the process).

Answer 9

What is pushing the elections is a difference in potential energy, much like the way you are being pushed/pulled to the earth by gravity. This generates a favorable probably for the electrons to move one way over another, this also partly explains why the electrons move "randomly" in a wire.