# What is voltage and how to look at it intellectually?

1. Jul 15, 2014

### kartikwat

What is voltage and how to look at it intellectually?

2. Jul 15, 2014

### UltrafastPED

Voltage is something that is measured between two points; if one is taken as ground (=zero volts), then you can state the voltage at each point wrt ground.

The equipotential surface, or surfaces of equal voltages are a most useful tool; the gradient of the potential function yields the force, and so the equipotential surfaces can be used to define the field lines for the forces.

Thus you can work your way back and forth between the electric field and the potential.

The electric field is derived directly from the forces, and is a vector field. The potentials (voltages) are scalars, and are related to the work required to move a charge from here to there - or from ground to here. The voltages convert to potential energy when multiplied by the amount of charge: Potential Energy = voltage difference x amount of charge.

At what level have you studied electromagnetism? What is your interest?

3. Jul 15, 2014

### kartikwat

I have not studied electromagnetism yet .my intrest is know the nature but my reading comprehension is not good

4. Jul 15, 2014

### UltrafastPED

Last edited by a moderator: May 6, 2017
5. Jul 15, 2014

### Staff: Mentor

One way is to break it down into the basics. Voltage is the difference in electric potential between two points.

Electric potential is the amount of electric potential energy a unitary charge would have at a point in space.

Electric potential energy is the amount of potential energy that a charge possesses.

So you need to understand what electric potential energy is and work up from there.

6. Jul 15, 2014

### kartikwat

Voltage

Hey drakkith, it is comprehensive to me till the point that voltage is difference in electric potential between 2 points but when it comes to circuitry part with battery included it is out of my comprehension .how to help it

7. Jul 15, 2014

### Staff: Mentor

Well, what's causing the trouble? Remember that a battery is a voltage source, so unless you are wondering about the specifics of how a battery operates you can simply replace the battery with any other voltage source giving the same voltage and the circuit is identical.

8. Jul 15, 2014

### kartikwat

Voktage

The cicruitry part gives me terror

9. Jul 15, 2014

### Staff: Mentor

Okay. Can you be a bit more specific with where you are having trouble?

10. Jul 15, 2014

### atyy

Voltage is the colloquial term for difference in electric potential between two nearby points, and also called the potential difference.

The electric potential is a number that we can assign to every point in space to characterize exactly an electric field that is not changing in time, or approximately an electric field that is only slowly changing with time. The potential itself does not directly tell us about the electric field, but the difference in potential between two nearby points does tell us directly about the electric field. If the potential at point A is higher than at point B, a small positive test charge placed at point A will accelerate towards point B. The bigger the difference in potential between point A and point B, the larger will be the acceleration of the small positive test charge.

Let's say you have two metal plates separated by air. On plate A you put positive charge, on plate B you put negative charge. The electric potential at plate A will be higher than at plate B, indicating that the positive charge on plate A is attracted to plate B. If the potential difference is not too large, the air between the plates is an insulator and the charge will not move. If we connect the two plates with a wire, the positive charge on plate A will flow through the wire towards plate B and neutralize the charge there. After some time, there will not be any more charge on either plate, and the current will stop flowing.

A battery is like the two plates with positive and negative charge, except that unlike the plate, a chemical reaction keeps on producing positive charge at one terminal, and negative charge at the other terminal, so that the current does not quickly stop.

Last edited: Jul 15, 2014
11. Jul 15, 2014

### kartikwat

See,when battery is connected in circuits,based on my reading battery creates potential difference which start the charge to move. when the charge reaches capacitor in series what happens after that is creating a trouble

12. Jul 16, 2014

### Staff: Mentor

Let's label our 2 capacitors as C1 and C2 and connect plate 1 of C1 to the negative terminal of the battery, and plate 2 of C2 to the positive terminal. When you connect the battery current begins to flow throughout the entire circuit, not just in one spot. (In the example below, I'm using electrons as my charges)

Charges begin to move on and off the plates of the capacitors. Charges move from the negative terminal of the battery and build up on plate 1 of C1, negatively charging it. Charges move off of plate 2 of C1, positively charging it, and move to plate 1 of C2, negatively charging it. Charges move off of plate 2 of C2 to the positive terminal of the battery, positively charging the plate. (All this effectively happens simultaneously)

The end result is that the plates end up charged similarly to the diagram in your attachments in your capacitor thread.

13. Jul 16, 2014

### Delta²

So, you understand what is happening when the battery charges one capacitor, but you have problem understanding the charging of capacitors in series?

14. Jul 16, 2014

### kartikwat

Voltage

Yes you are saying right and what causes potential drop when charge passes through capacitor

Last edited: Jul 16, 2014
15. Jul 16, 2014

### Staff: Mentor

I believe it is the buildup of the electric field in the capacitor. When you first connect the battery the capacitor is uncharged and acts like a short. Therefore it has very little voltage drop across it. As the plates charge the electric field builds up, requiring more work to move each charge and increasing the voltage drop across the capacitor. Someone correct me if I'm wrong.

16. Jul 16, 2014

### kartikwat

Voltage

@drakkith what happens if we introduce mica between the plates of capacitor (1)when the voltage supply remain connected(2)after the supply was disconnected

17. Jul 16, 2014

### Staff: Mentor

What's mica?

18. Jul 16, 2014

### Delta²

Well thats another theoretical problem, if the potential creates the field or the field creates the potential but i believe in most textbooks about classical electromagnetism, first they define the electric field (as being created by electric charges) and then they define the potential (as the line integral of electric field between a point and the infinite).

However nowdays the scalar and the vector potential of the electromagnetic field are considered to be more fundamental than the field.

19. Jul 16, 2014

### kartikwat

Voltage

Mica is A dielectric

20. Jul 16, 2014

### CWatters

Individual charges do not pass through a capacitor. Charge arrives on one plate and repels like charges from the other plate. You end up with a different charge on each plate. It's this that causes a voltage to appear between the plates.

21. Jul 16, 2014

### UltrafastPED

Mica is a very important mineral; it often appears as shiny specks, but also forms large sheets. In the sheet form it can be split into very fine, very clean layers.

See https://en.wikipedia.org/wiki/Mica

22. Jul 16, 2014

### Staff: Mentor

That's a complicated issue that's probably best left until you know more about electronics.

23. Jul 16, 2014

### Staff: Mentor

Let's make an analogy, by first talking about gravity instead of electricity.

Suppose we have a 1 kg object resting on the ground. We lift it to a height of h = 2 m and hold it stationary there. We have to do work against the gravitational force: W = Fh = mgh = (1 kg)(9.8 m/s2)(2 m) = 19.6 J. We say the object now has 19.6 J of gravitational potential energy with respect to the ground. (I hope you've already studied potential energy in mechanics so this is just a review for you!)

An object with a mass of 2 kg at that height would have twice as much potential energy, 39.2 J. The potential energy of an object at a given height depends on its mass. However, we can say that any object at a height of 2 m has 19.6 J of potential energy per kg of mass. We can say that a location 2 m above the ground has a gravitational potential (note I didn't say "energy") of 19.6 J/kg. If we wanted to give the unit "J/kg" a name, we might call it a "gravolt." Any object placed at that height has gravitational potential energy equal to its mass times the gravitational potential ("gravoltage") at that location.

Now imagine that instead of a gravitational field, we have an electric field E in the downwards direction with a magnitude of 9.8 N/coulomb; and an object with +1 coulomb of charge. To lift the object to a certain height, we have to do work against the electrical force: W = Fh = qEh. The rest of the discussion is the same as above, except that we rename:

mass --> charge (and kg --> coulomb)
gravitational potential energy --> electric potential energy
gravitational potential --> electric potential
"gravoltage" --> voltage (J/kg --> J/coulomb = volt)

24. Jul 16, 2014

### atyy

Yes, indeed, that's a very important point.

25. Jul 16, 2014

### DiracPool

Last edited by a moderator: May 6, 2017