# Electric Potential Difference

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1. Jun 16, 2016

### EmilyBergendahl

1. The problem statement, all variables and given/known data
During a thunder storm, movement of water molecules within clouds creates friction which causes the bottom of the clouds to become negatively charged. This means that the bottom of the cloud and the ground begin to act like parallel plates. Once the electric field between the clouds and the ground becomes larger than 3.0 x 10^6 N/C, lightning will strike.
a. If the bottom of the cloud is 150 m above the ground, determine the electric potential difference between the ground and the cloud.
b. Say 6.24 x 10^18 electrons move between the ground and the cloud during a lightning strike. How much electric potential energy does this represent? Are the electrons gaining or losing energy?

2. Relevant equations
E = V/d
q = Ne
V = E_E/q

3. The attempt at a solution
a.
E = V/d
V = Ed
V = (3.0 x 10^6 N/C)(150 m)
V = 4.5 x 10^8 V

b.
q = Ne
q = (6.24 x 10^18)(-1.6 x 10^19)
q = -0.9984 C
q = -1.0 C

V = E_E/q
E_E = Vq
E_E = (4.5 x 10^8 V)(-1.0 C)
E_E = -4.5 x 10^8 J

How can I tell if the electrons are gaining or losing energy?

2. Jun 16, 2016

### rude man

Math looks good.

Where are the electrons after the strike? Does this represent a change in the potential energy of the electrons? If so, in which direction?

3. Jun 16, 2016

### EmilyBergendahl

Thank you for the response!

I'm not sure if I'm understanding it completely, but if the electrons are moving from the ground (positive plate) to the cloud (negative plate), the electrons would be gaining energy?

4. Jun 16, 2016

### cnh1995

5. Jun 16, 2016

### EmilyBergendahl

Okay. If the bottom of the clouds are negatively charged, that means that an excess of electrons is present. If more electrons move into the clouds, the electrons in the cloud are gaining energy?

6. Jun 16, 2016

### EmilyBergendahl

I don't think I'm visualizing this properly...

7. Jun 16, 2016

### cnh1995

Well, I don't know the exact mechanism of lightning phenomenon but from this,
I imagine the clouds are negatively charged before the lightening strike. This makes the ground positive w.r.t clouds.

8. Jun 16, 2016

### cnh1995

If the clouds are negative and ground is positive, how will be the flow of charge?

9. Jun 16, 2016

### EmilyBergendahl

In the strike, the electrons will be moving from the cloud to the ground. Therefore, the electrons in the cloud are losing energy?

10. Jun 16, 2016

### cnh1995

Yes. When a -ve charge is brought closer to a +ve charge, it loses energy and when it is taken away from the +ve charge, it gains energy.

11. Jun 16, 2016

### EmilyBergendahl

Thank you so much again!

12. Jun 16, 2016

### cnh1995

Note that this is about potential energy of the electrons.

13. Jun 16, 2016

### rude man

Yes. Potential energy. As the cloud becomes more and more negatively charged it takes more and more energy to move an electron from the ground to the cloud.

When lightning finally strikes the potential energy is dissipated into light, sound and heat, and who knows what else (e.g. flying debris).

EDIT: the analogy to a capacitor with parallel plates is actually not good. In the capacitor the plates assume a NET charge, in lightning neither the cloud nor the earth do. The movement of the cloud forces separation of charges so that the bottom of the cloud becomes - and the proximate earth is induced +. I'm not sure that the electrons actually travel from the cloud to ground as the problem postulates. This is actually a not well understood science even today as I understand it.

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Last edited: Jun 16, 2016