Electric Potential

Chapter 21

1. How high up are you going? (What is your change in elevation?)

2. How much energy will it take to get there? (How much potential energy will you have once you get there?)

When going for a hike there are two things to consider.

Which hiker is doing more work to get to the top of the mountain? (which hiker will have more potential energy when at the top?)

Even though they are climbing the same mountain the hiker with the heavier (more massive) load will be doing more work (or will have more potential energy at the top)

If the hiker trips, his potential energy will convert into kinetic energy and he will fall down the hill.

Work Done on the Ball Energy (Potential) Gained

Kinetic Energy

Electric Potential & Electric Potential Energy

Energy & Electric FieldsGravity is similar to two unlike charges cause

they attract each other just like masses do.When unlike charges are separated, it takes

work.That work is stored in the charges as Electric

Potential EnergyThe larger the charge, the larger the PEJust as a hiker hiking up a hill does work and that

work is stored as Gravitational Potential Energy.

Work Done equals Energy Gained

However, no one really talks about how much work someone did climbing a mountain or how much potential energy they had once they got there. Instead everyone talks about the elevation of the peak.

The same is true for charged objects.

With hiking, people talk about heightWith charges, people talk about potential

The difference in electric Potential is the work done moving a positive test charge between two points in an electric field.

However, it is rarely a positive test charge moving. Usually it is an actual charge (Q) so the electric potential difference is measured as…

The difference in electric potential is the ratio of the work needed to move a charge to the strength of that charge.

V = Change in potential

Units: Joules/Coulomb= Volts

WonQ = Work on a charge being moved (J)

Q = charge being moved (C)

• Positive work is done moving a charge farther away from where it wants to be. This increases V.

• Negative work is done when a charge moves towards where it wants to be. This is a decrease in V.

Masses will always try to move to lower their potential energy by decreasing their height. (This is why meatballs roll off the table and onto the floor…)

Charges will always move to lower their potential energy also by decreasing their potential difference. But it gets a bit trickier because the charge can either be positive or negative.

Charges move to decrease their potential ENERGY

Think of potential as having the potential to move.

---

A negative charge will move away from another negative and towards a positive all on it’s own. As it does this, its V decreases.

++

A positive charge will move away from another positive and towards a negative all on it’s own. As it does this, its V decreases.

+

The electric potential of an electron decreases by 600. V. a)How much work is done by the electron?b)In this scenario, would the electron be moving towards another negative charge or towards a positive one?V = -600. VQe- = -1.6x10-19 CW = ?

W = 9.6x10-17J

B) The electron is decreasing it’s electric potential so it would be moving towards a positive charge.

When going hiking, it is helpful to bring a map. Hikers use topographical maps.

Electric Potential due to a Point Charge

+ + + + + +

+

Equipotential Lines & Electric Field Lines (draw these)

The Electric Potential Produced by a Heart Beat

The lamp will not glow when it is held with both ends equidistant from the charged Van de Graaff generator. But when one end is closer to the dome than the other, a current is established and it glows. Why?

Consider equal potential lines

Consider equal potential linesHmmm…the

ends are at the same potential

Consider equal potential linesHmmm…Now

the ends are at a different potential

Why does anyone care about electric potential?

It’s all about the chemicals

What happens when two chemicals are mixed together?

sodium water+ =

They React!

Why do they react?All elements strive to have 8 valence electrons.

12

5 63

# of Valence electrons

4

2

7 8

Some elements do this by giving away valence electrons

K -> K+ + e-

Some elements do this by gaining valance electrons

S + 2e- -> S2-

When a potassium atom (K) is placed next to a sulfur atom (S), they react and electrons flow from the potassium atom to the sulfur atom.

This means, there must be an electric potential difference between potassium & sulfur.

Batteries harness this flow of electrons.

In a battery, elements are placed close to each other, without touching, so they do not react.

A wire connects the two elements.

The wire allows the transfer of electrons from one element to the other.

This movement of electrons generates ELECTRICITY!

As the zinc (Zn) loses electrons, it goes from solid zinc into an ion dissolved in water.

As the dissolved hydrogen ions (H+) gains electrons, they become hydrogen gas and leave the container.Eventually the chemicals run out, and the battery

is considered dead.

Rechargeable batteries

Rechargeable batteries can be “reset”

A dead rechargeable battery can be plugged into the wall.

The electric potential (voltage) from the WALL returns the electrons and chemicals to their original starting position to be used again.

This process is not perfect so rechargeable batteries eventually die as well.