Transcript W = 9.6x10 -17 J B) The electron is decreasing it`s electric potential

Electric Potential
Chapter 21
When going for a hike there are two things to consider.
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?)
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.
Electric Potential &
Electric Potential Energy
Work Done on the Ball
Kinetic Energy
Energy (Potential) Gained
Energy & Electric Fields
Gravity 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 Energy
The larger the charge, the larger the PE
Just 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 height
With 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.
WonQ
DV =
Q
WonQ = Work on a charge being
moved (J)
Q = charge being moved (C)
DV = Change in potential
Units: Joules/Coulomb = Volts
• Positive work is done moving a charge farther away
from where it wants to be. This increases DV.
• Negative work is done when a charge moves towards
where it wants to be. This is a decrease in DV.
Charges move to decrease
their potential ENERGY
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.
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 DV
decreases.
-
-
-
A positive charge will move away from another positive and
towards a negative all on it’s own. As it does this, its DV
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?
DV = -600. V
Qe- = -1.6x10-19 C
W=?
B) The electron is
decreasing it’s
electric potential so
it would be moving
towards a positive
charge.
WonQ
DV =
Q
W
onQ
J
-600 =
C -1.6 ´10-19 C
W = 9.6x10-17J
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 lines
Hmmm…the ends
are at the same
potential
Consider equal potential lines
Hmmm…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.
2
# of Valence electrons
1
2
3 4 5 6 7 8
Some elements do this by giving away valence electrons
K -> K+ + eSome elements do this by gaining valance electrons
S + 2e- -> S2When 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.