1. Current

2. Electric Potential Difference- VOLTAGE
Let’s first revisit gravitational potential energy…
What is gravitational potential energy dependent on? m
PE = mgh 2m
Work (energy) is required
to lift these rocks against
the force of gravity. 2h h

3. Work is also required to move charges in an electric field
Work is also required to move charges in an electric field. If the direction of an electric field is such that it opposes the motion of a charged particle, work must be done to move the particle in that direction. +
W = Fd + +
Work must be done to
keep 2 like charges
together or two opposite
charges apart. + + + + + + +
Doing work on a system transfers energy.
This influences the electric potential
(think potential energy) of the charge.
The electric potential of this positive
test charge changes as it is moved
closer to the positive metal sphere.

4. For each situation below determine if work is done on the test charge to move it from point A to point B. + +
Electric
Field
Lines A B + - - + B A
NO work is done.
The + charge is moving
with nature; work is not
required when it moves
with the electric field.
YES work is done.
The + charge is moving
against nature; work is
required when it moves
against the electric field.

5. Now let’s look at the electric potential of the charges
Now let’s look at the electric potential of the charges. Are the charges losing or gaining potential energy when they are moved from A to B? + +
Electric
Field
Lines A B + - - + B A
The positive test charge will naturally
move in the direction of the
electric field. No work is done
in moving it. The potential energy
will decrease (just like an object
falling with gravity).
The positive test charge does not
want to naturally move against the
electric field (same charges repel) so
work needs to be done to move it to point B.
The potential energy will increase
(just like raising an object up against gravity).

6. We can now conclude that the high energy location for a positive test charge is a location nearest the positive source charge; and the low energy location is furthest away.
Electric
Field
Lines
Lower PE +
Higher PE + +

7. Electric Potential Difference (V)
The electric potential difference between
two points in an electric field is
the work done per unit charge.
V = W q
1 J/C = 1 volt

8. If the electric potential difference between two locations is 1 volt, then 1 C of charge will gain 1 J of potential energy when moved between those two locations.
Electric
Field
Lines + + +

9. If 8 Joules of work are required to move 2 Coulombs of charge through a 3-ohm resistor, what is the potential difference across the resistor?
V = W q
V = 8J 2C
V = 4V

10. Units- Joules vs. eV 1.6 x 10-19 J = 1 eV (electronvolt)
How much work is done to move an elementary
charge (+/- e) against an electric field through a
potential difference of 1 volt?
V = W q
qV = Wq q
W = Vq
W = (1V)(1.6 x C)
= 1.6 x J
1.6 x J = 1 eV (electronvolt)
Electronvolts are like inches and Joules are like miles.

11. How much energy in eV is needed to move one electron through a potential difference of 1.0 x 102V?
W = energy
W = Vq
W = (1.0 x 102V)(-1.6 x 10-19C)
W = 1.6 x 10-17J
= 100 eV
1 eV = 1.6 x J

12. How many electronvolts are in 320 x 10-19J of energy?
1 eV = 1.6 x J
200 eV

13. eV and Joules are both units of ENERGY!

14. Static Electricity vs. Electric Current
Static electricity is a
one time event.
Electric current is a
constant flow.

15. Electric current (I) is the rate at which charge passes a given point in a electric circuit.
An electric circuit is a closed path
along which charged particles move.

16. Current I = Δq t I = amps (A) An ammeter is a device used to measure

17. Determine the current for these situations:
1. A cross section of wire is
isolated and 20 C of charge
are determined to pass
through it in 40 s.
2. A cross section of wire is
isolated and 2 C of charge
are determined to pass
through it in .5 s.
20 C
2 C
40 s
.5 s
I = Δq t
= 20 C
40 s
= .5 A
I = Δq t
= 2 C
.5 s
= 4 A

18. Requirements necessary for an electric current:
There must be a closed conducting path which extends from the
positive terminal to the negative terminal of a cell or battery
(combination of cells).

19. Requirements necessary for an electric current:
2. There must be a difference in electric potential between the two end
points in the circuit. The potential difference may be supplied by a cell
or battery. In other words you need an energy source.
The potential difference or voltage of
a circuit can be measured using a voltmeter.

20. Light Bulbs
Using only a light bulb, a wire, and a battery come up with 4 ways to create a complete circuit so that the light bulb lights up.

21. You need to create a closed circuit.

22. Conventional flow vs. THE TRUTH
Ben Franklin envisioned positive charges as the carriers of
charge. Because of this an early convention for the direction of an
electric current was established to be in the direction which positive
charges would move. The direction of an electric current is by
convention the direction in which a positive charge would move.
However, we now know that the positive charges (protons) are NOT
the charged particles that are moving. IT’S THE ELECTRONS.
And the negatively charged electrons would be moving in the
opposite direction.

23. Alternating Current Direct Current AC - The electrons in alternating
current flow in one direction, then
in the opposite direction—over
and over again. Electricity from
a power plant is alternating current.
DC - The electrons in direct
current flow in one direction. The
current produced by a battery is
direct current.

24. In the United States, the current flow alternates 120 times per second
In the United States, the current flow alternates 120 times per second. (In Europe it alternates 100 times per second.) The current supplied to your home by the local utility is alternating current.

25. War of Currents DC vs. AC EDISON TESLA WESTINGHOUSE
Edison carried out a campaign to
discourage the use of alternating
current, including spreading
disinformation on fatal AC
accidents, publicly killing animals,
and lobbying against the use of
AC in state legislatures. Edison
directed his technicians to preside
over several AC-driven killings of
animals, primarily stray cats and
dogs but also unwanted cattle and
horses. Acting on these directives,
they were to demonstrate to the
press that alternating current was
more dangerous than Edison's
system of direct current.
TESLA
WESTINGHOUSE