1. Electric Currents & Circuits
Chapter 19:
Electric Currents & Circuits
Chapter 25 opener. The glow of the thin wire filament of a light bulb is caused by the electric current passing through it. Electric energy is transformed to thermal energy (via collisions between moving electrons and atoms of the wire), which causes the wire’s temperature to become so high that it glows. Electric current and electric power in electric circuits are of basic importance in everyday life. We examine both dc and ac in this Chapter, and include the microscopic analysis of electric current.

2. Ohm’s “Law” Chapter Outline Electric Current
The Electric Battery
Electric Current
Ohm’s “Law”
Resistance, Resistors, Resistivity
Electric Power. Power in Household Circuits
Alternating Current
Microscopic View of Electric Current:
Current Density & Drift Velocity
Superconductivity??
Electrical Conduction in the Nervous System??

3. This is a simple Electric Cell.
The Electric Battery
Volta’s Original
Electric Cell
In 1801, Alessandro Volta discovered that
electricity could be created if dissimilar metals
were connected by a conductive solution called
an electrolyte.
This is a simple Electric Cell.
Figure Simple electric cell.

4. The Electric Battery Electric Cell Schematic of a modern
Figure Simple electric cell.
Schematic of a modern
Electric Cell

5. Count Alessandro Giuseppe Antonio Anastasio Volta
Alessandro Volta
February 18, March 5, 1827
Alessandro Volta (1745 – 1827).
Italian Physicist
Known for pioneering work in electricity. Born in
Como & educated in public schools. By 1800, he
had developed the so called voltaic pile, a
forerunner of the electric battery, which
produced a steady “stream” of electricity.

6. Alessandro Volta Emperor Napoleon
February 18, 1745 –
March 5, 1827
Volta demonstrating a
battery to
Emperor Napoleon
For his work in electricity, the SI electrical unit
known as the volt was named in his honor.
Also to honor him for this work, Napoleon
made him a Count in 1801.

7. Luigi Galvani: (Sept. 9, 1737 - Dec. 4, 1798)
Italian physiologist, anatomist, biologist.
A founder of electrochemistry.
Should be called a founder of
biophysics also.
Educated & taught in Bologna.
Made early discoveries that
advanced the study of electricity.
His work with frogs led to his
1781discovery of galvanic or
voltaic electricity.

8. Luigi Galvani: Experiments on frog legs!
He made the muscles of a dead
frog twitch when he touched them
with different metals carrying
current from an electric generator.
He incorrectly thought fluid in the
frog's body was the electric source.
Later, Volta proved that the
source of the electricity was a
reaction caused by the animal's
body fluids being touched by two
different types of metal.

9. Common Household Battery
Through chemical reactions,
a battery transforms it’s
internal chemical energy into
Electrical Energy.
The chemical reactions
within the cell create a
Potential Difference
between the terminals by
slowly dissolving them. This
can be maintained even if a
current is kept flowing, until
one or the other terminal is
completely dissolved.
Common Household
Battery
Technically, several cells
connected together make a
battery. Modern usage is to
refer to a single cell as a
battery also.

10. Average Electric Current
Assume charges are
moving perpendicular
to a surface of area A.
If Q is the amount of
charge that passes
through A in time t,
the average current is:
Notation in our book is:

11. The SI unit of electric current is
Electric Current is the rate of flow of
charge through a conductor. Average Current:
Instantaneous Current is denoted as::
The SI unit of electric current is
The Ampère, A: 1 A = 1 C/s.

12. André-Marie Ampère Formulated a law of electromagnetism, called
Jan. 22, June 10, 1836
French physicist. Founder
of Electromagnetism.
A prodigy who mastered all
known mathematics by age 12!
Professor of physics,
chemistry, & mathematics.
Formulated a law of electromagnetism, called
Ampère's Law, that describes the magnetic force
between two electric currents. An instrument he
devised to measure the flow of electricity was
later refined as the galvanometer.

13. Memoir on Mathematical Theory of Electrodynamic Phenomena
André-Marie Ampère
Jan. 22, June 10, 1836
Ampère's Law describes the
magnetic force between two
electric currents.(Later)
Chief published work: (1827):
Memoir on Mathematical Theory of Electrodynamic Phenomena
The SI unit of electric current, the
Ampere (A) is named for him.

14. A complete circuit is one where current can flow
all the way around. Note that the schematic
drawing doesn’t look much like the physical circuit!
Figure (a) A simple electric circuit. (b) Schematic drawing of the same circuit, consisting of a battery, connecting wires (thick gray lines), and a lightbulb or other device.

15. Example: Current - The Flow of Charge.
A steady current of 2.5 A exists in a wire for 4.0 min.
(a) Calculate the total charge which passes by a
given point in the circuit during those 4.0 min.
(b) How many electrons is this?
Solution: a. Charge = current x time; convert minutes to seconds. Q = 600 C.
b. Divide by electron charge: n = 3.8 x 1021 electrons.

16. Example: Current - The Flow of Charge.
A steady current of 2.5 A exists in a wire for 4.0 min.
(a) Calculate the total charge which passes by a
given point in the circuit during those 4.0 min.
(b) How many electrons is this?
ΔQ = IΔt
ΔQ = (2.5 C/s)(240 s)
ΔQ = 600 C
Solution: a. Charge = current x time; convert minutes to seconds. Q = 600 C.
b. Divide by electron charge: n = 3.8 x 1021 electrons.

17. Example: Current - The Flow of Charge.
A steady current of 2.5 A exists in a wire for 4.0 min.
(a) Calculate the total charge which passes by a
given point in the circuit during those 4.0 min.
(b) How many electrons is this?
ΔQ = IΔt
ΔQ = (2.5 C/s)(240 s)
ΔQ = 600 C
ΔQ = 3.8  1021 electrons
Solution: a. Charge = current x time; convert minutes to seconds. Q = 600 C.
b. Divide by electron charge: n = 3.8 x 1021 electrons.

18. How to connect a battery.
Conceptual Example
How to connect a battery.
What is wrong with each of the schemes shown for lighting a flashlight bulb with a flashlight battery and a single wire?
Solution: a. Not a complete circuit.
b. Circuit does not include both terminals of battery (so no potential difference, and no current)
c. This will work. Just make sure the wire at the top touches only the bulb and not the battery!

19. By convention, current is DEFINED as flowing
from + to -. Electrons actually flow in the opposite
direction, but not all currents consist of electrons.
Figure Conventional current from + to - is equivalent to a negative electron flow from – to +.