Coulombs, Voltage and Cells
1 coulomb (C) is the charge on
6.24 x 1018 electrons.
6 240 000 000 000 000 000 eCharles Augustin Coulomb
(1736 - 1806)
http://www.corrosiondoctors.org/Biographies/images/coulomb.jpg
Repulsive
Force
e
e
e
e
When electrons (-) are pushed
together they gain electrical
potential energy (stored energy)
Chemical Cells (we call “batteries”)
give electrons electrical potential
energy.
Pulls e-s into the
+ terminal
Pushes e-s out of
the - terminal
e- e- e- e- e- e-
e- e- e- e- e- e-
-
+
e-s have lower
potential energy
e-s have higher
potential energy
+-
Voltage
– Electrical Potential Energy per Coulomb.
Voltage is also called Potential Difference
(The electrical potential energy given to each
coulomb (6.24 x 1018 e-s) as they pass through
a cell)
Joule
1 volt = 1 Coulomb
Alessandro Volta (1745-1827)
http://www.acienciasgalilei.com/fotos/volta.jpg
A Voltmeter
30 V
1 cell with a
voltmeter
volts
V
Schematic Diagram
2 cells in series
V
volts
2 Cells in Series
2 cells in
Parallel
V
volts
2 Cells in Parallel
Do Activity 3 – Cells in Series and Parallel
How to connect a voltmeter
• Voltmeters must be connected across a
resistor (in parallel)
Voltage/Loop Law
• Around any loop:
• The total voltage gain (at the
battery) =
• The total voltage lost (at resistors)
Voltage / Loop Law
V = 3.0 V
What is the
voltage drop
at each
resistor?
Which
voltmeter will
measure
voltage
correctly?
V
=?
V
=?
ELECTRIC CURRENT
Electric current – the flow of e-’s
Electron flow is from
e- flow
+
e- flow
-+
(From History:)
Conventional Current
-flows from + Conventional current
+
Conventional current
The unit for current is the
Ampere (A)
coulomb
1 Ampere (A) = 1
second
1 coulomb = 6.24 x 1018 e-’s
Ampere = the # of charged particles
that pass a point per second
Symbol = A (amps)
Different amounts of current
Appliance
Radio
100 W lamp
TV
Toaster
Microwave oven
Oven
Current (Amps)
0.4
0.8
1.7
8.8
11.7
40
André-Marie Ampère
Born:20 January 1775
Died:10 June 1836
http://www.ieee-virtualmuseum.org/media/M1QFVQPEZIR4.jpg
An Ammeter
measures Current
in amperes (A) or
milliamperes (mA)
mA
1 mA = 0.001 A
0r
1 A = 1000 mA
An Ammeter must
be connected in
SERIES (in front
of or behind) in a
circuit
It “counts” the
charges passing
per second
How to connect an ammeter
a)_____mA
_____ A
a
b
b)_____mA
_____A
x 1000 (3 dp right)
Amperes
(A)
Milliamperes
(mA)
÷1000 (3 dp left)
Amperes (A)
Milliamperes (mA)
5.0 A
0.25 A
350 mA
8 000 mA
0.56 A
0.0020 A
2.0 mA
2.3 A
0.0050 mA
860 000 mA
Amperes (A)
Milliamperes (mA)
5.0 A
5 000 mA
0.25 A
350 mA
8 000 mA
0.56 A
0.0020 A
2.0 mA
2.3 A
0.0050 mA
860 000 mA
Amperes (A)
Milliamperes (mA)
5.0 A
5 000 mA
0.25 A
250 mA
350 mA
8 000 mA
0.56 A
0.0020 A
2.0 mA
2.3 A
0.0050 mA
860 000 mA
Amperes (A)
Milliamperes (mA)
5.0 A
5 000 mA
0.25 A
250 mA
0.35 A
350 mA
8 000 mA
0.56 A
0.0020 A
2.0 mA
2.3 A
0.0050 mA
860 000 mA
Amperes (A)
Milliamperes (mA)
5.0 A
5 000 mA
0.25 A
250 mA
0.35 A
350 mA
8.0 A
8 000 mA
0.56 A
0.0020 A
2.0 mA
2.3 A
0.0050 mA
860 000 mA
Amperes (A)
Milliamperes (mA)
5.0 A
5 000 mA
0.25 A
250 mA
0.35 A
350 mA
8.0 A
8 000 mA
0.56 A
560 mA
0.0020 A
2.0 mA
0.0020 A
2.0 mA
2.3 A
2 300 mA
0.0000050 A
0.0050 mA
860 A
860 000 mA
Conductors – materials that
electrons can pass through easily
Insulators – materials that
electrons cannot pass through
easily
A closer look at insulators and
conductors
• Conductors: electrons loosely bound to
nuclei
– Electrons flow easily when voltage is
applied
– Eg: most metals
• Insulators: electrons tightly bound to nucleus
– Electrons do not flow easily
– Eg: glass and rubber (non-metals)
Drawing electrical circuits
• We don’t want to spend time drawing
pictures of actual batteries and motors
• Instead we use schematic diagrams
– These use symbols to represent a circuit
Too time consuming!
Drawing electrical circuits cont.
• Using schematics, the circuitry of a flashlight is
much easier to represent
• This figure also shows the basic components of
any circuit: an energy source, conducting wires,
a control for turning current on and off, and a
load device
Schematic Circuit Diagrams
resistor
cell
A
V
Ammeter
Voltmeter