Electromagnetism

1. ELECTRIC
CURRENT

2. HOW DO CHARGES FLOW?
• “Charges flow whenever there is a difference between their potential
energy (Electrical Potential Energy)”

3. WHAT IS ELECTRICAL POTENTIAL ENERGY?
An object has potential energy by virtue of its location, say in a force
field.
For example, doing work by lifting an object increases its
gravitational potential energy.
a. In an elevated
position, the ram has
gravitational potential
energy. When
released, this energy
is transferred to the
pile below.
b. Similar energy transfer occurs for
electric charges.

4. WHAT IS ELECTRICAL POTENTIAL ENERGY?
 To push a positive test charge closer to a
positively charged sphere, we will expend
energy to overcome electrical repulsion.
 Work is done in pushing the charge against
the electric field.
 The energy a charge has due to its location in
an electric field is called
electrical potential energy.
 If the charge is released, it will accelerate
away from the sphere and electrical potential
energy transforms into kinetic energy.

5. The SI unit of measurement for
electric potential is the volt,
named after the Italian physicist
Allesandro Volta.
The symbol for volt is V.
Potential energy is measured in
joules and charge is measured in
coulombs.
Electric Potential

6. CAPACITORS
Electrical energy can be stored in a
device called a capacitor.
• Computer memories use very tiny
capacitors to store the 1’s and 0’s of
the binary code.
• Capacitors in photoflash units store
larger amounts of energy slowly and
release it rapidly during the flash.
• Enormous amounts of energy are
stored in banks of capacitors that
power giant lasers in national
laboratories.

7. The simplest capacitor is a pair of conducting
plates separated by a small distance, but not
touching each other.
• Charge is transferred from one plate to
the other.
• The capacitor plates then have equal and
opposite charges.
• The charging process is complete when
the potential difference between the
plates equals the potential difference
between the battery terminals—the
battery voltage.
• The greater the battery voltage and the
larger and closer the plates, the greater
the charge that is stored.

8.  In practice, the plates may be thin metallic foils separated by a thin
sheet of paper.
 This “paper sandwich” is then rolled up to save space and may be
inserted into a cylinder.

9. CAPACITANCE
•Capacitance is the ability of a capacitor to store
charges.
•The SI unit of the capacitance is farad(F) named after
Michael Faraday. 1 F = 1C/V.
•𝐶 =∈
𝐴
𝑑
A= area of one plate
d= the distance between the
plate
∈= the permittivity of some
common dielectric material

10. PROBLEM SOLVING:
• A capacitor consist of two square metal plates, each
measuring 5𝑥10−2
m on a side. In between the plates is a
sheet of mica measuring 1𝑥10−4
m thick. What is the
capacitance of this capacitor?
• The parallel plates of an air capacitor are separated by 2.25
mm. Each plate carries a charge of 6.50𝑥10−9
. The magnitude
of the electric field of the plates is 4.75𝑥105
V/m. Find the
capacitance.

11. RESISTORS
• Are electrical components used to limit the amount of current flow.
• It is measured by Ohm( Ω), named after Georg Simon Ohm, a German
physicist.
• The stripes on these resistors are color coded to indicate the resistance in ohms.

12. RESISTOR READING
4- band
• 1st digit
• 2nd digit
• Multiplier
• Tolerance
5- band
• 1st digit
• 2nd digit
• 3rd digit
• Multiplier
• Tolerance
6- band
• 1st digit
• 2nd digit
• 3rd digit
• Multiplier
• Tolerance
• Temperature Coefficient

13. RESISTOR READING
COLOR DIGIT MULTIPLIE
R
TOLERANC
E
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Gray
White
Gold
Silver
None
0
1
2
3
4
5
6
7
8
9
100 =1
101 =10
102 =100
103 =1 000
104
=10 000
105 =100 000
106
=1 000 000
107 =10 000 000
108 =100 000 000
109
=1 000 000
000
0.1
0.01
±1%
±2%
±3%
±4%
±5%
±10%
±20%

14. OHM’S LAW

15.  Electric resistance is measured in units called ohms.
 Georg Simon Ohm, a German physicist, tested wires in circuits to
see what effect the resistance of the wire had on the current.
The relationship among voltage, current, and resistance is called
Ohm’s law.
I
V
R
÷
X
÷
“Ohm’s law states that the current in a circuit is directly proportional to
the voltage impressed across the circuit, and is inversely proportional to
the resistance of the circuit. “

16. The relationship among the units of measurement is:
A potential difference of 1 volt impressed across a circuit that
has a resistance of 1 ohm will produce a current of 1 ampere.
If a voltage of 12 volts is impressed across the same circuit,
the current will be 12 amperes.

17.  The resistance of a typical lamp cord is much less than 1 ohm, while
a typical light bulb has a resistance of about 100 ohms.
 An iron or electric toaster has a resistance of 15 to 20 ohms.
 The low resistance permits a large current, which produces
considerable heat.
Ohm’s Law

18. From Ohm’s law, we can
see that current depends on
the voltage applied, and
also on the electric
resistance of the human
body.

19. PROBLEM:
1. How much current is drawn by a lamp that has a resistance of 100
ohms when a voltage of 50 volts is impressed across it?
2. If the resistance of your body were 100,000 ohms, what would be
the current in your body when you touched the terminals of a 12-
volt battery?
3. If your skin were very moist, so that your resistance was only 1000
ohms, and you touched the terminals of a 24-volt battery, how
much current would you draw?

20. ELECTRIC
CIRCUIT
Any path along
which electrons
can flow is a
circuit.

21. Mechanical things seem to
be easier to figure out for
most people than electrical
things. Maybe this is
because most people have
had experience playing with
blocks and mechanical toys.
Hands-on laboratory
experience aids your
understanding of electric
circuits. The experience
can be a lot of fun, too!

22. A flashlight consists of a reflector cap, a light bulb,
batteries, and a barrel-shaped housing with a switch.
A Battery and a Bulb
 Any path along which electrons can flow is a circuit.
 A gap is usually provided by an electric switch that can be
opened or closed to either cut off or allow electron flow.

23. There are several ways to connect the battery and bulb
from a flashlight so that the bulb lights up.
The important thing is that there must be a complete
path, or circuit, that
• includes the bulb filament
• runs from the positive terminal at the top of the
battery
• runs to the negative terminal at the bottom of the
battery
A Battery and a Bulb

24. Electrons flow
• from the negative part of the battery through the wire
• to the side (or bottom) of the bulb
• through the filament inside the bulb
• out the bottom (or side)
• through the wire to the positive part of the battery
The current then passes through the battery to complete
the circuit.
A Battery and a Bulb

25. a. Unsuccessful ways to light a bulb.
b. Successful ways to light a bulb.
A Battery and a Bulb
“For a continuous flow of electrons, there must
be a complete circuit with no gaps.”

26. Electrons do not pile
up inside a bulb, but
instead flow through
its filament.
A Battery and a Bulb

27. Most circuits have more than one device that receives
electrical energy.
These devices are commonly connected in a circuit in one
of two ways, series or parallel.
• When connected in series, the devices in a circuit
form a single pathway for electron flow.
• When connected in parallel, the devices in a circuit
form branches, each of which is a separate path for
electron flow.
Electric Circuits

28. In this simple series circuit, a 9-volt battery provides
3 volts across each lamp.
Series Circuits

29. In this parallel circuit, a 9-volt battery provides 9 volts across
each activated lamp. (Note the open switch in the lower
branch.)
Parallel Circuits

30. Characteristics Series Parallel
Path of electron Electric current has a
single pathway through
the circuit.
There are separate pathways for
current, one through each lamp.
A break in any one path does not
interrupt the flow of charge in
the other paths.
Electrical devices
across each wire
The addition of more
lamps results in a greater
circuit resistance. This
decreases the current in
the circuit (and in each
lamp), which causes
dimming of the lamps.
The light intensity for each lamp
is unchanged as other lamps are
introduced (or removed).
Although changes of resistance
and current occur for the circuit
as a whole, no changes occur in any
individual branch in the circuit.
Disadvantages If one device fails in a
series circuit, current in
the whole circuit ceases
and none of the devices
will work.
From the battery’s perspective,
the overall resistance of the
circuit is decreased.

31. ELECTRIC CIRCUIT EQUATIONS:
• SERIES
𝐼𝑇 = 𝐼1 = 𝐼2 … = 𝐼𝑛
𝑉𝑇 = 𝑉1 + 𝑉2 … + 𝑉
𝑛
𝑅𝑇 = 𝑅1 + 𝑅2 … + 𝑅𝑛
•PARALLEL
𝐼𝑇 = 𝐼1 + 𝐼2 … + 𝐼𝑛
𝑉𝑇 = 𝑉1 = 𝑉2 … = 𝑉
𝑛

1
𝑅𝑇
=
1
𝑅1
+
1
𝑅2
… +
1
𝑅𝑛

32. PROBLEM:
1. Three resistance 2Ω, 4Ω, and 8Ω are in series. If the
current through the combination is 0.5A. What is the
voltage across each resistance?
2. Three resistance 3Ω, 4Ω, and 12Ω are in parallel and
voltage of 6V is applied to the combination. Find the
current through each branch and the total resistance in
the combination.