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Ohm’s Law, Power
and Energy
EE 2010: Fundamentals of Electric Circuits
Mujahed AlDhaifallah
Ohm’s Law!
Current flows because Electrons flow from
one point to another.
 What causes the flow of electrons?

Ohm’s Law!
Current flows because Electrons flow from
one point to another.
 What causes the flow of electrons?

 Voltage
(potential difference)
Ohm’s Law!
Current flows because Electrons flow from
one point to another.
 What causes the flow of electrons?

 Voltage

(potential difference)
More voltage results in more or less
electrons?
Ohm’s Law!
Current flows because Electrons flow from
one point to another.
 What causes the flow of electrons?

 Voltage

(potential difference)
More voltage results in more or less
electrons?
 More
electrons
Ohm’s Law!

So does that mean that Current is
proportional to Voltage?
Ohm’s Law!

So does that mean that Current is
proportional to Voltage?
 Yes

The more the voltage the more is the
current
Ohm’s Law

What is the effect of resistance?
Ohm’s Law
What is the effect of resistance?
 Do more electrons flow when there is a
high resistance?

Ohm’s Law
What is the effect of resistance?
 Do more electrons flow when there is a
high resistance?

 No
Ohm’s Law
What is the effect of resistance?
 Do more electrons flow when there is a
high resistance?

 No

The higher the resistance, the less is the
current.
Ohm’s Law


What is the effect of resistance?
Do more electrons flow when there is a high
resistance?
 No


The higher the resistance, the less is the current.
Current is inversely proportional to the
resistance.
Ohm’s Law
I = V/R
 V=IR
 R=V/I

Ohm’s Law

An element with R = 0 is
called a short circuit, as for a
short circuit, v = iR = 0,
current could be any value.

an element with R =∞ is
known as an open circuit for
an open circuit, i =v/R =0,
voltage could be any value
Question

In the circuit shown, calculate the current i.
Power

Power dissipated by charge flowing
through a resistor
P
= VI
 P = V2/R
 P = I2 R
Question

In the circuit shown, calculate the power p.
Energy

Energy = Power x Time
Question

How much energy (in kilowatthours) is
required to light a 60-W bulb continuously
for 1 year (365 days)?
Devices
EE 2010: Fundamentals of Electric Circuits
Mujahed AlDhaifallah
Battery

Chemical Reactions to produce potential
difference
 Alkaline
and lithium-iodine primary cells
 Lead Acid secondary cell
 Nickel-Cadmium Secondary cell
 Nickel-Hydrogen and Nickel-Metal Hydride
Secondary cells

Solar Cells
Power Supply

Used very frequently in all devices.
 Transform
the AC supply into a lower voltage
 Rectify it (?)
Independent voltage source
provides a specified voltage
 (independent of the current through the
element)

Independent Current Sources

Supplies a fixed amount of current
 It
is the dual of the battery

In a battery voltage is constant, but current drains
out
Dependent source

is a voltage or current generator whose
source quantity depends on another circuit
variable (current or voltage)
Dependent source
There are four
types of dependent
sources:
 VCVS, VCCS,
CCVS, CCCS
 Vx: controlling
voltage
 Ix: controlling
current

Example

Compute the power that is absorbed or
supplied by the elements in the following
network.
Ammeters
Device to measure current
 The wire in which current is to be
measured is broken up, and are joined via
an ammeter.
 What should be the resistance of the
ammeter?

Voltmeters
Devices to measure voltage
 Connected in a parallel fashion across the
device where there is a need to measure
potential difference.
 What should be the resistance of a
voltmeter?

Ohmmeters
Used to measure the resistance of a
device.
 Connected across the two pins of a
resistor
 Also used to check the continuity of
networks.

Wattmeters
Used to measure the dissipation of power
in a circuit element.
 Includes both an ammeter and a
voltmeter.
 Displays the multiplication of both
measurements.

Fuses and Circuit Breakers




The power supply to the homes is not ideal.
If it goes above a specified level, it can burn the
devices. May result in Fire or Smoke.
Fuse wires melt if they experience a large
current.
In a breaker, a large current results in a large
enough strength in an inbuilt electromagnet to
draw the switch open