Ohm’s Law, Power and Energy

EE 2010: Fundamentals of Electric CircuitsMujahed 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 = VIP = V2/RP = I2R

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 CircuitsMujahed AlDhaifallah

Battery

Chemical Reactions to produce potential differenceAlkaline and lithium-iodine primary cellsLead Acid secondary cellNickel-Cadmium Secondary cellNickel-Hydrogen and Nickel-Metal Hydride

Secondary cells Solar Cells

Power Supply

Used very frequently in all devices. Transform the AC supply into a lower voltageRectify 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