Ohm’s Law Basic lecture 3 -Electrical Engineering -EEE

Subject: Basic Electrical Engineering
Topic: Ohm’s Law Basic 3 lecture
Subject Code: EEE-4201
Teacher: Md. Rezaul Karim 

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Basic Electrical Engineering

Ohm’s Law

The ratio of potential difference of (v) between any two points on a conductor to the current (I) flowing between them, is constant, provided the temperature of the conductor does not change

·         In other words, V=IR  

1. Exmple: A nine volt battery supplies power to cordless curling iron with a resistance of 18 ohms. How much current is flowing through the curling iron? 

Solution:

We know, V=IR

or,   I= v/R

or,   I= 9/18 = 0.2Amps

Problem’s based on ohm’s Law

Problem

1.     A current of 0.5 A is flowing through the resistance of 10 Ohm’s. Find the potential difference between its ends

2.     A supply Voltage of 220V is applied to a 100 Ohm’s resistor. Find the current flowing through it.

3.     Calculate the resistance of the conductor if a current of 2A flows through it when the potential difference across its ends is 6V

4.     Calculate the current and resistance of a 100W, 200V electric bulb.

5.     A circuit is mad e of 0.4 Ohm’s wire, a 150 ohm’s and 120 ohm’s rheostat connected in series. Determine the total resistance of the circuit

Three resistances of values 2 Ohm’s, 3 ohm’s and 5 ohm’s are connected in series across 20 V. D.C  Supply. Calculate a. Equivalent resistance of the circuit, B. The total current of the circuit, C. The Voltage drop across each resistor and d. The power dissipated in each resistor

Power (P=VI)

Electric Power is the rate at which energy transferred. We measure power in units of Jules/Sec, also known a Watts

  1 watt = 1 Jule/Sec

  Power, P=dE/dq).(dq/dt) = VI

  Electrical power is the product of voltage times current in units of watts

Example: A 9V battery connected across a 10 Ohm’s resistor. Calculate the power across resistor.

 

Electrical Energy (E=Pt)

•         Electrical energy is defined as an electric charge that lets work be accomplished. The energy is measured in watt-seconds as the power measured in watts and time in seconds. Often it is measured in kilowatt-hours as we can observe in our home electricity meter.

 

•         Electrical energy = power × time

•         1Ws =1 J               

•         1Wh=1W*3600 s=3600 J          

•           1kWh=1000Wh= 1000*3600 J= 3.6*10⁶  J    (1 B.O.T unit= 1 kWh)

 

Problem: Consider an electric bulb or lamp rated at 100W is connected to a supply source of 220V. Find out the current flowing to the load, the resistance of the lamp and the energy dissipated in 2 Hours. If per unit cost is 5.5 Tk., then calculate the total cost

Solution:

•         From the power formula we know that P = VI

Then the current flowing through the lamp is, I = 100 / 220

I = 0.45A

•         From the ohms law, Resistance R = V/I

•         R = 220/ 0.45= 489 ohms.

•         Energy dissipated, E = Power * time

•         E = V * I * t= 220*0.45* (2)= 198 Wh=0.198 kWh

Cost=0.198*5.5=1.1 taka

Defining standard electrical unit

Ampere : The ampere is that constant current which—if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum—would produce between these conductors a force equal to 2⨯10^-7 N per meter of length

Coulomb: One coulomb is defined as the amount of charge flowing when the current is 1 ampere.

1ampere=1coulomb/second or equivalently,

1coulomb=1ampere⋅second

 

Defining standard electrical unit

Volt: The size of 1 volt is officially defined as the potential difference between two points of a wire carrying a current of 1 ampere when the power dissipated in the wire is 1 watt.

1volt=1watt/ampere
1volt=1joule/coulomb (in terms of energy and charge)

 

Ohm: One ohm is defined as the resistance between two points of a conductor when 1 volt is applied and a current of 1 ampere is flowing

ohm=volt/ampere

Watt: the watt is the power which in one second gives rise to energy of 1 joule.

1watt=1joule/second

Circuit

Circuit: An electric circuit is a closed path which allows the electrons to flow through it continuously

 

Short Circuit: if you connect a wire directly from the positive to the negative side of a power supply, you’ll create what is called a short circuit

Open Circuit:. The opposite of a short circuit is an open circuit. 

This is a circuit where the loop isn’t fully connected

Circuit Contd.

Series Circuits: Two components are in series if they share a common node and if the same current flows through them. Here’s an Example circuit with three series resistors. 

• Equivalent Resistance of the series combination R=R1+R2+R3 
• There’s only one way for the current to flow in the above circuit.
• Parallel circuits: If components share two common nodes, the are in parallel. Here’s an example  schematic of three resistors in parallel with a batarry. 
• Equivalent Resistance of the parallel combination 1/R= 1/R1 + 1/R2+1/R3

Parallel Circuits:  If components share two common nodes, They are in parallel. Here an Example schematic of  three resistors in parallel with battery