Electromagnetism Lecture#08 MUHAMMAD MATEEN YAQOOB THE UNIVERSITY OF LAHORE SARGODHA CAMPUS

Review of V, I, and R Voltage:The amount of energy per charge available to move electrons from one point to another in an electric circuit. The unit is the volt (V) Current: The rate of flow of charges (electrons). The unit is the Ampere (A) Resistance: Opposition to current flow. The unit is the ohm (Ω) MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

OHM’S LAW Lecture Objective The Relationship of Current, Voltage, and Resistance Calculating Current Calculating Voltage Calculating Resistance MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

OHM’s LAW The most important fundamental law in electronics is Ohm’s law, which relates voltage, current, and resistance. Georg Simon Ohm ( ) studied the relationship between voltage, current, and resistance and formulated the equation that bears his name. In terms of current, Ohm’s law states MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

THE RELATIONSHIP OF CURRENT, VOLTAGE, AND RESISTANCE I α V Constant Resistance Effect on the current of changing the voltage with the resistance at a constant value. Less V, less I More V, More I MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

THE RELATIONSHIP OF CURRENT, VOLTAGE, AND RESISTANCE I α 1/R Constant Voltage Effect on the current of changing the resistance with the voltage at a constant value. Less R, more IMore R, less I MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

OHM’s LAW Ohm's law states that current is directly proportional to voltage and inversely proportional to resistance. I α V Constant Resistance I α 1/R Constant Voltage where: I = current in amperes (A) V = voltage in volts (V) R = resistance in ohms (Ω) MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Ohm’s Law power consumption through a resistance Some practical every day examples of this basic rule are: base board heaters, electric frying pans, toasters and electric light bulbs. The heater consumes power producing heat for warmth, the frying pan consumes power producing heat for general cooking, the toaster consumes power producing heat for cooking toast, and the electric light bulb consumes power producing heat and more important light. A further example is an electric hot water system. All are examples of Ohm’s Law. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

The Linear Relationship of Current and Voltage In resistive circuits, current and voltage are linearly proportional. Linear means that if one of the quantities is increased or decreased by a certain percentage, the other will increase or decrease by the same percentage, assuming that the resistance is constant in value. V= 10V, V=30V MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Example Assume that you are measuring the current in a circuit that is operating with 25 V. The ammeter reads 50 mA. Later, you notice that the current has dropped to 40 mA. Assuming that the resistance did not change, you must conclude that the voltage source has changed. How much has the voltage changed, and what is its new value? MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

CALCULATING CURRENT How many amperes of current are in the following circuit? MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Units with Metric Prefixes In electronics, resistance values of thousands of ohms or even millions of ohms are common. Example Calculate the current. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

CALCULATING VOLTAGE In the circuit of following Figure, how much voltage is needed to produce 5 A of current? MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

CALCULATING VOLTAGE How much voltage will be measured across the resistor ? MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

CALCULATING RESISTANCE In the circuit of following Figure, how much resistance is needed to draw 3.08 A of current from the battery? MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Ohm’s Law-Problems Solving Figure shows a graph of current versus voltage for three resistance values. Determine R 1, R 2, and R 3. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Which circuit in Figure has the most current? The least current? 15.2 mA, 19.2 mA, 21.3 mA MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

ENERGY AND POWER Energy is the ability to do work. Power is the rate at which energy is used. Where P = power in watts (W) W = energy in joules (J) t = time in seconds (s) One watt (W) is the amount of power when one joule of energy is used in one second. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

An amount of energy equal to 100 J is used in 5 s. What is the power in watts? MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

The Kilowatt-hour (kWh) Unit of Energy Determine the number of kilowatt-hours (kWh) for each of the following energy consumptions: (a) 1400 W for 1 h (b) 2500 W for 2 h (c) 100,000 W for 5 h MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

POWER IN AN ELECTRIC CIRCUIT The amount of power dissipated in an electric circuit is dependent on the amount of resistance and on the amount of current, expressed as follows: P=I 2 R Power dissipation in an electric circuit results in heat energy given off by the resistance. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

POWER IN AN ELECTRIC CIRCUIT Watt’s Laws MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Calculate the power in each of the following three circuits. 20W, 188W, 2.5W MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

RESISTOR POWER RATINGS The power rating is the maximum amount of power that a resistor can dissipate without being damaged by excessive heat buildup. The power rating of a resistor is directly related to its surface area. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Determine whether the resistor in each circuit of following has possibly been damaged by overheating W, W, 2.5W MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

POWER SUPPLIES Ampere-hour Ratings of Batteries Batteries convert chemical energy into electrical energy. Because of their limited source of chemical energy, batteries have a certain capacity that limits the amount of time over which they can produce a given power level. This capacity is measured in ampere-hours. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Ampere-hour Ratings of Batteries The ampere-hour (Ah) rating determines the length of time that a battery can deliver a certain amount of average current to a load at the rated voltage. For example, a 12 V automobile battery may be rated for 70 Ah at 3.5 A. This means that it can produce an average of 3.5 A for 20 h at the rated voltage. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

RESISTORS IN SERIES When connected in series, resistors form a "string" in which there is only one path for current. A series circuit provides only one path for current between two points so that the current is the same through each series resistor. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

CURRENT IN A SERIES CIRCUIT The current is the same through all points in a series circuit. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

TOTAL SERIES RESISTANCE The total resistance of a series circuit is equal to the sum of the resistances of each individual series resistor. Total resistance increases with each additional series resistor MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Series Resistance Formula For any number of individual resistors connected in series, the total resistance is the sum of each of the individual values. Where n= 1,2,3………………. R T = R 1 + R 2 + R Rn MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Determine the value of R 4 in the circuit of following figure? 1OkΩ MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Equal-Value Series Resistors When a circuit has more than one resistor of the same value in series, there is a shortcut method to obtain the total resistance: R T = nR where n is the number of equal-value resistors and R is the resistance value. Find the R T of eight 22 Ω resistors in series. 176 Ω MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

APPLICATION OF OHM'S LAW The basic concepts of series circuits and Ohm's law can be applied to series circuit analysis. 1. Current through any of the series resistors is the same as the total current. 2. If you know the total applied voltage and the total resistance, you can determine the total current by Ohm's law. 3. If you know the voltage drop across one of the series resistors (Rx ), you can determine the total current by Ohm's law. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

APPLICATION OF OHM'S LAW 4. If you know the total current, you can find the voltage drop across any of the series resistors by Ohm's law. 5. The polarity of a voltage drop across a resistor is positive at the end of the resistor that is closest to the positive terminal of the voltage source. 6. The current through a resistor is defined to be in a direction from the positive end of the resistor to the negative end. 7. An open in a series circuit prevents current; and, therefore, there is zero voltage drop across each series resistor. The total voltage appears across the points between which there is an open. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

SERIES CIRCUIT ANALYSIS Find the current in the given circuit. 129Ω, 194mA MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

RESISTORS IN PARALLEL When two or more resistors are individually connected between two separate points, they are in parallel with each other. A parallel circuit provides more than one path for current. Each current path is called a branch, and a parallel circuit is one that has more than one branch. Resistors in parallel. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Calculate the total parallel resistance between points A and B of the circuit in the following Figure. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

The Case of Two Resistors in Parallel The total resistance for two resistors in parallel is equal to the product of the two resistors divided by the sum of the two resistors. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

The Case of Equal-Value Resistors in Parallel MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Quiz#3 will be from lecture #08 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE