1. Electromagnetism Lecture#6 Instructor: Engr. Muhammad Mateen Yaqoob

2. Electric Current So far we have been confined to study of charges in equilibrium situations, or electrostatics. We now consider situations involving electric charges that are not in equilibrium. We use the term electric current, or simply current, to describe the rate of flow of charge through some region of space. For example, the battery in a flashlight produces a current in the filament of the bulb when the switch is turned on. A variety of home appliances operate on alternating current. In these common situations, current exists in a conductor, such as a copper wire. It also is possible for currents to exist outside a conductor. For instance, a beam of electrons in a television picture tube constitutes a current. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

3. Electric Current The amount of flow depends on the material through which the charges are passing and the potential difference across the material. Whenever there is a net flow of charge through some region, an electric current is said to exist. To define current more precisely, suppose that charges are moving perpendicular to a surface of area A (This area could be the cross-sectional area of a wire, for example.) MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

4. Current (I) Electrical current is the rate of flow of charges where: I = current in amperes (A) Q = charge in coulombs (C) t = time in seconds (s) the rate of flow of charge. Random motion of free electrons in a material. Electrons flow from negative to positive when a voltage is applied across a conductive or semiconductive material. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

5. Definition of Current One ampere (1 A) is the amount of current that exists when a number of electrons having a total charge of one coulomb (1 C) move through a given cross-sectional area in one second (1 s). MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

6. Electric Current The SI unit of current is the ampere (A), 1 A of current is equivalent to 1 C of charge passing through the surface area in 1 s. It is conventional to assign to the current the same direction as the flow of positive charge. In electrical conductors, such as copper or aluminum, the current is due to the motion of negatively charged electrons. Therefore, when we speak of current in an ordinary conductor, the direction of the current is opposite the direction of flow of electrons. If the ends of a conducting wire are connected to form a loop, all points on the loop are at the same electric potential, and hence the electric field is zero within and at the surface of the conductor. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

7. Example MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

8. Resistance (R) Resistance is the opposition to current. Definition of resistance One ohm (1 Ω) of resistance exists if there is one ampere (1 A) of current in a material when one volt (1 V) is applied across the material. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

9. Conductance (G) The reciprocal of resistance is conductance, symbolized by G. It is a measure of the ease with which current is established. The formula is Unit is siemens. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

10. Types of Resistor Fixed Resistor MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

11. Resistor Color Code MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

12. Resistor 4-band color code MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

13. Example What is the resistance and tolerance of each of the four-band resistors? 5.1 k  ± 5%  k  ± 5% 47  ± 10% 1.0  ± 5% Tolerance= 0.255KΩ 4.845------------5.355 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

14. Superconductors There is a class of metals and compounds whose resistance decreases to zero when they are below a certain temperature Tc, known as critical temperature. These materials are known as superconductors. The resistance–temperature graph for a superconductor follows that of a normal metal at temperatures above Tc. When temperature is at or below Tc, resistivity drops suddenly to zero. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

15. Superconductors This phenomenon was discovered in 1911 by the Dutch physicist Heike Kamerlingh-Onnes (1853– 1926) as he worked with mercury, which is a superconductor below 4.2 K. Two kinds of superconductors are recognized. The more recently identified ones are essentially ceramics with high critical temperatures, whereas superconducting materials such as those observed by Kamerlingh-Onnes are metals. If a room-temperature superconductor is ever identified, its impact on technology could be tremendous. The value of Tc is sensitive to chemical composition, pressure, and molecular structure. It is interesting to note that copper, silver, and gold, which are excellent conductors, do not exhibit superconductivity. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

16. OHM’s LAW The most important fundamental law in electronics is Ohm’s law, which relates voltage, current, and resistance. Georg Simon Ohm (1787-1854) 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

17. 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

20. 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

21. 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

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

23. 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

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

25. 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

26. Which circuit in Figure has the most current? The least current? MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

27. 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

28. 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

29. 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

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

31. Calculate the power in each of the following three circuits. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

32. 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

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

34. 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

35. 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 3 +... + Rn MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

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