1 circuit variables1 circuit variables.pdf
- 1. Electric Circuits I Circuit Variables 1 Dr. Firas Obeidat
- 2. 2 Dr. Firas Obeidat – Philadelphia University Introduction electrical systems are found everywhere, such as homes, schools, hospitals, factories, transportation … etc. Any electrical system can be represented by electrical circuit and mathematical equations to study and analyze its physical behavior. An electric circuit is an interconnection of electrical elements. In electrical circuit analysis, we often find ourselves seeking specific currents, voltages, or powers. Pay close attention to the role of “+” and “−” signs when labeling voltages, and the significance of the arrow in defining current; they often make the difference between wrong and right answers.
- 3. 3 Dr. Firas Obeidat – Philadelphia University Units and Scales In order to state the value of some measurable quantity, we must give both a number and a unit, such as “3 meters.” The most frequently used system of units is International System of Units (SI).
- 4. 4 Dr. Firas Obeidat – Philadelphia University Units and Scales Comparison of units of the various systems of unit.
- 5. 5 Dr. Firas Obeidat – Philadelphia University Units and Scales The SI uses the decimal system to relate larger and smaller units to the basic unit, and employs prefixes to signify the various powers of 10.
- 6. 6 Dr. Firas Obeidat – Philadelphia University Units and Scales Powers of Ten
- 7. 7 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Charge the most basic quantity in an electric circuit is the electric charge. all matters are made of fundamental building blocks known as atoms and that each atom consists of electrons, protons, and neutrons. charge e on an electron is negative and equal in magnitude to (-1.602 ×10-19 C). while a proton carries a positive charge of the same magnitude as the electron (+1.602 ×10-19 C). In 1 C of charge, there are (1/1.602 ×10-19=6.24×1018) electrons. Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C).
- 8. 8 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Charge
- 9. 9 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Current Electric current is the time rate of change of charge, measured in amperes (A). Or the net amount of charge that passes through the wire per unit time. 𝒊 = 𝒅𝒒 𝒅𝒕 1 Ampere = 1 Coulomb per second (C/s). If the current does not change with time, but remains constant, we call it a direct current (DC). A direct current (DC) is a current that remains constant with time. A common form of time-varying current is the sinusoidal current or alternating current (AC). Alternating current (AC) is a current that varies sinusoidally with time. DC current AC current i = current in amperes q = charge in coulombs t = time in sec
- 10. 10 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Current The direction of current flow is conventionally taken as the direction of positive charge movement. In fig.a he direction of the arrow and the value 3 A indicate either that a net positive charge of 3 C/s is moving to the right or that a net negative charge of −3 C/s is moving to the left each second. In fig.b there are again two possibilities: either −3 A is flowing to the left or +3 A is flowing to the right.
- 11. 11 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Current
- 12. 12 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Current Safety Considerations It is important to realize that even small levels of current through the human body can cause serious, dangerous side effects. Most individuals can withstand currents up to perhaps 10 mA for very short periods of time without serious side effects, any current over 10 mA should be considered dangerous. currents of 50 mA can cause severe shock, and currents of over 100 mA can be fatal. In most cases the skin resistance of the body when dry is sufficiently high to limit the current through the body to relatively safe levels for voltage levels typically found in the home. when the skin is wet due to perspiration, bathing, etc., or when the skin barrier is broken due to an injury, the skin resistance drops dramatically, and current levels could rise to dangerous levels for the same voltage shock.
- 13. 13 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Voltage Voltage (or potential difference) is the energy required to move a unit charge through an element, measured in volts (V). 𝟏 𝑽𝒐𝒍𝒕 = 𝟏 𝒋𝒐𝒖𝒍𝒆 𝒄𝒐𝒖𝒍𝒐𝒎𝒃 =𝟏 𝒏𝒆𝒘𝒕𝒐𝒏−𝒎𝒆𝒕𝒆𝒓 𝒄𝒐𝒖𝒍𝒐𝒎𝒃 let us suppose that a dc current is sent into terminal ‘a’, through the general element, and back out of terminal ‘b’. Let us also assume that pushing charge through the element requires an expenditure of energy. We then say that an electrical voltage (or a potential difference) exists between the two terminals, or that there is a voltage “across” the element. v= 𝒅𝒘 𝒅𝒒 v = voltage in volt w = energy in joule t = time in sec
- 14. 14 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Voltage The plus (+) and minus (-) signs are used to define reference direction or voltage polarity. The vab means that Point a is at a potential of vab volts higher than point b. vab=-vba For example, in fig.a and fig.b are two representations of the same voltage. In fig.a, point a is +9V above point b; in Fig.b, point b is -9V above point a. A constant voltage is called a DC voltage, whereas a sinusoidally time-varying voltage is called an AC voltage. A dc voltage is commonly produced by a battery; ac voltage is produced by an electric generator.
- 15. 15 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Power Power is the time rate of expending or absorbing energy, measured in watts (W). 𝒑 = 𝒅𝒘 𝒅𝒕 = 𝒅𝒘 𝒅𝒒 × 𝒅𝒒 𝒅𝒕 =vi If the power has a (+) sign, power is being delivered to or absorbed by the element. If the power has a (-) sign, power is being supplied by the element. When the current enters through the positive polarity of the voltage as in fig.a. In this case, p=+vi implies that the element is absorbing power. However, if p=-vi as in fig.b the element is releasing or supplying power. P=power in watts w= energy in joule q = charge in coulombs i = current in amperes v = voltage in volt t = time in sec
- 16. 16 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Power Two cases of an element with an absorbing power of 12 W: (a)p=4×3=12 W, (b) p=4×3=12W. Two cases of an element with an absorbing power of 12 W: (a)p=-4×3=-12 W, (b) p=-4×3=-12W. Passive sign convention is satisfied when the current enters through the positive terminal of an element and p=+vi. If the current enters through the negative terminal, p=-vi.
- 17. 17 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Power
- 18. 18 Dr. Firas Obeidat – Philadelphia University Basic Electrical Quantities - Power
- 19. 19