Lecture 2 F d Fundamentals l off Electrical El i l Engineering User Name and Password for Course Website: User Name: ee101 Password: d dsw3QC8Z d 3QC8 Introduction 1. Define current, voltage, and power, including their units. 2. Calculate p power and energy, gy, as well as determine whether energy is supplied or y a circuit element. absorbed by 3. State and apply basic circuit laws. 4. Solve for currents, voltages, and powers in simple circuits. Electrical Current Electrical current is the time rate of flow of electrical charge through a conductor or circuit element. The units are amperes (A), which are equivalent to coulombs per second d (C/ (C/s). ) Electrical Current ddq (t ) i (t ) = dt t q (t ) = ∫ i (t )dt + q (t0 ) t0 Direct Current Alternating Current When a current is constant with time, we say that th t we have h di t current, direct t abbreviated as dc. On the other hand, a currentt that th t varies i with ith time, ti reversing i direction periodically, is called alternating c rrent abbreviated current, bb i t d as ac. . Voltages The voltage associated with a circuit element is the energy transferred per unit of charge that flows through the element. The units of voltage are volts (V), which are equivalent i l t to t joules j l per coulomb l b (J/C). (J/C) “uphill: battery” “downhill: resistor” Power and Energy gy p(t ) = v (t )i (t ) Watts t2 w = ∫ p(t )dt t1 Joules Current is flowing in the passive configuration If the current flows opposite to the passive configuration, the power is given by p = -vi Pa = iava = (2A)*(12V) = 24W, energy is being absorbed Pb = -iibvb = -(1A)*(12V) (1A)*(12V) = -12W, 12W energy is i being b i supplied li d Pc = icvc = (-3A)*(12V) = -36W, energy is being supplied Power and Energy gy v(t) = 12 V i(t) = 2e-t A p(t) = v(t)i(t) = 24e-t ∞ w= ∞ ∫ p(t )dt = ∫ 24e 0 0 −t dt = −24e −∞ − (−24e ) = 24 J 0 ? • An ideal voltage source has a voltage vx =12 V independent of the load • An ideal conductor requires that vx = 0 Resistors and Ohm’s Law a b v = iR vab = iab R The units of resistance are Volts/Amp which are called ohms . The symbol for ohms is omega: Ω “ohms” George g Simon Ohm, 1789-1854 V I= R In 1805 Ohm entered the University of Erlangen but he became rather carried away with student life. Rather than concentrate on his studies he p much time dancing, g, ice skating g and p playing y g billiards. spent http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Ohm.html Conductance 1 G= R i = Gv The units of conductance are 1/Ω or Ω-1. The units are called “siemens” siemens Resistance Related to Physical Parameters R= ρL A ρ is the resistivity of the material used to fabricate the resistor. The units of resitivity are ohm-meters (Ω m) (Ω-m) Power dissipation in a resistor 2 v p = vi = Ri = R 2 KIRCHHOFF S CURRENT KIRCHHOFF’S LAW • The net current entering g a node is zero. • Alternatively, the sum of the currents entering a node equals the sum of the currents leaving a node. Gustav Kirchhoff (1824-1887) Kirchhoff's Current Law The principle of conservation of electric charge implies that: At any point in an electrical circuit where charge density is not changing in time, the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point. A charge density changing in time would mean the accumulation of a net positive or negative charge, which typically cannot happen to any significant degree because of the strength of electrostatic forces: the charge buildup would cause repulsive forces to disperse the charges. http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Kirchhoff.html (a) Currents into the node = 1A+3A = 4A current out of the node ia (b) C Currents into i the h node d = 3A+1A+i 3A 1A ib = current out off the h node d = 2A so ib=2A-4A=-2A (c) Currents into the node = 1A+ic+3A+4A = current out of the node= 0 amps so ic =-8A Series Connection Which elements are in series? KIRCHHOFF S VOLTAGE KIRCHHOFF’S LAW The algebraic Th l b i sum off the h voltages l equals l zero for any closed path (loop) in an electrical l i l circuit. i i Loop 1: -vva+vvb+vvc = 0 Loop 2: -vc-vd+ve = 0 Loop 3: va-vve+vd-vvb = 0 Parallel Connection KVL through A and B: -va+vb = 0 → va = vb KVL through A and C: -va - vc = 0 → va = -vc -3V - 5V + vc = 0 → vc = 8V -8V - (-10V) + ve = 0 → ve = -2V Which elements are in series? Which elements are in parallel? Find the current, voltage and power for each element: Power PR = iRvR = (2A)(10V) = 20W (power dissipated) = iR2R = VR2/R / PS = -vSiS = -(10V)(2A) = -20W (power supplied) PR+ PS = 0 Example • What is the current iR flowingg through g the resistor? • What is the power for each element in the circuit? • Which elements are absorbing power? • Since all of the elements are in series, the same current iR= 2A runs through each of them • The voltage drop across the resistor: vR= iR = ((2A)(5Ω) )( ) = 10V • Apply KVL: vc = vR + 10 = 20V • The Th power dissipated di i d in i eachh element: l pR = iR2R = (2A)2(5Ω) = 20W (absorbing) pvs= iv = (2A)(10V) = 20W (absorbing) pcs= -iv = -(2A)(20V) = 40W (supplying) Example Use Ohm’s law, KVL and KCL to find Vx IT R1 IR2 IR3 R2 R3 Oh ’ Law: Ohm’s L VR4 = (1A)(5Ω) = 5V = VR2 = VR3 IR2 = 5V/10Ω = 0.5A IR3 = 5V/5Ω = 1A KCL: IT = IR2 + IR3 + IR4 = 0.5A + 1A + 1A = 2.5A )( ) = 12.5V Ohm’s Law: VR1= ITR1 = ((2.5A)(5Ω) KVL: Vx = VR1 + VR4 = 12.5V+5V = 17.5V R4