lecture21 Ohm's Law (2.1) Kirchhoff's Laws (2.2) Prof. Phillips January 24, 2003

lecture22 Resistors A resistor is a circuit element that dissipates electrical energy (usually as heat) Real-world devices that are modeled by resistors: incandescent light bulbs, heating elements (stoves, heaters, etc.), long wires Resistance is measured in Ohms (  )

lecture23 Ohm’s Law v(t) = i(t) R- or -V = I R p(t) = i(t)v(t) = i 2 (t) R = v 2 (t)/R Always dissipating or absorbing power (p>0) The Rest of the Circuit Rv(t)v(t) i(t)i(t) + –

lecture24 Example: a 25W Bulb If the voltage across a 25W bulb is 120V, what is its resistance? R = V 2 /P = (120V) 2 /25W = 576  What is the current flowing through the 25W bulb? I = V/R = 120V/576  = A = 208 mA

lecture25 Thought Question When I measured the resistance of a 25W bulb, I got a value of about 40 . What’s wrong here? Answer: The resistance of a wire increases as the temperature increases. For tungsten, the temperature coefficient of resistivity is 4.5x10 -3 / o K. A light bulb operates at about 5000 o F.

lecture26 Open Circuit What if R=  ? i(t) = v(t)/R = 0 The Rest of the Circuit v(t)v(t) i(t)=0 + –

lecture27 Short Circuit What if R=0? v(t) = i(t) R = 0 The Rest of the Circuit v(t)=0 i(t)i(t) + –

lecture28 Class Example

lecture29 Kirchhoff’s Laws Kirchhoff’s Current Law (KCL) –sum of all currents entering a node is zero –sum of currents entering node is equal to sum of currents leaving node Kirchhoff’s Voltage Law (KVL) –sum of voltage (drops) around any loop in a circuit is zero

lecture210 KCL (Kirchhoff’s Current Law) The sum of currents entering the node is zero: Analogy: mass flow at pipe junction i 1 (t) i 2 (t)i 4 (t) i 5 (t) i 3 (t)

lecture211 Class Examples

lecture212 KVL (Kirchhoff’s Voltage Law) The sum of voltages around a loop is zero: Analogy: pressure drop thru pipe loop v1(t)v1(t) + + – – v2(t)v2(t) v3(t)v3(t) +–+–

lecture213 KVL Polarity A loop is any closed path through a circuit in which no node is encountered more than once Voltage Polarity Convention –A voltage encountered + to - is positive –A voltage encountered - to + is negative

lecture214 Class Examples

lecture215 Electrical Analogies (Physical)

lecture216 EE Subdisciplines Communication/Signal Processing Controls Electronic Circuits Electromagnetics Power Solid State

lecture217 Power Generation of electrical energy Storage of electrical energy Distribution of electrical energy Rotating machinery-generators, motors

lecture218 Electromagnetics Propagation of electromagnetic energy Antennas Very high frequency signals Fiber optics

lecture219 Communications/Signal Proc. Transmission of information electrically and optically Modification of signals –enhancement –compression –noise reduction –filtering

lecture220 Controls Changing system inputs to obtain desired outputs Feedback Stability

lecture221 Solid State Devices –Transistors –Diodes (LEDs, Laser diodes) –Photodetectors Miniaturization of electrical devices Integration of many devices on a single chip

lecture222 Digital Digital (ones and zeros) signals and hardware Computer architectures Embedded computer systems –Microprocessors –Microcontrollers –DSP chips

lecture223 Personal Systems Scaling