* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Presentation_W04D2
Survey
Document related concepts
Valve RF amplifier wikipedia , lookup
Electric charge wikipedia , lookup
Electric battery wikipedia , lookup
Power MOSFET wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Operational amplifier wikipedia , lookup
Galvanometer wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Opto-isolator wikipedia , lookup
Power electronics wikipedia , lookup
Surge protector wikipedia , lookup
Electrical ballast wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Rechargeable battery wikipedia , lookup
Current source wikipedia , lookup
Current mirror wikipedia , lookup
Transcript
Class 10: Outline Hour 1: DC Circuits Hour 2: Kirchhoff’s Loop Rules P10- 1 Last Time: Capacitors & Dielectrics P10- 2 Capacitors & Dielectrics Capacitance To calculate: 1) Put on arbitrary ±Q 2) Calculate E 3) Calculate V Q C V Energy 2 E Q2 1 1 2 U Q V C V uE d 3r o d 3r 2 2C 2 2 Dielectrics E dA S free qinside 0 CFilled with Dielectric C0 P10- This Time: DC Circuits P10- Examples of Circuits P10- 5 Current: Flow Of Charge Average current Iav: Charge Q flowing across area A in time t Q I av t Instantaneous current: differential limit of Iav dQ I dt Units of Current: Coulombs/second = Ampere P10- 6 Direction of The Current Direction of current is direction of flow of pos. charge or, opposite direction of flow of negative charge P10- 7 Current Density J J: current/unit area I J Iˆ A Î points in direction of current I J nˆ dA J d A S S P10- 8 Why Does Current Flow? If an electric field is set up in a conductor, charge will move (making a current in direction of E) Note that when current is flowing, the conductor is not an equipotential surface (and Einside ≠ 0)! P10- 9 Microscopic Picture Drift speed is velocity forced by applied electric field in the presence of collisions. It is typically 4x10-5 m/sec, or 0.04 mm/second! To go one meter at this speed takes about 10 hours! How Can This Be? P10-10 Conductivity and Resistivity Ability of current to flow depends on density of charges & rate of scattering Two quantities summarize this: s: conductivity r: resistivity P10- 11 Microscopic Ohm’s Law E rJ r or J sE 1 s r and s depend only on the microscopic properties of the material, not on its shape P10-12 Demonstrations: Temperature Effects on r P10-13 PRS Questions: Resistance? P10-14 Why Does Current Flow? Instead of thinking of Electric Field, think of potential difference across the conductor P10-15 Ohm’s Law What is relationship between V and current? b V Vb Va E d s E a E V / J r r I J A r V I IR A P10-16 Ohm’s Law V IR R r A R has units of Ohms (W) = Volts/Amp P10-17 Examples of Circuits P10-18 Symbols for Circuit Elements Battery Resistor Capacitor Switch P10-19 Sign Conventions - Battery Moving from the negative to positive terminal of a battery increases your potential V Vb Va Think: Ski Lift P10-20 Sign Conventions - Resistor Moving across a resistor in the direction of current decreases your potential V Vb Va Think: Ski Slope P10-21 Sign Conventions - Capacitor Moving across a capacitor from the negatively to positively charged plate increases your potential V Vb Va Think: Ski Lodge P10-22 Series vs. Parallel Series Parallel P10-23 Resistors In Series The same current I must flow through both resistors V I R1 I R2 I ( R1 R2 ) I Req Req R1 R2 P10-24 Resistors In Parallel Voltage drop across the resistors must be the same V V1 V2 I1R1 I 2 R2 IReq V V V I I1 I 2 R1 R2 Req 1 1 1 Req R1 R2 P10-25 PRS Questions: Light Bulbs P10-26 Kirchhoff’s Loop Rules P10-27 Kirchhoff’s Rules 1. Sum of currents entering any junction in a circuit must equal sum of currents leaving that junction. I1 I 2 I 3 P10-28 Kirchhoff’s Rules 2. Sum of potential differences across all elements around any closed circuit loop must be zero. V E d s 0 Closed Path P10-29 Internal Resistance Real batteries have an internal resistance, r, which is small but non-zero Terminal voltage: V Vb Va I r (Even if you short the leads you don’t get infinite current) P10- Steps of Solving Circuit Problem 1. 2. 3. 4. 5. Straighten out circuit (make squares) Simplify resistors in series/parallel Assign current loops (arbitrary) Write loop equations (1 per loop) Solve P10-31 Example: Simple Circuit You can simplify resistors in series (but don’t need to) What is current through the bottom battery? P10-32 Example: Simple Circuit Start at a in both loops Walk in direction of current 2 I1R I1 I 2 R 0 I 2 I1 R 0 Add these: 2 I1 R 0 I1 R We wanted I2: I 2 I1 R I 2 I2 0 R I1 P10-33 Group Problem: Circuit Find meters’ values. All resistors are R, batteries are HARDER EASIER P10-34 Power P10-35 Electrical Power Power is change in energy per unit time So power to move current through circuit elements: d d dq P U qV V dt dt dt P I V P10-36 Power - Battery Moving from the negative to positive terminal of a battery increases your potential. If current flows in that direction the battery supplies power I Psupplied I V I P10-37 Power - Resistor Moving across a resistor in the direction of current decreases your potential. Resistors always dissipate power V Pdissipated I V I R R 2 2 P10-38 Power - Capacitor Moving across a capacitor from the positive to negative plate decreases your potential. If current flows in that direction the capacitor absorbs power (stores charge) 2 dQ Q d Q dU Pabsorbed I V dt C dt 2C dt P10-39 Energy Balance Q IR 0 C Multiplying by I: 2 Q dQ d 1 Q 2 2 I I R I R C dt dt 2 C (power delivered by battery) = (power dissipated through resistor) + (power absorbed by the capacitor) P10-40 PRS Questions: More Light Bulbs P10-41