* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Electric Current
Power electronics wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Electric battery wikipedia , lookup
Operational amplifier wikipedia , lookup
Negative resistance wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Superconductivity wikipedia , lookup
Surge protector wikipedia , lookup
Opto-isolator wikipedia , lookup
Lumped element model wikipedia , lookup
Galvanometer wikipedia , lookup
Thermal runaway wikipedia , lookup
Electromigration wikipedia , lookup
RLC circuit wikipedia , lookup
Power MOSFET wikipedia , lookup
Electrical ballast wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Two-port network wikipedia , lookup
Current source wikipedia , lookup
Rectiverter wikipedia , lookup
Current • Electric Current (I) – The rate at which charge flows through a perpendicular surface dQ I dt C/s or Ampere, A The current has the same direction as the flow of positive charge. Current • Direct Current (DC) – The constant current in magnitude and direction • Alternating Current (AC) – The current changing in magnitude and direction all the time Current Q (nAvd t )q Q (nAvd t )q I av nqvd A t t vd : drift speed of charge q n : number of mobile charge carriers per unit volume Quiz #1 • Consider positive and negative charges moving horizontally through the four regions shown in the figure. Rank the current in these four regions, from lowest to highest. current density • The current density (J) – the current per unit area I J nqvd A J nqvd current density • A current density J and an electric field E are established in a conductor whenever a potential difference is maintained across the conductor J E Ohm’s law : conductivity Resistance J V Vb Va El = 1/ : Resistivity I l A l l I IR A l Resistance (.m) R A Resistivity Resistors Resistance Resistance and Temperature Conductor 0 1 (T T0 ) R R0 1 (T T0 ) 0 T temperature coefficient of resistivity Resistance and Temperature Semiconductor superconductor • a class of metals and compounds whose resistance decreases to zero when they are below a certain temperature Tc, known as the critical temperature superconductor • A small permanent magnet levitated above a disk of the superconductor Ba2Cu3O7, which is at 77 K. Electrical Power (P) • The rate at which energy is delivered to a resistor (V )2 P I V I 2R R Example #7 • For the two lightbulbs shown in figure, rank the current values at points a through f, from greatest to least Direct Current Circuits • Electromotive Force • Resistors in Series and Parallel • Kirchhoff’s Rules • RC Circuits • Electrical Meters Electromotive Force • Describing not a force but rather a potential difference in volts • A battery is called either a source of electromotive force, or more commonly, a source of emf Electromotive Force • The emf of a battery is the maximum possible voltage that the battery can provide between its terminals Circuit Diagram V Va Vb Ir r = internal resistance IR Ir I Rr R = load resistance Power I I R I r 2 2 Example #8 • A battery has an emf of 12.0 V and an internal resistance of 0.05 . Its terminals are connected to a load resistance of 3.00 . Resistors in Series V IR1 IR2 I ( R1 R2 ) IReq Req R1 R2 Resistors in Parallel V I I1 I 2 R1 V R2 V Req 1 1 1 Req R1 R2 Example #9 • Four resistors are connected as shown in the figure, – Find the equivalent resistance between points a and c – What is the current in each resistor if a potential difference of 42 V is maintained between a and c Example #10 • Three resistors are connected in parallel as shown in the figure 28.11 . A potential difference of 18.0 V is maintained between points a and b – Find the current in each resistor – Calculate the power delivered to each resistor and the total power delivered to the combination of resistors – Calculate the equivalent resistance of the circuit Kirchhoff’s Rules • Junction rule. – The sum of the currents entering any junction in a circuit must equal the sum of the currents leaving that junction I in I out Kirchhoff’s Rules • Loop rule. – The sum of the potential differences across all elements around any closed circuit loop must be zero CosedLoop V 0 Example #11 • A single-loop circuit contains two resistors and two batteries, as shown in the figure (Neglect the internal resistances of the batteries. – Find the current in the circuit. – What power is delivered to each resistor? – What power is delivered by the 12-V battery? Solution #11 The 12-V battery delivers power Example #12 • Find the currents I1, I2, and I3 in the circuit shown in the figure. Solution #12 RC Circuits • Charging a Capacitor q IR 0 C RC Circuits • Charging a Capacitor I0 R current at t = 0 Q C maximum charge t RC q(t ) Q 1 e RC time constant of the circuit I I 0e t RC RC Circuits • Discharging a Capacitor q IR 0 C q(t 0) Q Charge at t = 0 q(t ) Qe t RC t dq(t ) Q RCt I (t ) e I 0e RC dt RC Electrical Meters • The Galvanometer – the main component in analog meters for measuring current and voltage Electrical Meters • The Ammeter – A device that measures current Electrical Meters • The Voltmeter – A device that measures potential difference Quiz #2 • Find the equivalent resistance between points a and b in the figure • If a potential difference of 34.0 V is applied between points a and b. calculate the current in each resistor.