dc circuits
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CircuitsTRANSCRIPT
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PHY-2049Current & CircuitsFebruary 08
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NewsQuiz TodayExamination #2 is on Wednesday of next week (2/4/09)It covers potential, capacitors, resistors and any material covered through Monday on DC circuits.No review session on Wednesday Exam Day!
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A closed circuitHot, Hot Hot
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Power in DC Circuit
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Lets add resistors .
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Series CombinationsSERIES Resistors
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The rod in the figure is made of two materials. The figure is not drawn to scale. Each conductor has a square cross section 3.00 mm on a side. The first material has a resistivity of 4.00 103 m and is 25.0 cm long, while the second material has a resistivity of 6.00 103 m and is 40.0 cm long. What is the resistance between the ends of the rod?
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Parallel Combination??
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Whats This???In the figure, find the equivalent resistance between points (a) F and H and [2.5] (b) F and G. [3.13]
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(a) Find the equivalent resistance between points a and b in Figure P28.6. (b) A potential difference of 34.0 V is applied between points a and b. Calculate the current in each resistor.
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Power Source in a CircuitThe ideal battery does work on charges moving them (inside) from a lower potential to one that is V higher.
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A REAL Power Sourceis NOT an ideal batteryE or Emf is an idealized device that does an amount of work E to move a unit charge from one side to another.Internal Resistance
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A Physical (Real) BatteryInternal Resistance
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Which is brighter?
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Which is Brighter Which is Brighter???
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Back to PotentialChange in potential as one circuitsthis complete circuit is ZERO!
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Consider a circuit.This trip around the circuit is the same as a path through space.
THE CHANGE IN POTENTIAL FROM a AROUND THE CIRCUIT AND BACK TO a is ZERO!!
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To rememberIn a real circuit, we can neglect the resistance of the wires compared to the resistors.We can therefore consider a wire in a circuit to be an equipotential the change in potential over its length is slight compared to that in a resistorA resistor allows current to flow from a high potential to a lower potential.The energy needed to do this is supplied by the battery.
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NEW LAWS PASSED BY THIS SESSION OF THE FLORIDUH LEGISLATURE.LOOP EQUATIONThe sum of the voltage drops (or rises) as one completely travels through a circuit loop is zero.Sometimes known as Kirchoffs loop equation.NODE EQUATIONThe sum of the currents entering (or leaving) a node in a circuit is ZERO
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TWO resistors again
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A single real resistor can be modeledas follows:ADD ENOUGH RESISTORS, MAKING THEM SMALLERAND YOU MODEL A CONTINUOUS VOLTAGE DROP.
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We start at a point in the circuit and travel around until we get back to where we started.If the potential rises well it is a rise.If it falls it is a fall OR a negative rise.We can traverse the circuit adding each rise or drop in potential.The sum of all the rises around the loop is zero. A drop is a negative rise.The sum of all the drops around a circuit is zero. A rise is a negative drop.Your choice rises or drops. But you must remain consistent.
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Take a trip around this circuit.
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Circuit Reductioni=E/Req
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ReductionComputes i
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Another Reduction Example
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BatteryA battery applies a potential difference between its terminals.Whatever else is connected (circuits, etc.), the potential between the points remains the same: the battery potential.
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Take a trip around this circuit.
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Multiple Batteries
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START by assuming a DIRECTION for each CurrentLets write the equations.
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In the figure, all the resistors have a resistance of 4.0 W and all the (ideal) batteries have an emf of 4.0 V. What is the current through resistor R?
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Consider the circuit shown in the figure. Find (a) the current in the 20.0- resistor and (b) the potential difference between points a and b.
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Using Kirchhoffs rules, (a) find the current in each resistor in Figure P28.24. (b) Find the potential difference between points c and f. Which point is at the higher potential?
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The Unthinkable .
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RC CircuitInitially, no current through the circuitClose switch at (a) and current begins to flow until the capacitor is fully charged.If capacitor is charged and switch is switched to (b) discharge will follow.
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Close the SwitchI need to use E for ENote RC = (Volts/Amp)(Coul/Volt) = Coul/(Coul/sec) = (1/sec)
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Really Close the SwitchI need to use E for ENote RC = (Volts/Amp)(Coul/Volt) = Coul/(Coul/sec) = (1/sec)
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This is a differential equation.To solve we need what is called a particular solution as well as a general solution.We often do this by creative guessing and then matching the guess to reality.You may or may not have studied this topic but you WILL!
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Time Constant
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Result q=CE(1-e-t/RC)
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q=CE(1-e-t/RC) and i=(CE/RC) e-t/RC
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Discharging a Capacitorqinitial=CE BIG SURPRISE! (Q=CV)iiR+q/C=0