Coursehero >> Pennsylvania >> Drexel >> PHYS 102

Course Hero has millions of student submitted documents similar to the one below including study guides, homework solutions, papers, exam answer keys and textbook solutions.

Circuits RC A direct current circuit may contain capacitors and resistors, the current will vary with time When the circuit is completed, the capacitor starts to charge The capacitor continues to charge until it reaches its maximum charge (Q = C) Once the capacitor is fully charged, the current in the circuit is zero 1 Quiz 2 on Wednesday Chapter 20 2 Quiz 2 Wednesday Chapter 20 3 Charging an RC Circuit As the plates are being charged, the potential difference across the capacitor increases At the instant the switch is closed, the charge on the capacitor is zero Once the maximum charge is reached, the current in the circuit is zero The potential difference across the capacitor matches that supplied by the battery 4 Charging Capacitor in an RC Circuit The charge on the capacitor varies with time q = C(1 e-t/RC) = Q(1 e-t/RC) is the time constant =RC The current can be found -t RC I (t ) = R e 5 Time Constant, Charging The time constant represents the time required for the charge to increase from zero to 63.2% of its maximum has units of time The energy stored in the charged capacitor is Q = C2 6 Discharging Capacitor in an RC Circuit When a charged capacitor is placed in the circuit, it can be discharged q = Qe-t/RC The charge decreases exponentially 7 Discharging Capacitor At t = = RC, the charge decreases to 0.368 Qmax In other words, in one time constant, the capacitor loses 63.2% of its initial charge The current can be found Both charge and current decay exponentially at a rate characterized by = RC dq Q -t RC I (t ) = = -Io e , Io = dt RC 8 Consider the circuit in Active Figure 21.25a and assume that the battery has no internal resistance. (i) Just after the switch is closed, the potential difference across which of the following is equal to the emf of the battery? 1. 2. 3. C R neither C nor R 33% 33% 33% 10 ne ith er C no rR C R 9 Consider the circuit in Active Figure 21.25a and assume that the battery has no internal resistance. (ii) After a very long time, the potential difference across which of the following is equal to the emf of the battery? 1. 2. 3. C R neither C nor R 33% 33% 33% 10 ne ith er C no rR C R 10 Problem 21.59. The values of the components in a simple series RC circuit containing a switch (Fig. 21.25) are C = 1.00 F, R = 2.00 106 , and = 10.0 V. At the instant 10.0 s after the switch is closed, calculate (a) the charge on the capacitor, (b) the current in the resistor, (c) the rate at which energy is being stored in the capacitor, and (d) the rate at which energy is being delivered by the battery. 11 Atmosphere as a Conductor Lightning and sparks are examples of currents existing in air Earlier examples of the air as an insulator were a simplification model Whenever a strong electric field exists in air, it is possible for the air to undergo electrical breakdown in which the effective resistivity of the air drops and the air becomes a conductor 12 Creating a Spark A (a) molecule is ionized as a result of a random event Cosmic rays and other events produce the ionized molecules (b) The ion accelerates slowly and the electron accelerates rapidly due to the force from the electric field This is if there is a strong electric field In a weak field, they both accelerate slowly and eventually neutralize as they recombine 13 Creating a Spark, cont (c) The accelerated electron approaches another molecule at a high speed (d) If the field is strong enough, the electron may have enough energy to ionize the molecule during the collision 14 Creating a Spark, final (e) There are now two electrons to be accelerated by the field Each of these electrons can strike another molecule and repeat the process The result is a very rapid increase in the number of charge carriers available in the air and a corresponding decrease in the resistance of the air 15 Lightning Lightning occurs when a large current in the air neutralizes the charges that established the initial potential difference Typical currents during lightning can be very high Stepped leader current is in the range of 200 300 A Peak currents are about 5 x 104 A Power is in the billions of watts range 16 Fair Weather Currents The average fair-weather current in the atmosphere is about 1000 A This is spread out over the entire globe The average fair-weather charge density is 2 x 10-12 A / m2 During the lightning stoke, J ~ 105 A/m2 Fair-weather current is in the opposite direction from the lightning current 17 Chapter 22 Magnetic Forces and Magnetic Fields A Brief History of Magnetism 13th century BC Chinese used a compass Uses a magnetic needle Probably an invention of Arab or Indian origin 800 BC Greeks Discovered magnetite attracts pieces of iron 19 A Brief History of Magnetism, 2 1269 Pierre de Maricourt found that the direction of a needle near a spherical natural magnet formed lines that encircled the sphere The lines also passed through two points diametrically opposed to each other He called the points poles 20 A Brief History of Magnetism, 3 1600 William Gilbert Expanded experiments with magnetism to a variety of materials Suggested the earth itself was a large permanent magnet 1750 John Michell Magnetic poles exert attractive or repulsive forces on each other These forces vary as the inverse square of the separation 21 A Brief History of Magnetism, 4 1819 Hans Christian Oersted Pictured, 1777 1851 Discovered the relationship between electricity and magnetism An electric current in a wire deflected a nearby compass needle Andr -Marie Amp re Deduced quantitative laws of magnetic forces between currentcarrying conductors Suggested electric current loops of molecular size are responsible for all magnetic phenomena 22 A Brief History of Magnetism, final 1820's Faraday and Henry Further connections between electricity and magnetism A changing magnetic field creates an electric field Maxwell A changing electric field produces a magnetic field 23

Find millions of documents here - Study Guides, Homework Solutions, Papers, Exam Answer Keys and more. Course Hero has millions of course related materials that will enable you to learn better, faster and get an A in all your courses.
Below is a small sample set of documents: