current electricity http://www.physicsclassroom.com/class/circuits

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  • Slide 1
  • Current Electricity http://www.physicsclassroom.com/class/circuits
  • Slide 2
  • Electric energy provides the means to transfer large quantities of energy over great distances with little loss. Producing Electric Energy
  • Slide 3
  • Because electric energy can so easily be changed into other forms, it has become indispensable in our daily lives. Producing Electric Energy
  • Slide 4
  • Potential Energy If you do work against gravity, the gravitational field stores that energy as Gravitational Potential Energy, GPE If you do work against an electrostatic force, the electric field stores that energy as electric Potential Energy, EPE Low PE High PE
  • Slide 5
  • Electric Potential Energy Electrical potential energy is the energy contained in a configuration of charges. Like all potential energies, when it goes up the configuration is less stable; when it goes down, the configuration is more stable. The unit is the Joule.
  • Slide 6
  • Electric Potential Energy Electrical potential energy increases when charges are brought into more unstable configurations. +++ Lower PE Higher PE d FeFe + Moving q1 closer to q2 requires work and that will increase the PE of the charge. Work against electric force increases electric PE Stable Unstable
  • Slide 7
  • Electric Potential Energy Electrical potential energy decreases when charges are brought into more stable configurations. + - Higher PELower PE d FeFe + - q1 will naturally move or fall towards q2 in the direction of E. No work is required and the PE of the charge will decrease. Work with the electric force decreases electric PE Stable Unstable
  • Slide 8
  • Electric Potential Energy Electrical potential energy, EPE, is stored or lost as charges, q, move in an electric field, E. EPE is dependent on both the location in the E field and the amount of charge, q moved. E is not the same for every situation and it changes in space. It depends on the configuration of the source charges
  • Slide 9
  • Electric Potential Difference + + + + + + - - - - - - Lower V Higher V + Moving a charge in an electric field requires work or energy input A t every location, a charge has a position- dependent potential Potential difference is simply the difference in potential at any 2 points Electric Potential greatest at? A B FeFe
  • Slide 10
  • Circuit Lower V High V Charges flow from high to low V through conducting wire This flow of positive charge is called conventional current The flow stops when the potential difference between A and B is zero. A B
  • Slide 11
  • Potential Difference and Current Lower V High V To be a circuit, charges must flow continuously thru a loop, returning to their original position and cycling thru again. To do so requires energy input, a charge pump that raises the electric potential of the charge
  • Slide 12
  • Electric Circuits A circuit is simply a closed loop through which charges can continuously move Flow of charge is CURRENT
  • Slide 13
  • 1. Voltaic or galvanic cell converts chemical E to electric E. A battery is made up of several galvanic cells connected together. 2. Photovoltaic cell, or solar cell changes light energy into electric energy. Producing Electric Current (Charge Pump)
  • Slide 14
  • Requirements of a Circuit
  • Slide 15
  • 1. Closed conducting loop that extends from the positive to negative terminal What do the 4 successful arrangements have in common?
  • Slide 16
  • Requirements of a Circuit 1. Closed conducting loop that extends from the + to - terminal What do the 4 successful arrangements have in common? 2. There must be an energy source that maintains an electric potential difference across the ends of the circuit.
  • Slide 17
  • Electric Circuits A circuit is simply a closed loop through which charges can continuously move
  • Slide 18
  • Current Once the two requirements of a circuit are met, charge will flow. The flow or movement of charge is called CURRENT
  • Slide 19
  • Current The flow or movement of charge is called CURRENT. Electric current is represented by I It is the overall rate of flow of electric charge, q/t. I = q/t Unit of current: Ampere (A) 1 A = 1 C/s
  • Slide 20
  • Current A 2 mm long cross section of wire is isolated and 20 C of charge is determined to pass through it in 40 s. I = _____________ A 20C/40s = 0.5
  • Slide 21
  • Conventional Current Direction The particles that carry charge through a wire are mobile electrons which move in a direction opposite the electric field. Ben Franklin, who conducted extensive scientific studies in both static and current electricity, envisioned positive charges as the carriers of charge. The convention has stuck and is still used today. The direction of an electric current is by convention the direction in which a positive charge would move. Electrons would actually move through the wires in the opposite direction.
  • Slide 22
  • Problem A long wire is connected to the terminals of a battery. In 5.0 sec, 5.8 x 10 20 electrons pass a cross section along the wire. a) Determine the current in the wire (if you need any extra information, ask your classmates). b) If the electrons flow from left to right, in which direction is the current? Opposite, right to left
  • Slide 23
  • A typical flashlight battery will produce a 0.5-A current for about 3 h before losing its charge. Determine the total number of electrons that have moved past a cross section of wire connecting the battery and light bulb.
  • Slide 24
  • Circuit Components Cell + - + - Battery Wire Light bulb Switch Resistor
  • Slide 25
  • Sample Problem Draw a single loop circuit that contains a cell, a light bulb and a switch. Name the components + - bulb switch cell
  • Slide 26
  • Series Circuit How do the brightnesses compare? Which circuit has the greater current flow? Does the charge get used up? + - + - 123 brighte r dimmer
  • Slide 27
  • I + - + - Parallel Connections As the number of light bulbs increases, what happens to the current through the circuit? 3I More Iless I Increases, There are more pathways so less resistance
  • Slide 28
  • Circuit Connections
  • Slide 29
  • Series Connections If one resistor is turned off (a light bulb goes out), what happens to the other resistors in the circuit? If one resistor goes out, there is no longer a closed loop for current flow and all other devices in series will go out. There is an OPEN CIRCUIT
  • Slide 30
  • Parallel Connections If on resistor is turned off (a light bulb goes out), what happens to the other resistors in the circuit? If one resistor goes out, there is still a closed loop for current flow and so the other devices in series will stay on
  • Slide 31
  • Series or Parallel? The light bulbs are identical and have identical resistance, R. Which configuration produces more light? Which way do you think the headlights of a car are wired? More pathways, Less resistance, More current More light
  • Slide 32
  • Circuit Components Cell + - + - Battery Wire Light bulb Switch Resistor
  • Slide 33
  • Circuit Components Voltmeter Ammeter V A Ohmmeter
  • Slide 34
  • Measuring Current Current to be measured must pass through the ammeter, so it must be placed in SERIES mode in the circuit. Ideally ammeters have ZERO resistance so that they do not affect the energy of the circuit Ammeter measures current
  • Slide 35
  • Measuring Voltage Voltmeter measures voltage Does NOT require the current to pass through it. It must be placed in parallel to the circuit element. Ideally voltmeters have INFINITE resistance so that they do not draw current away from circuit.
  • Slide 36
  • The amount of current in a circuit depends on BOTH the potential difference across the circuit, V, AND the total resistance in the circuit, R. Resistance LOAD Energy Source II An electron traveling through the wires and loads of a circuit encounters resistance, R. Resistance is a hindrance to the flow of charge. VV
  • Slide 37
  • The table lists some of the factors that impact resistance. Resistance
  • Slide 38
  • Resistors are devices designed to have a specific resistance. Resistors are devices put in circuits to reduce the current flow Resistors may be made of graphite, semiconductors, or wires that are long and thin. Resistors
  • Slide 39
  • Slide 40
  • Current flow does NOT depend only on voltage. Charge traveling through the wires and loads of a circuit encounters resistance, R. Resistance is a hindrance to the current. The higher the resistance, the smaller the current. To produce electric current, I, a potential difference, V, is required. Simon Ohm established experimentally that the current in a metal wire is proportional to the potential difference applied to its ends.
  • Slide 41
  • Ohms Law Resistance Units: Ohms Current Units: Amperes Electric potential Units: Volts v Every element in a circuit obeys Ohms Law

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