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Learn about potential difference, energy transfer, electrical power, major sources of EMF, Ohm's Law, and more in DC circuit analysis. Understand the concepts with clear explanations and examples.
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Electrical Circuit (DC) analysis Sections 5.1-5.2: pages 186-201; 208-214
Potential difference (in a circuit) Energy GAINED by electrons: • The electrons in a wire gain potential energy when passing through the cell or battery. • Electromotive force (emf) is the difference in the energy per unit charge (J·C-1, or Volts (V)) that the electrons have gained while passing through the cell or battery • NOT a force! • e.m.f is the work per unit charge made available (supplied) by an electrical source
Potential difference (in a circuit) Energy USED by electrons • Potential Drop (difference): is the measure of the amount of energy per unit charge that has been used by an element within the circuit. (i.e. an electron uses energy to pass through a resistor; the amount of energy per unit charge (voltage) used is equivalent to the “Potential Drop” or “Voltage Drop” across that resistor.) • Resistors: energy transitions mainly to internal (thermal) • Lamps: energy transitions to radiant and internal/thermal
Electrical Power • Power: the rate at which work is done; the rate at which energy is transferred/used • In a conductor with a potential difference, V, and a current, I, passing through it: The amount of charge: The energy transferred: • Power used in a circuit:
Major sources of e.m.f • Electromagnetic • When a coil of wire is rotated in a magnetic field, an induced current is produced • Power stations use generators such as these to produce a current • Chemical • Oxidation-reduction reactions transfer electrons between chemicals • Dry cells, fuel cells, and batteries
Major sources of e.m.f • Photoelectric effect • Electrons are emitted from certain metal surfaces when high frequency light is directed onto their surfaces • Photocells used in watches, clocks, automatic doors, etc. • Piezoelectric effect • Certain crystals can produce a charge on one side when placed under stress; a potential difference results • Used in Crystal microphones
Major sources of e.m.f • Thermoelectric effect • When two pieces of certain metals are wound together and one end is heated while the other is cooled, a current is produced • Thermocouples—used in temperature-measuring devices (typically high-temperature measurements)
Review: What is necessary for a DC Circuit? • a Complete Circuit has: • A conducting pathway connecting two ends of … • A Source of Potential Difference, Allowing current to flow through a… • Resistive Electrical Componentthat consumes electrical power • Note: DC = direct current current always flows in the same direction
Conventional Current vs. Electron Flow • Historically, the direction of the flow of charge carriers was defined to be from positive to negative • It is now known that current is a flow of electrons from negative to positive, however… • CONVENTIONAL CURRENT STILL IS DRAWN AND INTERPRETED AS FLOWING FROM POSITIVE TO NEGATIVE!
Lab Break: Pivot Interactives • Get a computer and log into your Pivot Interactives account • Complete the activity: Relationship between Current and Potential Drop in Conductors • Complete all data and analysis questions online during today’s and tomorrow’s class period.
Post-Lab: Current and Voltage • How were the current and the voltage drop measured in the interactive video lab? • Ammeter: a device used to measure the current flowing in a circuit or branch of a circuit • Always connected in series with the element for which the current is being measured • Ideal ammeter has zero resistance • Voltmeter: A device used to measure the potential drop across a circuit element • Always connected in parallel with the element for which the potential drop is being measured • Ideal voltmeterhas extremely (infinitely) high resistance Draw a circuit diagram for last week’s lab set-up
Post-Lab: Current and Voltage • What was the general relationship between the potential drop across your resistor and the current flowing through the resistor?
Ohm’s Law • “Provided the physical conditions, such as temperature, are kept constant, the resistance is constant over a wide range of applied potential differences, and therefore the potential difference [across the resistor] is directly proportional to the current flowing [through the resistor].”
Ohmic devices (ohmic conductors) • Those devices which obey the linear relationship of Ohm’s Law at all potential differences • Temperature of this device will NOT change due to the heating effect of the current as voltage changes • No device is truly completely Ohmic • Reserved for only an ideal resistor • Most devices would be considered Non-Ohmic
Potential Dividers • When 2 or more resistors are in series, the total potential supplied to the circuit is used between them • The potential is divided between them in a ratio determined by the ratio of the resistances. • The same current will flow through each; the potential drop will possibly be different. • Example:
Variable Resistors • A resistor which can be adjusted in order to change the current in a circuit in a controlled manner. • Also can be used as a potential divider • Common names/types: • Potentiometer – total potential drop is constant; potential drop across terminals of the V.R. changes due to resistance changes = potential divider • Rheostat – used to control current flow; potential remains constant; changing resistances changes current • Example uses: • Dimmer Switches; volume control knobs; speed of motor rotations; etc. • Circuitry symbol:
