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Transcript
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 𝑾 𝑷= ∆𝒕 • 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: 𝒒 𝑰= ∆𝒕 𝑾 𝑽= 𝒒 𝑃= 𝑊 ∆𝑡 = ∆𝒕 = 𝒒 𝑰 𝑾 = 𝑽𝒒 𝑉𝑞 𝑞 𝐼 = 𝐼𝑉 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 temperaturemeasuring devices (typically high-temperature measurements) a Complete Circuit has: A conducting pathway connecting two ends of … A Source of Potential Difference, Allowing current to flow through a… Resistive Electrical Component that consumes electrical power Note: DC = direct current current always flows in the same direction 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! Post-Lab: Current and Voltage • How were the current and the voltage drop measured in last week’s 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 voltmeter has 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 Non-Ohmic vs. Ohmic devices— Current vs. Voltage Potential Dividers Variable Resistors Semiconducting Diodes Thermistors Circuit Analysis using Ohm’s Law