February 16, 2010 Potential Difference and Electric Potential.
Potential difference (in a circuit) - Physics Rocks! · Potential difference ... •What was the...
Transcript of Potential difference (in a circuit) - Physics Rocks! · Potential difference ... •What was the...
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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
𝑷 =𝑾
∆𝒕• In a conductor with a potential difference, V, and a
current, I, passing through it:
The amount of charge: 𝑰 =𝒒
∆𝒕 ∆𝒕 =
𝒒
𝑰
The energy transferred: 𝑽 =𝑾
𝒒𝑾 = 𝑽𝒒
• Power used in a circuit: 𝑃 =𝑊
∆𝑡=
𝑉𝑞𝑞
𝐼
= 𝐼𝑉
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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)
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
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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?
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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
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Potential Dividers Variable Resistors
Semiconducting Diodes Thermistors
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Circuit Analysis using Ohm’s Law