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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