Download Ohm*s Law and Electrical Power

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