16.3
Ohm’s Law / Energy and Power / Electric Meters
Voltage
Within a battery, a chemical reaction occurs that transfers electrons
from one terminal to another terminal.
This potential difference across the terminals is called the voltage.
Ohm’s Law
 Voltage produces a flow of charge, or current, within a
conductor.
 The flow is restrained by the resistance it encounters.
 The rate at which energy is transferred by electric current is
power.
Ohm’s Law
The relationship among voltage, current, and resistance is
called Ohm’s Law.
Ohm’s Law states:
Current in a circuit is directly proportional to the voltage
across the circuit, and is inversely proportional to the
resistance of the circuit.
Ohm’s Law
Voltage
(Volts)
(Joules / Coulomb)
Current
(Amperes)
(Coulomb / Sec)
Drop across a
resistive device
Current passing
through a
resistive device
Resistance
(Ohms Ω)
Ohm’s Law
Ohm’s Law
Ohm’s Law
Example #1
The current in a wire is 24 amperes when connected to a 1.5
volt battery. Find the resistance of the wire.
R = 0.0625 Ω
Ohm’s Law
Example #2
In a simple electric circuit, a 24 Ω resistor is connected across a
6 volt battery. What is the current in the circuit?
I = 0.25 A
Ohm’s Law
Example #3
A high-beam filament of an automobile headlight carries a current
of 4.5 amps. The voltage difference between its terminals is 12 V.
Calculate the resistance of the filament.
R = 2.667 Ω
Ohm’s Law
Example #4
Calculate the current when a 12-V battery is connected across a 4
Ω resistor.
I=3A
Ohm’s Law
I=2A
V = 12 V
R = 6Ω
I=4A
V = 12 V
R=3Ω
Electric Circuits
Electric Circuits
In an electric circuit, an energy source and an energy consuming
device are connected by conducting wires through which electric
charges move.
Electric Circuits
Electric Circuits are typically represented using diagrams know
as schematics.
Schematics are simplified, standard representation in which
common circuit elements are represented with specific
symbols, and wires connecting the elements in the circuits are
represented by lines.
Circuit Symbols
Electric Circuits
In order for current to flow through a circuit, you must have a
source of potential difference.
Typical sources of potential difference are batteries (which are
just two or more cells connected together), and power
supplies (electron pumps).
In drawing a cell or battery on a circuit schematic, remember
that the longer side of the symbol is the positive terminal.
Electric Circuits
 Electric circuits must form a complete conducting path
(closed loop) in order for current to flow.
 In the example circuit shown below left, the circuit is
incomplete because the switch is open, therefore no current
will flow and the lamp will not light.
Electric Circuits
 In the circuit below, the switch is closed, creating a closed
loop path. Current will flow and the lamp will light up.
Energy and Power
Just like mechanical power is the rate at which mechanical
energy is expended, electrical power is the rate at which
electrical energy is expended.
Formula(s):
P  IV
2
P  I IR  I R
V
V 
P   V 
R
R
2
SI Unit of Power: Watts (W)
Energy and Power
Example #1
A 110 volt toaster over draws a current of 6 amps on its highest
setting as it converts electrical energy into thermal energy. What
is the toaster’s power rating?
P = 660 W
Energy and Power
Example #2
A potential drop of 50 volts is measured across a 250 Ω resistor.
What is the power in the resistor?
P = 10 W
Energy and Power
Example #3
How much electrical energy is required to move 4µC charge
through a potential difference of 6 volts?
W = 2.4 x 10-5 J
Energy and Power
Example #4
What is the resistance of a 60 watt light bulb operated at 120
volts?
R = 240 Ω
Electric Meters - Voltmeters
 Voltmeters are tools used to measure the potential
difference (voltage) between two points in a circuit.
 The voltmeter is connected in parallel with the element to
be measured, meaning an alternate current path around the
element to be measured and through the voltmeter is
created.
 Voltmeters have very high resistance so as to minimize the
current flow through the voltmeter and the voltmeter's
impact on the circuit.
Electric Meters - Voltmeters
In the diagram below, a voltmeter is connected to correctly
measure the potential difference (voltage) across the lamp.
Electric Meters - Ammeters
 Ammeters are tools used to measure the current in a circuit.
 The ammeter is connected in series with the circuit, so that
the current to be measured flows directly through the
ammeter.
 Ammeters have very low resistance to minimize the
potential drop (voltage) through the ammeter and the
ammeter's impact on the circuit, so inserting an ammeter
into a circuit in parallel can result in extremely high
currents and may destroy the ammeter.
Electric Meters - Ammeters
In the diagram below, a ammeter is connected to correctly
measure the current flowing through the circuit.
Electric Meters
Electric Meters - Voltage
Electric Meters - Series
Electric Meters
Electric Meters
Electric Meters
Example #1
In the electric circuit diagram, possible locations of an ammeter
and voltmeter are indicated by circles 1, 2, 3, and 4. Where should
the ammeter be located and where should a voltmeter be located
to correctly measure the total current and voltage?
Electric Meters
Example #2
Which circuit diagrams below correctly shows the connection of
ammeter A and voltmeter V to measure the current through and
potential difference across resistor R?
Electric Meters
Example #3
A student uses a voltmeter to measure the potential difference
across a resistor. To obtain a correct reading, the student must
connect the voltmeter:
a.
b.
c.
d.
In series with the resistor
In parallel with the resistor
Before connecting the other circuit components
After connecting the other circuit components