Download 4 - Electric Circuits

Electric Circuits
The three basic components of an electric circuit:



Electric Current
Electric Resistance
Potential Difference (Voltage)
Electric Current

Electric Current: The flow of
electric charge
 The
loosely bound outer
electrons of conductors
carry the charge through
circuits
 Protons tightly bound to the
nuclei of atoms
Electric Current

current = charge / time
or
I = q/t
Units: Amps (A)
 An
amp is the flow of 1 C of
charge per second
 NOTE: 1 C = the charge of
6,240,000,000,000,000,000
electrons
Electric Current

Usually the number of electrons
entering a wire is the same as
the number leaving
 This gives the wire a net
charge of zero
Voltage Sources


Voltage Source: A device which
provides a potential difference in
order to keep current flowing
 Dry/Wet Cells: Convert chemical
energy to electrical energy
 Generators: Convert mechanical
energy to electrical energy
The voltage available to electrons
moving between terminals is
called electromotive force, or emf.
Voltage Sources
Note:
Voltage
flows across a
circuit
Current
circuit
flows through a
Current vs. Voltage
Current – Flow rate
Measured in Amperes
Amount of flowing water
 Voltage – Potential
Measured in Volts
Water Pressure

Electric Resistance

Electric Resistance: The ability of a
material to resist the flow of charge
 Units: Ohms (W)
 The amount of charge that flows
through a circuit depends on two
things:
Voltage provided by source
Electric resistance of the
conductor
Resistors

What is the resistance of this
resistor?
Review

What is the resistance of this
resistor?
Resistance in a Wire
Electric Resistance Factors



Thick wires have less resistance
than thin wires
Short wires have less resistance
than long wires
Higher temperatures usually
cause more resistance
 The resistance in some
materials becomes almost
zero at very low temperatures
Ohm’s Law

The current in a circuit is
 Directly
proportional to the
voltage across the circuit
 Inversely proportional to the
resistance of the circuit
Therefore:
Current = Voltage/Resistance
or
I = V/R
Ohm’s Law: Sample Problems

How much current flows in a 1000-ohm
resistor when 1.5-volts are impressed
across it?
V = 1.5V
R = 1000Ω
Ohm’s Law: Sample Problems

How much current flows in a 1000-ohm
resistor when 1.5-volts are impressed
across it?
V = 1.5V
R = 1000Ω
Ohm’s Law: Sample Problems

How much resistance allows an impressed
voltage of 6-Volts to produce a current of
0.006-Amps?
V = 6V
R=?
Ohm’s Law: Sample Problems

How much resistance allows an impressed
voltage of 6-Volts to produce a current of
0.006-Amps?
V = 6V
Ohm’s Law: Sample Problems
V=?
Ohm’s Law: Sample Problems
V = 45V
Electric Power

Electric Power

How much power is being dissipated across
a 10Ω resistor when it draws 3A of current
form a 30V battery?
V = 30V
P = IV
= (3A)(30V)
I = 3A
= 90W
R = 10Ω , P = 90W
Electric Power

Electric Power

What is the power when a
voltage of 60V drives a load
with a resistance of 20Ω?
Electric Power

How much current does a100W lamp draw when connected
to 120 V?
Electric Power

Kilowatt-Hour (kWh): The
amount of energy consumed
in 1 hour at a rate of 1 kW
Example: In a place where
energy is 5¢ per kWh, a 100
W light bulb can be lit for 10
hours for 5¢
Jacksonville Electric Authority
JEA charges 12¢ per kWh if
you use less than 2,750 kWh a
month and 13¢ if you use
more than 2,750 kWh a month.
Electrical Tools

Voltmeter


Ammeter


Measures potential difference
Measures the flow of charge
Ohmmeter

Measures the resistance to the
flow of charge
Ohm’s Law and Electric Shock


The current flowing through the
human body depends on two
factors:
 Voltage applied to the body
 Resistance of the body
The resistance of the human body
could range from 100W (soaked in
salt water) to 500,000W (very dry)
 The lower the resistance, the
greater the shock
Ohm’s Law and Electric Shock
Current
Effect
0.001 A
(1 mA)
Can be felt
0.005 A
(5 mA)
0.010 A
(10 mA)
Painful
Involuntary muscle contractions
(spasms)
0.015 A
(15 mA)
Loss of muscle control
0.070 A
(70 mA)
If through the heart, serious disruption.
More than 1 second, probably fatal
End