16.1
Electrical Current
Electric Current
Electric Current
When the ends of an electric conductor are at different
electric potentials, charge flows from one end to the other
Flow of Charge
 Charge flows when there is a potential difference, or
difference in potential (voltage), between the ends of a
conductor. The flow continues until both ends reach the
same potential.
 When there is no potential difference, there is no longer
a flow of charge through the conductor.
 To attain a sustained flow of charge in a conductor, one
end must remain at a higher potential than the other.
 The situation is analogous to the flow of water.
Flow of Charge
Water flows from higher pressure to lower pressure. The flow
will cease when the difference in pressure ceases.
Flow of Charge
Water continues to flow because a difference in pressure is
maintained with the pump. The same is true of electric current.
Electric Current
 Electric current is the flow of electric charge.
 In solid conductors, electrons carry the charge through
the circuit because they are free to move.
Electric Current
Measuring Current
 Electric current is measured in amperes, symbol A.
 An ampere is the flow of 1 coulomb of charge per second.
 When the flow of charge past any cross section is 1 coulomb
(6.25 x 1018 electrons) per second, the current is 1 ampere.
Electric Current
Formula:
Current = (charge / time)
I=Q/t
I = Current (amps)
Q = amount of charge (coulombs)
t = time (seconds)
Units:
coulombs / second = Amperes or Amps
1C/1s=1A
Electric Current
Most small currents are expressed in:
milli-amperes (mA): 1 mA = 1 x 10-3 A
micro-amperes (µA): 1 µA = 1 x 10-6 A
Voltage Sources
Voltage Sources
 Charges do not flow unless there is a potential difference.
 Something that provides a potential difference is known as a
voltage source.
 Batteries and generators are examples of voltage sources
that are capable of maintaining a continuous flow of
electrons.
Voltage Sources
Steady Voltage Sources
 In a battery, a chemical reaction releases electrical
energy.
 Generators—such as the alternators in automobiles—
convert mechanical energy to electrical energy.
 The electrical potential energy produced is available at
the terminals of the battery or generator.
Voltage Sources
The potential energy per coulomb
of charge available to electrons
moving between terminals is the
voltage.
The voltage provides the “electric
pressure” to move electrons
between the terminals in an
electric circuit.
Voltage Sources
Low
Potential
High
Potential
Electron
Pump
When the potentials are equal, the current stops flowing.
To increase the potential of the electrons, an electron
pump must convert (do work) another form of energy
into electrical potential energy.
+
-
Voltage Sources
Distinguishing Between Current and Voltage
There is often some confusion between charge flowing through a
circuit and voltage being impressed across a circuit.
Consider a long pipe filled with water:
• Water will flow through the pipe if there is a difference in
pressure across the pipe or between its ends.
• Water flows from high pressure to low pressure.
What are two voltage sources used to provide the
energy that allows charges to move steadily?
Batteries and Generators
Types of Current
There are two types of current:
1.
2.
DC Current
AC Current
Types of Current
DC Current
By DC, we mean direct current, which refers to a flow of charge
that always flows in one direction.
• A battery produces direct current in a circuit because the
terminals of the battery always have the same sign of
charge.
• Electrons always move through the circuit from the
negative terminal toward the positive terminal.
Types of Current
AC Current
Alternating current (AC), as the name implies, is electric current
that repeatedly reverses direction.
• Electrons in the circuit move first in one direction and then
in the opposite direction.
• They alternate back and forth about relatively fixed
positions.
Which way does current flow?
Conventional Current
Flow of current from positive terminal to the negative
terminal.
Electron Current
Flow of current from the negative terminal to the positive
terminal.
Direction of Current
Direction of Current:
Current in Electric Circuits
For current to flow, an electric circuit
needs to be a closed loop.
Basic electric circuits consist of three
things:
• Electron pump (Battery)
• Device that reduces potential (User)
• Conducting connection (Wires)
What affects current flow?
1.
Potential Difference (Voltage)
2. Resistance (Slowing or reduction of the flow of current )
Assessment Questions
Example #1
A charge of 30 Coulombs passes through a 24 Ω resistor in 6
seconds. What is the current through the resistor?
Assessment Questions
Example #2
Charge flowing at a rate of 2.5 x 106 elementary charges per
second is equivalent to a current of:
Assessment Questions
Example #3
The current though a light bulb is 2 amps. How many coulombs
of electric charge pass through the light bulb in one minute?
Assessment Questions
Example #4
The current going from the cathode to the screen in the
television picture tube is 5.0 x 10-5 amperes. How many
electrons strike the screen in 5 seconds?
Assessment Questions
Example #5
A current of 5 A flowed in a copper wire for 20 seconds.
a. How many coulombs of charge passed through the wire
at this time?
b. How many electrons flowed through this wire at this
time?
Assessment Questions
Example #6
Electric charge will flow in an electric circuit when:
a.
b.
c.
d.
electrical resistance is low enough.
a potential difference exists.
the circuit is grounded.
electrical devices in the circuit are not defective.
Assessment Questions
Example #7
The electric current in a copper wire is normally composed of:
a.
b.
c.
d.
electrons
protons
ions
amperes