Day 13 Current Electricity
LO: Current electricity involves continuously
moving electrons
LO: Definition of “Current” and “Amps”
AGENDA: Do Now
Do Now
Notes
HWp 609(1-5) and p 615(1-6)
Read Holt p. 608-611
Electrical Devices
Look around you. Chances are that there is an electrical
device nearby - a cell phone, a computer, a projector something that needs electrical energy to operate.
CD- Player
Typical Batteries and the Symbol
Used to Represent Them in Electric
Circuits
Electromotive Force (emf)
The energy needed to run a CD player, for
instance, comes from batteries.
Within a battery, a chemical reaction occurs that transfers
electrons from one terminal (leaving it positively charged) to another terminal
(leaving it negatively charged).
Because of the positive and negative charges on the battery terminals, an
electric potential difference exists between them. The maximum potential
difference is called the electromotive force* (emf) of the battery.
The electric potential difference is also known as the voltage, V.
The SI unit for voltage is the volt, after Alessandro Volta (1745-1827) who
invented the electric battery. 1 volt = 1 J/C.
Emf’s or Voltages of Common Batteries
•Car battery = 12 V
•AAA, AA, C, D = 1.5 V
•9-volt battery = 9 V
•Lantern battery = 6 V
Electric Current
The electric current is the amount of charge per unit time that passes
through a surface that is perpendicular to the motion of the charges.
Q
I 
.
t
The SI unit of electric current is the ampere (A), after the French mathematician André
Ampére (1775-1836). 1 A = 1 C/s. Ampere is a large unit for current. In practice
milliampere (mA) and microampere (μA) are used.
Direction of Current Flow
Electric current is a flow of electrons. In a circuit, electrons actually flow
through the metal wires.
Conventional electric current is defined using the flow of positive
charges.
It is customary to use a conventional current I in the opposite direction
to the electron flow.
AC and DC
•If the charges move around a circuit in the
same direction at all times, the current is said
to be direct current (dc), which is the kind
produced by batteries.
•In contrast, the current is said to be
alternating current (ac) when the charges
move first one way and then the opposite way,
changing direction from moment to moment.
Outlets give us ac voltage.
Electrical Resistance
When electric current flows through a metal wire there exists a hindrance to
the flow, known as electrical resistance.
This is because as the electrons move through they will collide with the atoms
of the conductor.
The SI unit of resistance is the ohm (Ω), after Georg Simon Ohm (1787-1854),
a German physicist, who discovered Ohm’s law, which will be discussed in
the next section.
A resistor is a material that provides a specified resistance in an electric
circuit.
Ohm’s Law
Ohm’s Law
Georg Simon Ohm (1787-1854), a German physicist, discovered Ohm’s law in
1826.
This is an experimental law, valid for both alternating current (ac) and direct
current (dc) circuits.
When you pass an electric current (I) through a resistance (R) there will be a
potential difference or voltage (V) created across the resistance.
Ohm’s law gives a relationship between the voltage (V), current (I), and resistance
(R) as follows:
V=IR
Units
Quantity
Symbol
Unit
Name
Unit
Abbreviation
Current
I
Ampere
A
Voltage
V
Volt
V
Resistance
R
ohm
Ω
Flashlight
Resistance,R and Resistivity,ρ
The resistance of a conductor is directly proportional to
the length since the current needs to pass through all
the atoms in the length.
The resistance is inversely proportional to the crosssectional area since there is more room for the current
to pass through.
The above observations can be combined and the
resistance, R of the conductor is written as follows,
L
R .
A
Resistivity of Materials
Resistivity is an inherent property of a material, inherent in the same
sense that density is an inherent property.
The Heating Element of an Electric
Stove
Electrical Energy
•Our daily life depends on electrical energy.
•We use many electrical devices that transform
electrical energy into other forms of energy.
• For example, a light bulb transforms electrical
energy into light and heat.
•Electrical devices have various power
requirements.
Electric Power,P
Energy
P
.
time
Since the electrical energy is charge times voltage (QV), the above
equation becomes,
QV
P
.
t
Since the current is charge flow per unit time (Q/t), the above equation
becomes,
QV Q
P
 V  I V .
t
t
Since V = IR, the above equation can also be written as,
2
V
P  IV  I 2 R 
.
R
SI Unit of Power: watt(W)
Killowatt-hour (kWh)
The SI unit of power is watt, after James Watt (1736-1819), who developed
steam engines.
joule
J
watt  W 
 .
sec ond s
Utility companies use the unit kilowatt-hour to measure the electrical energy
used by customers. One kilowatt-hour, kWh is the energy consumed for one
hour at a power rate of 1 kW.
20.5 Alternating Current
V = V 0 sin 2 p f t
Alternating Voltage from the outlet
Effective voltage ≈ 115 V, called the RMS value.
Electrons in a Circuit
• With no voltage on a wire, free electrons
move rapidly, but in random directions.
• The drift speed is much smaller than the
average speed between collisions
• When a circuit is completed, the electric field
travels with a speed of about 2/3 the speed
of light, causing electrons throughout the
wire to move almost instananeously.
Electrons in a Circuit
• In a wire, excess charge moves to the
outside of the wire.
Electrons in a Circuit
• In a wire, excess charge moves to the
outside of the wire.
• Near the positive terminal of a battery,
there is a small deficit of electrons on
the wire’s surface.
Electrons in a Circuit
• In a wire, excess charge moves to the
outside of the wire.
• Near the positive terminal of a battery,
there is a small deficit of electrons on
the wire’s surface.
• Near the negative terminal, there is a
small excess.
Electrons in a Circuit
• This creates an electric field within the
wire.

E
Electrons in a Circuit
• This creates an electric field within the
wire.
• The electric field pushes electrons
throughout the volume of the wire toward
the more positive direction

E
Electrons in a Circuit
• This creates an electric field within the
wire.
• The electric field pushes electrons
throughout the volume of the wire toward
the more positive direction
• This creates a current from higher to lower
voltage

E
I
Resistance
Resistance in a Circuit
• The faster electrons flow in the wire, the
greater is the current.
Resistance in a Circuit
• It takes energy from the battery to push
electrons onto the negative end of the
wire and to pull electrons from the positive
end of the wire.
Resistance in a Circuit
• It takes energy from the battery to push
electrons onto the negative end of the
wire and to pull electrons from the positive
end of the wire.
• Electrons in the wire lose this energy in
colliding with atoms within the wire.
Resistance in a Circuit
• If the voltage across the wire is greater,
the electrons move faster.

The more the wire opposes the flow
of current, the greater is the
resistance of the wire.
Resistance
• Resistance is defined as the ratio of the
voltage across a conductor to the current
flowing through it.
V
R
I
Units of Resistance
• Units of resistance are ohms (Ω)
–1Ω=1V/A
Georg Simon Ohm
• 1787 – 1854
• Formulated the
concept of
resistance
• Discovered the
proportionality
between current and
voltages
Ohm’s Law
• Often, resistance remains constant over a
wide range of applied voltages or currents
• This statement has become known as Ohm’s
Law:
V=IR
• If resistance is constant in a material, it is said
to be ohmic.
Ohm’s Law
• For an ohmic
material, a graph of I
vs. V is a straight
line.
• The slope of the line
is 1/R.
Ohm’s Law
• Non-ohmic materials
are those whose
resistance changes
with voltage
Resistivity
Resistivity
• the resistance of an ohmic conductor is
proportional to its length, L, and inversely
proportional to its cross-sectional area, A
Lof proportionality and is called
• ρ is the constant
R
the resistivity of A
the material
Temperature Variation of
Resistivity
• For most metals, resistivity increases with
increasing temperature
– With a higher temperature, the metal’s atoms
vibrate with increasing amplitude
– The electrons lose more energy in collisions
to the faster-moving atoms
Temperature Variation of
Resistivity
• For most materials, resistivity increases
approximately linearly with temperature
over a limited temperature range, so
– where A and B are constants.
  A  BT