Physics
Flow of Charge
 Charge flows when there is a potential difference
 When the ends of an electric conductor are at different
electric potentials, charge flows from one end to the
other
 The flow of charge will continue until both ends reach a
common potential
Electric Current (I)
 Electric current = the flow of electric charge
 e- carry the charge through the circuit because they are
free to move throughout the atomic network
 Electric current is measured in amperes (A)
 ampere = the flow of 1 coulomb of charge per second
current = charge
time
I = Δq
Δt
What is the net charge of a current
carrying wire (as pictured above)?
A current-carrying wire has a net electric charge of zero
because the number of electrons entering one end is the
same as the number leaving the other.
Current (I) vs. Voltage (V)
 Charges flow when there is a potential difference (V)
 A voltage source provides a potential difference
 Ex: batteries, generators
 Charges flow through a circuit because of an applied
voltage across the circuit
Voltage (V) causes current (I)
If I increase the potential difference (voltage), will the current…
(a) increase or
(b) decrease?
Increase! V α I
Resistance (R)
 We now know:
 Amount of charge that flows in a circuit depends on the
voltage provided by the voltage source (V α I)
 But…
 The current also depends on the resistance (R) that the
conductor offers to the flow of charge—the electric
resistance
 Resistance is measured in ohms (Ω)
 Resistance (R) of a wire depends on:
 Conductivity of the material used in the wire
 Better conductors have less resistance
 Thickness of the wire
 Thick wires have less resistance than thin wires.
 Length of the wire
 Longer wires have more resistance than short wires
 Temperature
 For most conductors, increased temperature means increased
resistance
The longer an extension cord, the greater its resistance, and
Why will an electric drill operating on a very long extension
the more energy is dissipated in it, with less available for the
cord not rotate as fast as one operated on a short cord?
electric drill.
What do we know?
How is current (I) related to voltage (V)?
IαV
How is current (I) related to resistance (R)?
I α 1/R
Combining these findings…
I = V/R
Ohm’s Law
 The current in a circuit is directly proportional to the
voltage impressed across the circuit, and is inversely
proportional to the resistance of the circuit
I = V/R
V = I*R
Units:
V = volts (V)
I = amps (A)
R = ohms (Ω)
Examples
 If the voltage impressed across a circuit is constant but
the resistance doubles, what change occurs in the
current?
V = IR
V = 2R*_I
V = 2R * ½ I
The current is halved
 How much voltage is required to make 2 amperes flow
through a resistance of 8 ohms?
V = IR
V=2A*8Ω
V = 16 volts
Ohm’s Law and Electric Shock
 The damaging effects of electric shock are the result of
current passing through the body
 Body’s resistance = 100 ohms if you’re soaked with salt
water to about 500,000 ohms if your skin is very dry
Why don’t birds get fried?
 Every part of the bird’s body is at the same high
potential as the wire, so the bird feels no effects
 For the bird to receive a shock, there must be a
difference in potential between one part of its body
and another part
Jane falls from a bridge and manages to grab onto a high-voltage
power line, halting her fall. Will she be ok??
As long as Jane touches nothing else of different potential, she will receive no
shock at all.
Even if the wire is thousands of volts above ground potential and even if she hangs
by it with two hands, no charge will flow from one hand to the other.
If, however, you reach over with one hand and grab onto a wire of different
potential, ZAP!!
Source of e- in a circuit
 The source of electrons in a circuit is the conducting circuit
material itself
 Electrons DO NOT come from:
 electric outlets in the walls of homes
 power lines
 Power utilities do not sell electrons. They sell energy. You
supply the electrons.
 When you are jolted by an AC electric shock, the electrons
making up the current in your body originate in your body.
 Electrons do not come out of the wire and through your body
and into the ground; energy does.
Electric Power
 A charge moving in a circuit expends energy
 This may result in heating the circuit or in turning a
motor
 Electric power (P) = the rate at which electrical
energy is converted into another form such as
mechanical energy, heat, or light
 Measured in Watts (W)
 Electric power is equal to the product of current and
voltage
P = V*I
P = I2*R
Example
 Calculate the power supplied to an electric blanket
that carries 1.20 A when connected to a 120-V outlet.
P = V*I
P = 120 V*1.20 A
P = 144 W
Physics
Electric Circuits
 Circuit = Any path along which electrons can flow
 For a continuous flow of electrons, there must be a
complete circuit with no gaps
 A gap is usually provided by an electric switch that can
be opened or closed to either cut off or allow electron
flow
In Series
In Series
 Total resistance to current in the circuit is the sum of the
individual resistances along the circuit path
 Rtotal = R1 + R2 + R3
 Current passing through each electric device is the same

I1 = I2 = I3
 Vtotal = I*Rtotal
 Ohm’s Law applies across each individual device

V1 = I*R1
V2 = I*R2
V3 = I*R3
 Sum of the voltage drops across the individual devices is
equal to the total voltage supplied by the source

Vtotal = V1 + V2 + V3
Example 1: Series
 Find the total resistance of the three resistors
connected in series.
Example 2: Series
 What is the current through the battery?
R=4Ω
Example 3: Series
 Find the resistance of the unknown resistor, R.
In Parallel
In Parallel
 Overall resistance of the circuit is less than the resistance
of any one of the branches
 1/Rtotal = 1/R1 + 1/R2 + 1/R3
 Voltage is the same across each device
 V1 = V2 = V3
 Total current in the circuit equals the sum of the currents
in its parallel branches
 Itotal = I1 + I2 + I3
 Ohm’s Law applies across each individual device
 V = I1*R1 V = I2*R2
V = I3*R3
Example 1: Parallel
 Find the total resistance of the three resistors
connected in parallel.
Example 2: Parallel
 Find the current through the 2 ohm resistor.
Example 2: Parallel
 Three resistors are connected in parallel. If placed in a
circuit with a 12-volt power supply. Determine the
equivalent resistance, the total circuit current, and the
voltage drop across and current in each resistor.
Combining Resistors
 Series
 Parallel
Rtotal = R1 + R2 + R3
1
R Total

1
R1

1
R2

1
R3
Example 1: Circuits
 Find the total current passing through the circuit.
Example 2: Circuits
 Find the current passing through the 9 ohm resistor.