Water vs. Electric Current
There are many analogous properties
between water and electric current
Characteristics such as
• Voltage is a measure of electric
potential energy, just like height is a
measure of gravitational potential energy.
• Voltage is measured in volts (V).
Using a voltmeter
• A voltage difference of 1 volt means 1 amp of
current does 1 joule of work in 1 second.
Voltage source
Charges do NOT flow unless there is potential difference
A voltage source is needed to provide a sustained potential
difference i.e. batteries or generators
The battery or source is represented by an escalator which raises
charges to a higher level of energy.
Current is a flow of charge
• Electric current is measured in units called
amperes, or amps (A) for short.
• One amp is a flow of a certain quantity of
electricity in one second.
• The amount of electric current entering a
circuit always equals the amount exiting the
Current Electricity Example
• The continuous flow of charge
in a complete circuit.
Electrical resistance
• Resistance measures
how difficult it is for current to flow.
The ohm
• Resistance is measured in (W).
• One ohm is the resistance when a voltage of
1 volt is applied with a current of 1 amp.
Ohm’s Law  The formulae
Ohm Quiz
Ohm plate
Ohm on the range
Ohm sweet Ohm
Ice cream
Ohm Alone
Practice Quiz
1. If the resistance of your body were 100,000 ohms, what would be
the current in your body when you touched the terminals of a 12 volt
= 12 V / 100000
0.00012 A
2. If your skin were very moist so that your resistance was only 1000
ohms, and you touched the terminals of a 24 volt battery, how much
current would you draw?
= 0.024 A
= 24 V / 1000
Power (watts)
P = VI
Voltage (volts)
Current (amps)
Electric Power
The work done by an
electric current moving
through a circuit is given
Power = Volts x Amps
1 watt = 1 volt x 1 amp
Power Equations
Power (P)
Rate at which work is performed
Measured in watts (W)
• P = V I
P = (I R) I = I 2 R
Example problem
• A light bulb with a resistance of
1.5Ω is connected to a 1.5-V
battery in the circuit shown at
• Calculate the power used by the
light bulb.
• 1. Find the IT = V / R
= .5 A
• 2. Find the P = I V
= .75 watts
Power Dissipated in an Electricity Distribution System
150 miles
120 Watt
Light bulb
Power Plant
on Colorado River
12 Volt
Connection Box
• Estimate resistance of power lines: say 0.001 Ohms
per meter, times 200 km = 0.001 W/m  2105 m
= 20 Ohms
• We can figure out the current required by a single
bulb using P = VI so I = P/V
=120 Watts/12 Volts
= 10 Amps
Power Dissipated in an Electricity Distribution System
150 miles
120 Watt
Light bulb
Power Plant
on Colorado River
12 Volt
Connection Box
• Power in transmission line is
P = I 2R
= 102  20 = 2,000 Watts!!
• “Efficiency” is έ = 120 Watts/4120 Watts = 0.3%
Series Circuit
Charges can move having
one SINGLE PATH for the charges to
• SERIES circuits, current can only take one path.
If one of the items in the circuit is broken then
NO charge will be able to flow
Adding resistances in series
The total resistance of the circuit a.k.a
effective resistance is equal to the
sum of the individual resistances
Rtotal = R1 + R2 + R3...
Individual resistances Ω
Total resistance
( ohms Ω )
• The current is numerically equal to the voltage
supplied – Ohm’s Law
Because there is only
ONE possible path
Voltage in a series circuit
• Each separate resistance creates a
VOLTAGE DROP as the current passes through.
The voltage drop across each device depends on its
Total voltage divides among the devices
• As current flows along a series circuit, each type
of resistor has an effect
Voltage applied to Series Circuits
The sum of the potential drops equals the
potential rise of the source.
The total voltage is the sum of the voltage on
each component
Ohm’s Law & Circuits
An important caveat to Ohm's Law
All quantities (voltage, current, resistance, and
power) must relate to each other in terms of the
same two points in a circuit.
Solve the Problem
What is the voltage drop across each resistor?
Step 1 Calculate RT
RT = R1 + R2 + R3
= 18 k Ω
Step 2 Calculate IT
IT = VT / RT
= 5 x 10-4 amps
Step 3
Calculate V for each resistor
V1 = 1.5 V
V2 = 5 V
V3 = 2.5 V
Parallel circuit
There are Multiple pathways for the current to flow through
If one of the items in the circuit is broken then charge will move
through other paths & will continue to have
charges flow through them
Parallel Circuits
• In parallel circuits the current can take more than
one path.
• Because there are multiple branches, the current is
not the same at all points in a parallel circuit.
The inverse of the total resistance of the circuit
(also called effective resistance) is equal to the sum
of the inverses of the individual resistances
Example problem for resistance in parallel
• A circuit contains a 2 ohm resistor and a 4
ohm resistor in parallel.
• Calculate the total resistance of the circuit.
1.33 ohms
Voltage applied to Parallel Circuits
The potential drops of each branch equals the
potential rise of the source
Current applied to Parallel Circuits
The total current is equal to the sum of the
currents in the branches
Parallel Circuit Rules
Important thing to notice
the more branches you add to a parallel circuit
(the more things you plug in)
the lower the total resistance becomes
As the total resistance decreases, the total current increases.
Why are Parallel circuits found in most household electrical wiring?
Advantages of parallel circuits
Parallel circuits have two big advantages over
series circuits:
1. Each device in the circuit sees the full
battery voltage.
2. Each device in the circuit may be turned off
independently without stopping the current
flowing to other devices in the circuit.
Practice Problems
What is the total resistance?
27.5 V
What is the total voltage?
What is the voltage and current on A, B, and C?
Voltage is

similar documents