These notes correspond to the Current and Circuits
unit we started in class today, and continue through
page 4 of the outline.
By class time tomorrow, you should have the notes
for OUTLINE pages # 1-4 copied (Voltage through
Equivalent Resistance)
VOLTAGE
Electric potential difference is the work that must be
performed against electric forces to move a charge
between two points, divided by the charge. The unit
for this is called a volt (V).
Voltage = the amount of electric potential energy in
a system. Voltage difference is what makes the
current flow:
Current flows from an area of high voltage to low
voltage.
Voltage is also used as a measure of the difference in
energy in two places on a circuit.
CURRENT
Current = the flow of electricity in a circuit. Current is
measured in Amperes (Amps).
Current flows from positive to negative.
This means that the current will exit the positive end of the
battery and flow through the circuit, returning to the
negative end.
You can also think of current as the rate at which charges
are moving through a system.
When electrons flow, they do not move in a straight line.
They travel through wires by colliding with the atoms of
metal in the conductor.
RESISTANCE
Resistance = how easily the current will flow through a particular
object. Measured in Ω (Ohms).
Low resistance means current flows easily; high resistance means
that current cannot pass very well.
There are two ways to control the current in a circuit:
1) Increase resistance more resistance = less current flowing. Every object that uses
electricity in a circuit will add resistance.
Most wires have low resistance.
2) Reducing or increasing the amount of voltage
Resistance is the result of the collisions between the flowing
electrons and the metal atoms in the wire. These collisions are
inelastic; the friction from the collisions is the origin of the
resistance.
Resistance in wires depends upon 4 characteristics:
1) Length of the wire:
shorter wires provide less resistance
2) Diameter (cross-section) of the wire:
wider (larger) diameter provides less resistance
3) Type of material:
example: copper offers less resistance than iron
4) Temperature:
lower temperatures will offer less resistance
LOWER
GREATER
Ohm’s Law
Ohm’s law explains the relationship between the
voltage, current and resistance in a circuit.
The law can be stated in a formula:
V= I x R
Where V = voltage (volts)
I = current (Amps or Amperes)
R = resistance (Ohms) Ω
Circuits
A circuit is a closed pathway that allows current to flow
through.
There are two main types of circuits.
Series circuits have a single pathway.
Parallel Circuits have two or more separate pathways for
current to travel.
We will also look at complex circuits which involve dual use
of series and parallel connections in a single circuit.
Common Circuit Diagram Symbols –
Please copy ALL of these onto your outline!!
+
-
lamp
three batteries
+
Switch (open)
-
Voltmeter
single battery
Resistor
lamp
Ammeter
Series Circuit
In a series circuit, the flow of current has one path.
The amount of current is the same for any point in the circuit.
Each bulb added to this circuit reduces the overall
brightness of the circuit.
The voltage in this circuit is different because it will drop as
each resistor is encountered.
Resistance is calculated by adding the ohms for each object.
Any break or switch in the series circuit will cause all of the
lights to lose the flow of current.
Draw the following circuit and calculate the total
resistance and current using Ohm's Law.
current flows in
this direction
0.5 Ω
V=9V
9V +
-
I=
?
R = 2.5 Ω
2Ω
Voltage Drop = amount of volts a particular resistor will
consume
* You must calculate total current before you can determine
voltage drop!
The total drop in voltage over a circuit should equal the
amount of voltage leaving the battery.
By the time the current returns to the (-) end of the battery, the
voltage reading should be 0 volts.
To calculate voltage drop for any resistor, R, multiply the total
current for the circuit times the resistance at R.
EXAMPLE:
R = 2Ω
Itotal = 1.5 A
1) Based on the information provided, what is the voltage
drop for the resistor, R?
2) If the second resistor has the same resistance, what is the
total resistance for the circuit? What is the total voltage for
the circuit?
Electric Power:
The rate at which energy is transferred in a system.
Power is measured in Joules/second (1 J/s = 1 Watt)
P=IV
Power = Current × Voltage
unit = Watts*
*Did you know that the electrical energy consumption
in your house is measured in Kilowatt-hours?
Operational Costs for Electronic Devices
1 kilowatt hour is the equivalent of 1000 Watts delivered
continuously over 3600 seconds.
Most standard appliances will consume less than 1 kilowatt;
exceptions include hot water heaters, stoves, clothes dryers
and microwave ovens.
Power companies use P = IV to account for the loss of power
due to too much resistance (for example, when power is being
sent a long distance from the plant to someone's home).
They increase voltage and decrease the current to decrease the
resistance. This does not affect the amount of power being
delivered.
In a parallel circuit, the flow of current has more than one
path.
The amount of current is not the same for any point in the
circuit. Each bulb in this type of circuit should be equally
bright.
Adding bulbs will not affect this.
Voltage – is the same across each branch
Any break or switch in the parallel circuit will cause only the
lights in that branch of the circuit to shut off.
Resistance in Parallel Circuits
To find resistance in a parallel circuit:
1 = 1 + 1 + 1
Rtotal R1
R2
R3
This method for calculating parallel resistance is also known as Kirchoff’s
Law.
Where Rtotal is the total resistance in the circuit, and R1, R2 and R3 are
resistors in the circuit.
Example #1:
(IN PENCIL - Insert a circuit diagram for this problem in your notes in
the location indicated).
Please DO NOT attempt these calculations on your own,
just do the circuit diagram tonight!
For a parallel circuit with 2 branches and a 9 volt battery, what is the total
resistance if R1 = 8 Ohms and R2 = 5 Ohms?
Example #2
A 22 Ω resistor and a 33 Ω resistor are connected in
parallel over a 120 V source.
IN PENCIL: Insert illustration of the circuit as indicated:
Please DO NOT attempt these calculations on your own,
just do the circuit diagram tonight!
a)
b)
c)
d)
What is the total resistance?
What is the total current?
What is the current through R1?
What is the current in the 33Ω resistor?
Equivalent resistance of parallel branches.
There is another way of determining total resistance in a
parallel circuit (but only for certain cases).
If the resistance in each branch is equal, then the
total or equivalent resistance of the branches is
equal to the resistance of one branch divided by the
number of branches.