Download parallel circuits

What’s a Circuit?
 A circuit is a closed path where
positive charges flow from high to low
potential. They can be manipulated on
the way.
The Power Source
 Provides the difference in potential (potential
energy per unit charge). It is measured in volts
(remember this)
 A Cell (battery) is the easiest to see. It converts
chemical energy to electrical energy.
 This is also called “Electromotive force” (or emf)
 Think of “Voltage” as “pressure” the
charges are moving.
What is Electric Current?
 An electric current is the flow of charge through wires
and components.
 The greater the current, the more charges are moving!
+
-
 In which direction does the electrons flow?
 It flows from the negative terminal to the positive terminal.
 Measured in Amperes (we say Amps)
Conventional Current
+ + + Electron
eflow
Conventional flow
Electron flow: The direction
of e- flowing from – to +.
+
Conventional current:
The motion of +q from +
to – has same effect.
Electric fields and potential are defined in terms
of +q, so we will assume conventional current
(even if electron flow may be the actual flow).
Resistance
 When an object (like a light bulb) resists or
diminishes the flow of current, it has
resistance.
 Resistance is measured
in Ohms
Symbol for Ohms: Ω
Water Analogy to EMF
High
pressure
Constriction Low
pressure
Water
Flow
Water
Pump
Valve
High
Resistor Low
potential
potential
+
I
-
R
Switch
E
Source of
EMF
The source of emf (pump) provides the voltage
(pressure) to force electrons (water) through
electric resistance (narrow constriction).
Ohm’s Law
 Relates the voltage (Volts), current (Amps), and
resistance (Ohms) in a circuit.
V  IR
 V = Voltage (in Volts)
 I = Current (in Amps)
 R = Resistance (in Ω)
Power
 Power describes the rate
at which electrical energy
is transferred.
 It is measured in Watts
(W).
P  IV
 P = Power (Watts)
 I = Current (Amps)
 V = Voltage (Volts)
2
V
P  IV  I R 
R
2
A light bulb has a resistance of 30Ω.
What voltage would be required to run
4 Amperes of current through the bulb?
A. 120 V
B. 7.5 V
C. .13 V
D. Voltron
A toaster is connected to a 120-Volt
circuit and has 6 Amps of current
running through it. What is the
resistance of the toaster?
A. 720 Ω
B. 20 Ω
C. .05 Ω
D. Depends on how
brave he is.
If your body resistance is 100,000 Ω,
how much current will you experience if
you touch the terminals of a 12-Volt
battery?
A. 1,200,000 A
B. 8,333 A
C. .00012 A
D. Depends on how
good it taste
How much power is dissipated in a
toaster if it is connected to a 120Volt circuit and uses 8 Amps?
A. 960 W
B. 15 W
C. .067 W
D. Zero, it is powered by
imagination
A light bulb has a power rating of 60-Watts.
How much current would it pull if it has a
resistance of 15 Ohms?
A. 4 A
B. 2 A
C. 900 A
D. 30 A
The End!
Circuit Diagram
We draw electric circuits using specific symbols
(because quite frankly most people can’t draw)….
cell
lamp
switch
wires
Types of Circuits
There are two basic types of electrical
circuits;
SERIES CIRCUITS
PARALLEL CIRCUITS
SERIES CIRCUITS
The components are connected end-to-end, one
after the other.
They make a simple loop for the current to flow
round.
If one bulb ‘blows’ it breaks the whole circuit and
all the bulbs go out. (One charge gets stuck, they
all get stuck).
PARALLEL CIRCUITS
The components are connected side by side.
The current has a choice of routes.
If one bulb ‘blows’ there is still be a complete circuit to
the other bulb so it stays alight.
Parts of a Circuit
Electrical circuits often contain one or more resistors
grouped together and attached to an energy source,
such as a battery. The wires need to make a complete
circle from positive to negative potential.
Circuit Diagram
We draw electric circuits using specific symbols
(because quite frankly most people can’t draw)….
cell
lamp
switch
wires
Types of Circuits
There are two basic types of electrical
circuits;
SERIES CIRCUITS
PARALLEL CIRCUITS
SERIES CIRCUITS
The components are connected end-to-end, one
after the other.
They make a simple loop for the current to flow
round.
If one bulb ‘blows’ it breaks the whole circuit and
all the bulbs go out. (One charge gets stuck, they
all get stuck).
PARALLEL CIRCUITS
The components are connected side by side.
The current has a choice of routes.
If one bulb ‘blows’ there is still be a complete circuit to
the other bulb so it stays alight.
“Short Circuit”
 Just remember that electricity is lazy, and will always
take the path of least resistance.
 If something (usually a wire) provides a path around a
resistor, the electrons will take it!
COMPLEX CIRCUITS
Is made up of both series and parallel circuits combined.
This is what most circuits in the “real world” are like.
The End!
Applying Ohm’s Law
 It can be used to analyze
a whole circuit or a single
component.
 For the circuit shown
 What is the total current?
 What is the voltage across
a single resistor?
 What is the voltage across
two resistors?
What if the circuit isn’t so simple?
 Can we find the total
current through this circuit?
 What information do we
need?
 Voltage is given
 Resistance is going to be a
little tougher.
 We need to find the
“equivalent resistance” of
the whole circuit.
Equivalent Resistance
 Remember that in
series, we just add
the resistors
 R1+ R2+ R3+ …
 In parallel, we can
use the formula at
right.
 Do the parallel part
1st and then add it to
the series resistor.
Now, back to the original problem:
 We have a total
resistance of 15Ω
and a voltage of 9V
V
i
r
9V
i
 0 .6 A
15
Isn’t electricity supposed to be
dangerous?
 Well, that depends.
 You would think that 1000V
would be more dangerous
than 100V, right?
 It all comes down to Ohm’s
law
 It is actually the current that
kills, not the voltage.
V
i
r
Current kills…Voltage hurts
 As current flows through muscle tissue, the




muscle fibers contract.
Anything over 10mA can cause you to grab
onto a wire and not be able to let go.
At around 75mA, you are unable to breathe
Between 100 – 200mA, the heart fibrillates
Actually, above 200mA, your chances are
better because the heart seizes completely
What determines how much
current flows?
 That depends on the voltage and resistance
 Your skin usually has around 100,000 –
500,000 Ω of resistance when dry.
 If you get wet or sweaty, that resistance
goes way down.
 Whether you get hurt depends on the
voltage and your skin’s resistance
 This is why we could play with the batteries,
but it would be dangerous to use wall outlets
What amount of current would
be used if a 10Ω resistor was
connected to a 240 V circuit?
A. 2400 A
B. 2400 I
C. 24 I
D. 24 A
A light bulb with a resistance of
10 Ω is hooked up to a circuit.
The power used by the bulb is
160 Watts.
A. What is the voltage difference is in the
circuit?
B. How much current is flowing through the
light bulb
If a 20 ohm resistor is
connected to a 30 ohm resistor
in series. Which resistor will
have a higher current.
A. The 30 Ohm one
B. The 20 Ohm one
C. Both have the same
D. Not enough info
If a 20 ohm resistor is
connected to a 30 ohm resistor
in series. Which resistor will
have a higher voltage.
A. The 30 Ohm one
B. The 20 Ohm one
C. Both have the same
D. Not enough info
If a 20 ohm resistor is
connected to a 30 ohm resistor
in parallel. Which resistor will
have a higher current.
A. The 30 Ohm one
B. The 20 Ohm one
C. Both have the same
D. Not enough info
AC/DC
AC/DC
• Direct vs. Alternating
• A circuit containing a battery is a DC circuit.
• In a DC circuit the current always flows in the same
direction.
• The electricity that you get from the power company is
AC.
• In an AC circuit the current reverses direction
• The voltage reverses
periodically.
polarity 60 times a
second.
• The current through the
bulb also reverses
direction 60 times a
second.
Duracell
+
Effects of
Resistance
 Resistors oppose or “resist” the current flowing
through them.
 Light bulbs do as well. The filament inside the bulb glows
hot from resisting current.
 The longer the resistor or light bulb filament, the
more resistance. The more stuff the current has
to push through
 The thicker the wire the less resistance. It has more
room to pass through the material.
 Even wire has a resistance, but it is usually very
small
If a 20 ohm light bulb is
connected to a 30 ohm bulb in
parallel. Which light bulb will be
the brightest.
A. The 30 Ohm one
B. The 20 Ohm one
C. Both have the same
D. Not enough info
If a 20 ohm light bulb is
connected to a 30 ohm bulb in
series. Which light bulb will be
the brightest.
A. The 30 Ohm one
B. The 20 Ohm one
C. Both have the same
D. Not enough info
Electrical
Stored Energy
 Electrical energy can
be stored in a device
called a capacitor.
 Capacitors are found
in nearly all electronic
circuits.
 The simplest capacitor
is a pair of conducting
plates separated by a
small distance, but not
touching each other.
• When the plates are connected to
a power source, charge is
transferred from one place to the
other.
• This occurs as the positive battery
terminal pulls electrons from the
plate connected to it.
• The capacitor plates then have
equal and opposite charges.
Capacitors
 The charging process is complete when the
voltage between the plates equals the voltage
between the battery terminals.
 The greater the battery voltage and the larger
and closer the plates, the greater the charge
that is stored.
 The net charge on the capacitor is still zero.
Discharge Capacitors
 A charged capacitor is discharged when
the plates make contact.
 Capacitors can still shock you even
when the device is turned off (ex: TVs).
Capacitance
• Capacitance is the measure of the ability of a device to
store charge for a given voltage.
q=CV
C = capacitance, in Farads
q = magnitude of charge on each plate
V = voltage difference
• SI unit for capacitance is the farad, F
• 1 F = 1 C/V
Remember
 Because some charges are very small, we
will use smaller units.
 You will need to remember how to convert.
10-6 Coulomb = 1 microCoulomb (μC)
10-6 Farad = 1 microFarad (μF)
Series Circuits
 Only one path for electricity to flow!
• If connected in series, the capacitors charged in
•
•
•
•
consecutive fashion.
Components in a series circuit share the same charge
q total = q1 = q2 …
In a series circuit, adding more capacitors causes the
overall charged stored within the circuit to decrease.
Total Voltage is equal to the sum of the individual voltage
drops
V total = V1 + V2 +….
To find total capacitance in a series circuit, you have to so
something similar to a resistors in parallel because total
capacitance decreases
Parallel Circuits
 More than one path for electricity to flow!
 All capacitors in a parallel circuit are charged all at the same



•
time
Components in a parallel circuit share the same voltage
Vtotal = V1 = V2 …
Total charge stored in a parallel circuit is equal to the sum of
the individual branch charges
qtotal = q1 + q2 +….
For parallel circuits, as the number of capacitors increases,
the overall charge stored also increases.
To find total capacitance in a parallel circuit, add the
individual capacitance
C total = C1+ C2+ C3+ …