Download Direct Current (DC) Electric Circuits

Direct Current (DC)
Electric Circuits
Physics
Montwood High School
R. Casao
Electric Current I

An electric current is the movement of
positive and/or negative charges Q through
a conductor.
 Current I is the rate of charge movement
through a cross-sectional area of the
conductor.
Q
I
t
Electric Current I

The charge carriers in metallic conductors
are electrons.
Electric Current I
Charge is measured in coulombs C.
 The charge on an electron and a
proton is: Q = 1.602 x 10-19 C
 Current is measured in amperes A;

1C
1A 
s
Two Types of Current

DC current (direct current) is a
steady flow of current in one
direction.

AC current (alternating current) direction of current flow changes
many times a second. In the US, the
frequency of change is 60 Hz.
Therefore, the current changes
direction 60 times per second.
Electric Circuits

A simple electric
circuit will consist of:
– A source of energy (in
this case a battery).
– Conducting wires.
– A resistor R that uses
the energy.
– A switch to open/close
circuit.

The source of energy
has an internal
resistance r.
Electromotive Force (EMF)

Batteries, generators, and solar cells,
transform chemical, mechanical, and
radiant energy, respectively, into electric
energy. These are examples of sources of
EMF.

EMF is measured in Volts V;

The source of EMF provides the energy
the charge carriers will conduct through
the electric circuit to the resistor.
1J
1V 
1C
Potential Difference or Voltage V
Current in a circuit moves from an area
of high electric potential energy to an
area of low potential energy. This
difference in electric potential energy is
necessary for current to move through a
conductor.
 The positive terminal of a battery is the
high electric potential energy terminal
and the negative terminal is the low
electric potential energy terminal.
 Potential difference V is also measured in
volts.

Chemical Battery
Batteries separate
positive and
negative charges
by using a
chemical reaction.
Chemical potential
energy is
converted into
electrical energy.
Rechargeable Battery
Eventually the battery’s chemicals are consumed unless
the reaction can be reversed by passing a current into
the battery.
Automobile
battery is
recharged while
the gasoline
engine is running
since the engine
powers a generator
that produces a
recharging current.
Starting the car
Engine running
Potential Difference or Voltage V
Within the battery, a
chemical reaction
occurs that transfers
electrons from one
terminal to another.
 Because of the
positive and negative
charges existing on
the battery
terminals, a potential
difference (voltage)
exists between them.

Potential Difference or Voltage V



The battery creates an
electric field within and
parallel to the wire,
directed from the positive
toward the negative
terminal.
This field exerts a force on
the free electrons, causing
them to move. This
movement of charge is
known as an electric
current.
The current in the circuit
is shown to flow from the
positive terminal to the
negative terminal.
Potential Difference or Voltage V

EMF is the maximum amount of energy per
charge the battery can provide to the charge
carriers.
 Voltage is the energy per charge the charge
carriers have after moving through the
internal resistance r of the battery.
– Some of the energy added to the charge carriers
has to be used to travel through the battery.
– The remaining energy is carried to the resistors
outside the battery.
Batteries in Series and Parallel:
How can birds perch or squirrels run along high
voltage (1000’s of volts) wires and not be fried??
To receive a current (shock)
there must be a difference in
potential between one foot
and the other, but every part
of the bird or squirrel is at the
same potential as the wire. IF
they landed with one foot on
one wire and the other foot on
a neighboring wire at a
different voltage, ZAP!!!!!
Resistance R


Resistance is the
opposition to the flow
of charge through the
conductors.
Resistance of a solid
conductor depends
upon:
1. nature of the material
2. length of the conductor
3. cross-sectional area of
the conductor
4. temperature
Resistance



The resistance of a conductor is proportional to the
length.
– Resistance increases with increased length.
The resistance of a conductor is inversely
proportional to the cross-sectional area of the
conductor.
– Resistance decreases with increased crosssectional area.
Resistance is also dependent upon the temperature
of the conductor. Collisions of electrons with other
electrons and with atoms raises the temperature of
a material as the added heat energy causes the
electrons to move faster and hence collide more
often. This increases the resistance of the
conductor.
Resistance

Resistance is measured in ohms .
1V
1Ω 
1A
Resistivity  is related to the nature of the material.
Good conductors have low resistivity (or high
conductivity). Poor conductors have high
resistivity (or low conductivity).
 Unit for resistivity  is ·m.
 Resistance:

ρl
R
A

Resistivity:  = o + o ··(T – To)
Factors Affecting Resistance
Current
Water flows from the
reservoir of higher
pressure to the reservoir of
lower pressure; flow stops
when the pressure
difference ceases.
Water continues to flow because a
difference in pressure is
maintained with the pump.
Electric Current
Just as water current is flow of water molecules,
electric current is the flow of electric charge.
In circuits, electrons make up the flow of charge.
ON
OFF
Current and its Effect on the Human
Body
Current in Amps
Effect on human body
0.001 (1 mA)
Can be felt
0.005 (5 mA)
Painful
0.010 (10 mA)
Involuntary Muscle Spasms
0.015 (15 mA)
Loss of Muscle Control
0.070 (70 mA)
If through heart, serious disruption;
probably fatal if > 1 second
0.1-0.2 (100-200 mA)
Uncontrolled “twitching” of heart
> 0.2 (> 200 mA)
Heart stops, but may be able to be
revived easier than 0.1 – 0.2 A
Nervous System
Nervous systems in animals use
electrical currents to signal the
contraction and relaxation of
muscles.
Frog leg jumps when electrical
current passes through it.
Conduction in Human Heart
The most important electrical
signal in our body is the
periodic signal that contracts
and relaxes our heart muscle
to pump blood.
Without a constant flow of
blood the brain can suffer
permanent damage.
SA
AV
Conduction in Human Heart
The normal electrical conduction in the
heart allows the impulse that is
generated by the sinoatrial (SA) node
of the heart to be propagated to (and
stimulate) the myocardium (muscle of
the heart).
When the myocardium is stimulated, it
contracts, pumping blood in the body.
As the electrical activity is spreading
throughout the atria, it travels via
specialized pathways, known as
internodal tracts, from the SA node to
the Atrioventricular (AV) node.
The AV node functions as a critical delay
in the conduction system. Without this
delay, the atria and ventricles will
contract at the same time, and blood
won't flow effectively from the atria to
the ventricles.
SA
AV
Ohm’s Law
Ohm's Law deals with the
relationship between the voltage
and current in a conductor with
resistance R.
V
 Mathematically:

I
R
V  IR
types of circuit
There are two types of electrical circuits;
SERIES CIRCUITS
PARALLEL CIRCUITS
Series Circuit

Resistors can be
connected in series;
that is, the current
flows through them
one after another. The
circuit in Figure 1
shows three resistors
connected in series,
and the direction of
current is indicated by
the arrow.
Figure 1: Resistors
connected in series.
Series Circuit
Since there is only one
path for the current to
travel, the current
through each of the
resistors is the same. All
the charge carriers that
come out of the battery
must pass through each
resistor.
 I = I1 = I2 = I3

Series Circuit
Also, the voltage drops across
the resistors must add up to
the total voltage supplied by
the battery.
 The charge carrier will supply
energy to each resistor in the
circuit; the amount of energy
each resistor receives
depends upon the resistance
itself.
 The greater the resistance,
the more energy it uses.
 E = V1 + V2 + V3

Series Circuit

When working with circuits, it is
customary to simplify the circuit by
combining resistances in series into
a single equivalent resistor Req.
 The equivalent resistance is the
single resistance that could be used
in the circuit to replace the three
separate resistances.
 Req = R1 + R2 + R3
 If R1 = 2  , R2 = 4 , and
R3 = 8 , the equivalent resistance
Req = 2  + 4  + 8  = 14 
Parallel Circuits
Resistors can be
connected such that they
branch out from a single
point and join up again
somewhere else in the
circuit. This is known as
a parallel circuit.
 Each of the three
resistors is another path
for current to travel
between points A and B.

Parallel Circuits



Resistors in parallel have the
same voltage drop across
them. Voltage is constant in
parallel.
E = V1 = V2 = V3
The charge carriers come out
of the battery carrying the
same joules of energy per
coulomb of charge and when
they reach the junction point,
some charge carriers (i1) will go
through R1, some will go
through R2 as i2, and the rest
go through R3 as i3.
Parallel Circuits




Each charge carrier has the
the same joules/coulomb no
matter which resistor it
passes through, which is
why the voltage V is constant
in parallel.
The sum of the currents in
each parallel branch equals
the total current entering the
parallel branch of resistors.
In this example:
i = i1 + i2 + i3
Usually: IT = I1 + I2 + I3
Parallel Circuits
 At
every junction
point in a
parallel circuit,
the current that
enters the
junction point
must equal the
current that
exits the
junction point.
Parallel Circuits

Resistances in parallel are
also simplified into an
equivalent resistance Req.
1
1
1
1



R eq R1 R 2 R 3

My suggestion involves the
reciprocal (x-1) calculator key:

Req  R1
1
 R2
1
 R3

1 1
Parallel Circuits

If R1 = 2  , R2 = 4 ,
and R3 = 8 , the
equivalent resistance
Req = [(2 )-1 + (4  )-1
+ (8  )-1 ] -1 =
1.14286 
Toll Road—Circuit Analogy
Toll Booth Explanation
 Adding
toll booths in series
increases resistance and slows the
current flow.
 Adding toll booths in parallel
lowers resistance and increases
the current flow.
Electric Power






Electric power P is the rate of doing electrical
work.
Power is the product of current and voltage.
P = V·I
V2
2
P

I

R
P

Unit: Watt, W
R
1 W = 1 Joule/sec = 1 Volt·Amp
The total power in a series combination of
light bulbs and in a parallel combination of
light bulbs is the sum of the individual
wattages. Ex.: two 60 W light bulbs will
dissipate 120 W in a series combination as
well as in a parallel combination.
Electric Energy
Electric companies sell
you electrical energy.
Your energy
consumption is
computed by expressing
power in kilowatts and
time in hours. Energy is
sold to you in units of
kW-hr.
$ amount
 E = P·t
Cost  E 
 E = V·I·t
kW  hr

Ammeters and Voltmeters

Ammeters are used to
measure current and must
be placed in series with
the circuit component you
want to measure the
current through.
 Voltmeters are used to
measure the voltage drop
across the resistor or the
circuit and must be placed
in parallel with the
component you want to
measure the voltage drop
across.
Fuses & Circuit Breakers
Fuse is designed to melt (due to ohmic heating) when
current is too large.
Circuit breaker does same job without needing
replacement; flip the switch to reconnect.
Fuse
Circuit
Breaker
Fuses
•A fuse is a ribbon of wire
with a low melting point
•If the current gets too
large, the wire melts or
blows out
•When fuses blow out,
they open the circuit
•Once the circuit is open,
electricity cannot flow
through it
Breakers
•A circuit breaker works like a
fuse, but doesn’t need to be
replaced after it opens the
circuit
•Circuit breakers are made of
2 different metals
•If the current gets too large,
the metal moves and acts like a
switch to open the circuit
•You can reset the circuit
breaker which closes the circuit
so electricity can flow
TAKS Warning - Parts of a Light Bulb
•In order to light the
bulb, electricity travels
through one contact, up
the support wires,
through the tungsten
filament and back down
the other support wire
to complete the circuit.
•What do you think
happens in a light bulb
when it “burns out”?