Download Chapter 7 Notes

Chapter 7
Electricity
Electric charge
 Electric charge is an inherent physical
property of certain subatomic particles that is
responsible for electrical and magnetic
phenomena.


Charge is represented by the symbol q.
The SI units of charge is the coulomb (C).
 Like mass, electric charge is a fundamental
property of matter.

Unlike mass, there are two types of charge:

Positive and Negative.
2
Electric charge, cont’d
 Recall that every atom is composed of
electrons surrounding a nucleus.

The nucleus contains:



protons — positive charge.
neutrons — no charge.
Electrons — negative charge.
 An atom’s atomic
number is the number
of protons in the
nucleus.
3
Electric charge, cont’d
 An atom’s charge is
neutral if it has the
same number of
protons and electrons.
 An atom is said to be
ionized if the number
of electrons and
protons is different.
4
Electric charge, cont’d
 If the protons outnumber the electrons, there
is more positive charge.

We call such an atom a positive ion.
 If the electrons outnumber the protons, there
is more negative charge.

We call such an atom a negative ion.
5
Electric force and Coulomb’s
law
 When we bring electric charges together, they
exert a force on each other.




Positive charges attract negative charges.
Positive charges repel positive charges.
Negative charges attract positive charges.
Negative charges repel negative charges.
 We paraphrase this as:
Like charges repel, unlike charges attract.
6
Electric force and Coulomb’s
law, cont’d
 The law that describes the forces between
electric charges is called Coulomb’s law.
 Coulomb’s Law states that the force acting on
each of two charged objects is directly
proportional to the net charges on the object
and inversely proportional to the square of
the distance between them:
q1q2
F 2
d
7
Electric force and Coulomb’s
law, cont’d
 The force on q1 is equal and opposite to the
force on q2.


If the charges increase, the force increases.
If the distance increases, the force decreases.
8
Electric force and Coulomb’s
law, cont’d
 The proportionality constant has a value of
9×109 N-m2/C2.
 Coulomb’s law is then:

9 N  m  q1q2
F   9 10
 2
2
C  d

2



F is in newtons,
q1 & q2 are in coulmbs, and
d is in meters.
9
Electric field
 The electrostatic force is similar to gravity.

The force can act through a distance without
the two charges having to touch.
 Just as we talk about the gravitational field,
we can also define an electric field.
force on a charged object
electric field strength 
charge on the object
 The electric field lines indicate the direction
that a positive charge would move.
10
Electric field, cont’d
 Here are some illustrations of electric field
lines surrounding a
positive and a negative
charge.


The field lines point
away from the positive
charge.
The field lines point
toward the negative
charge.
11
Electric field, cont’d
 If the field points to the right:

A positive charge would also travel to the right.


The positive charge “thinks” there is a negative
charge at the end of the line.
A negative charge would travel to the left.

The negative charge
“thinks” there is a
positive charge at
the start of the line.
12
Electric field, cont’d
 An electrostatic precipitator is an example of
using electrostatics.


Negatively charged wire charge the soot
passing between the
plates.
The now negative
soot particles are
attracted to the
positively charged
plates.
13
Electric currents
 An electric current is a flow of charged
particles.


It is the rate of flow of electric charge.
The amount of charge that flows by per
second:
charge
q
current 
 I
time
t

The SI unit is the ampere (A or amp):

1 amp is 1 coulomb per second.
14
Electric currents
 Here are some
examples of
electric current.
 Positive current
is in the direction
of positive
charge flow.
15
Resistance
 Resistance is a measure of the opposite to
current flow.





Resistance is represented by R.
The SI units of resistance is the ohm (O).
A conductor is any substance that readily
allows charge to flow through it.
An insulator is any substance through which
charge does not readily flow.
A semiconductor are substances that fall
between the two extremes.
16
Resistance, cont’d
 Resistance of a wire depends on:




Composition. The particular substance from
which the object is made.
Length. The longer the wire, the higher the
resistance.
Diameter. The thinner the wire, the higher the
resistance.
Temperature. The higher the temperature, the
higher the resistance.
17
Resistance, cont’d
 Resistance is similar to friction.





Resistance inhibits the flow of electric charge.
Electrons typically produce the current in
metals.
The electrons collide with the atoms of the
metal.
This slows them down.
They also lose some energy to the atoms.

The metal gets hotter.
18
Resistance, cont’d
 Superconductivity is a phenomenon in which
a substance provides zero resistance to the
flow of electric charge.

It typically only occurs at rather low
temperatures.


The temperature below which
a substance superconducts is
called is critical temperature.
The latest superconductors
require temperatures
around 140 K (-207ºF).
19
Electric current and Ohm’s law
 An electric current will flow through a wire
only if an electric field is present to exert a
force on the charges.


We typically use a “power supply” to provide
the electric field and therefore the force.
The power supply forces
charges out one terminal,
through the wire, and
back into the second
terminal.
20
Electric current and Ohm’s law,
cont’d
 Voltage is the work that a charged particle
can do divided by the size of the charge.

It is the energy per unit charge given to
charged particles by a power supply.
work
V
q

E
 V
q
The SI unit is the volt (V).

One volt equals one joule per coulomb.
21
Electric current and Ohm’s law,
cont’d
 The flow of charge in an electric circuit is
similar to the flow of water through a closed
path.

The power supply acts like the water pump.



It adds energy to make the current flow.
The resistance corresponds to the narrow
section of pipe.
The current is like
the flow of water.
22
Electric current and Ohm’s law,
cont’d
 Ohm’s law specifies that the current in a
conductor is equal to the voltage applied to it
divided by the resistance:
V
I
R



or V  IR
V is the voltage through the conductor,
I is the current passing through the conductor,
and
R is the conductor’s resistance.
23
Example
Example 7.1
A light bulb used in a 3-volt flashlight has a
resistance equal to 6 ohms. What is the
current in the bulb when it is switched on?
24
Example
Example 7.1
ANSWER:
The problem gives us:
V 3V
R6 
The current through the bulb is
V 3V
I 
 0.5 A.
R 6
25
Example
Example 7.2
A small electric heater has a resistance of 15
ohms when the current in it is 2 amperes.
What voltage is required to produce this
current?
26
Example
Example 7.2
ANSWER:
The problem gives us:
R  15 
I 2A
The necessary voltage is
V  IR   2 A 15  
 30 V.
27
Electric current and Ohm’s law,
cont’d
 A series circuit has only one path for the
current to flow.


The voltage across the first bulb, plus the
voltage across the second, etc., must equal
the battery’s voltage.
The current through each bulb is the same as
the current passing through the battery.
28
Electric current and Ohm’s law,
cont’d
 If the circuit is interrupted, then current no
longer flows through the circuit.

If one bulb goes bad, then all the bulbs go
dim.
29
Electric current and Ohm’s law,
cont’d
 A parallel circuit has multiple paths for the
current to flow.


The current through the first bulb, plus the
current through the second, etc., must equal
the current through the battery.
The battery voltage is the same voltage on
each bulb.
30
Electric current and Ohm’s law,
cont’d
 If one bulb goes out, the other bulbs remain
lit.

There is still a closed path for the electricity to
flow through the circuit.
31
Example
Example 7.3
Three light bulbs are connected in a parallel
circuit with a 12-volt battery. The resistance of
each bulb is 24 ohms. What is the current
produce by the battery?
32
Example
Example 7.3
ANSWER:
The problem gives us:
V  12 V
R  24 
Since the bulbs are connected in parallel, they
each have the same voltage.
So the current through each bulb is:
V 12 V
I 
 0.5 A.
R 24 
33
Example
Example 7.3
ANSWER:
The total current necessary is the sum of the
currents through each bulb.
There are three bulbs, so:
I battery  0.5 A  I bulb  I bulb  I bulb  3I bulb
I bulb  13 (0.5 A)  0.17 A
34
Power and energy in electric
circuits
 The power output of a circuit is the rate at
which energy is delivered to the circuit.
 The power equals:

the energy given to each coulomb of charge,
multiplied by the number of coulombs that
pass per second.


The energy given to each coulomb of charge is
the voltage.
The number of coulombs that pass per second is
the current.
35
Power and energy in electric
circuits, cont’d
 This means the power through a circuit can
be written as
power  voltage  current
P  VI
 The electrical resistance of many substances
causes them to get hot.
 This type of heating is called ohmic heating.
36
Example
Example 7.4
In Example 7.1, we computed the current that
flows in a flashlight bulb. What is the power
output of the batteries?
37
Example
Example 7.4
ANSWER:
The problem gives us:
So the power consumed is
V 3V
R6
I  0.5 A
P  IV   0.5 A  3 V 
 1.5 W
38
Example
Example 7.4
DISCUSSION:
This means the batteries supply 1.5 joules of
energy each second.
39
Power and energy in electric
circuits, cont’d
 The current through the
filament causes it to get
very hot.

The filament is a very
thin wire.

It is a coil wrapped into
a coil.
 The thicker supporting
wires do not get as hot.
40
Power and energy in electric
circuits, cont’d
 A fuse uses ohmic heating to monitor the
current through the
circuit.


If too much current
flows, the fuse gets
too hot.
It essentially melts
and breaks the
circuit.
41
Example
Example 7.5
An electric hair dryer is rated at 1,875 watts
when operating on 120 volts. What is the
current flowing through it?
42
Example
Example 7.5
ANSWER:
The problem gives us:
V  120 V
P  1,875 W
The power is given by
P  IV .
Since we want the current, we divide by V:
P 1,875 W
I 
 15.6 A.
V
120 V
43
Example
Example 7.5
DISCUSSION:
The wiring in the house and the hair dryer’s
cord must be large enough to handle 15.6 A
without overheating.
44
Power and energy in electric
circuits, cont’d
 It is more efficient for the power company to
use very high voltage and low current to
transmit power.



Cross-country power lines use several
hundred-thousand volts.
This allows smaller currents to be used.
With smaller current, smaller wires can be
used without fear of excessive ohmic heating.

This saves money of cable, the supporting
structures, etc.
45
Power and energy in electric
circuits, cont’d
 Recall our definition of power:

The change in energy per unit time.
 This means we can write energy as the
power multiplied with the elapsed time.

The energy change equals the rate at which
energy is transferred time how long it is
transferred.

This is similar to the change in distance equaling
the rate at which position changes multiplied with
the time over which the position changes.
46
Power and energy in electric
circuits, cont’d
 So we can write the energy in terms of the
power:
E
P   E  Pt
t

So if we know the power a circuit uses and
how long the circuit operates, we can
determine the energy used by the circuit.
47
Power and energy in electric
circuits, cont’d
 Power companies do not actually charge you
for power.
 They charge you
for energy.
1 kWh  1 kW 1 h
 1, 000 W  3600 s
 3, 600, 000 J
 So a kilowatt-hour
is really energy.
48
Example
Example 7.6
If the hair dryer discussed in Example 7.5 is
used for 3 minutes, how much energy does it
use?
49
Example
Example 7.6
ANSWER:
The problem gives us:
t  180 s
P  1,875 W
The energy is given in terms of power as:
E  Pt  1,875 W 180 s 
 337,500 J.
50
Example
Example 7.6
DISCUSSION:
This is about the energy:



required to melt 2 lb of ice, or
of a small car traveling at 60 mph, or
of a 150 lb person falling 170 floors.
51
AC and DC
 There are two types of current flow:

direct current (DC) represents current flow
that is always in the same direction.


Batteries provide DC.
alternating current (AC) represents current
flow that alters direction periodically.

Wall outlets provide AC.
 A power adapter converts the AC from the
outlet to DC to charge a battery or power
some device.
52
AC and DC, cont’d
 Here is an example of DC provided by a
battery to a light bulb.


The current always passes from the positive
battery terminal,
through the bulb,
and then to the
negative terminal.
The current is
constant in time.
53
AC and DC, cont’d
 Here is an example of AC provided by a wall
outlet to a light bulb.


The current always passes from the positive
terminal, through the bulb, and then to the
negative
terminal.
But the
terminals swap
position over
time.
54
AC and DC, cont’d
 So the current passing through the bulb
changes direction.


Our wall outlets operate at 60 Hz.
So the current changes direction 120 times
each second.
55
AC and DC, cont’d
 Here is one advantage of AC over DC.



A transformer is a device that can increase or
decrease AC voltage.
If the transformer increases the voltage, it is
called a “step up” transformer.
If the transformer decreases the voltage, it is
called a “step down” transformer.
 But there is an important consideration.
56
AC and DC, cont’d
 The power into the transformer must (ideally)
equal the power out.



Recall that electrical power is P = IV.
If the voltage is increased (stepped-up), the
current must be decreased by the same factor.
If the voltage is decreased (stepped-down),
the current must be increased by the same
factor.

Basically, energy must be conserved.
57