Transcript Electric Charge

Preparatory Physics
(Chapters 3-4(PYPY001)
Coordinator:
Prof.Dr.Hassan A.Mohammed
CHAPTER 3
Energy
3.1 WORK
The work done on the object is defined as the product of the applied
force and the parallel distance through which the force acts:
work = force × distance
W = Fd
UNITS OF WORK
W = (newton)(meter)
W = (N)(m)
The newton-meter is called a joule (J)
1 joule = 1 newton-meter
The units for a newton are kg∙m/s2, and the unit for a meter is m.
It therefore follows that the units for a joule are kg∙m2/s2.
EXAMPLE 3.1
How much work is needed to lift a 5.0 kg backpack to a shelf 1.0 m
above the floor?
SOLUTION
To lift the backpack requires a vertically upward force
equal to the weight of the backpack. F = w
m = 5.0 kg g = 9.8 m/s2
w = mg
w = (5.0 × 9.8) kg m / s 2 = 49 kg⋅m/s 2 = 49 N
F = w = 49 N d = 1.0 m
W = Fd
= (49 N)(1.0 m) = (49 × 1.0) (N·m) = 49 N·m = 49 J
POWER
The rate at which energy is transformed or the
rate at which work is done is called power.
Power is measured as work per unit of time,
power = work / time
P=W/t
In the metric system, power is measured in
joules per second. The unit J/s, is called a
watt (W).
The watt (Figure 3.5B) is used with metric prefixes for
large numbers: 1,000 W = 1 kilowatt (kW) and
1,000,000 W = 1 megawatt (MW).
It takes 746 W to equal 1 horsepower.
One kilowatt is equal to about 1 1/3 horsepower.
Electrical energy is measured by power (kW)
multiplied by the time of use (h). The kWh is a unit of
work, not power. Since power is P = W / t then it
follows that W = Pt
So power times time equals a unit of work, kWh
EXAMPLE 3.3
An electric lift can raise a 500.0 kg mass a distance of 10.0 m in 5.0 s.
What is the power of the lift?
m = 500.0 kg g = 9.8 m/ s 2 h = 10 m
P = mgh / t
= (500.0 kg)(9.8 m∙ s 2 )(10.0 m) /5 s
= 9,800 N·m/s = 9,800 J/s=9,800 W= 9.8 Kw
The power in horsepower (hp) units would be
9,800 W/ 746 W = 13 hp
3.2 MOTION, POSITION, AND ENERGY
Energy can be defined as the ability to do work.
When work is done on something, a change occurs in its energy
level i.e potential energy or kinetic energy.
POTENTIAL ENERGY
The energy that an object has because of its position is called potential energy
(PE). Potential energy is defined as energy due to position.
This is called gravitational potential energy, since it is a result of gravitational
attraction. There are other types of potential energy, such as that in a
compressed or stretched spring.
Note the relationship between work and energy in the example.
You did 10 J of work to raise the book to a higher shelf.
For the metric unit of mass, weight is the product of the mass of an
object times g, the acceleration due to gravity, so
KINETIC ENERGY
The energy of motion is known as kinetic energy. Kinetic energy can be
measured in terms of (1) the work done to put the object in motion or (2) the
work the moving object will do in coming to rest.
Kinetic energy is proportional to the square of the velocity (22 = 4; 32 = 9).
The kinetic energy (KE) of an object is
kinetic energy = 1/ 2 (mass)(velocity) 2
KE = 1/ 2 mv 2
Kinetic energy is measured in joules.
EXAMPLE 3.7
A 7.00 kg bowling ball is moving in a
bowling lane with a velocity of 5.00
m/s. What is the kinetic energy of the
ball?
m = 7.00 kg v = 5.00 m/s
KE = 1/ 2 mv 2
= 1/2 (7.00 kg) (5.00 m/s ) 2
= 1/2 (7.00 × 25.0) kg × m 2/s 2
= 1/ 2 175 kg· m 2/s2
=187.5N.m
=187.5 joule
ENERGY CONVERSION
Potential energy can be converted to kinetic energy and vice versa.
The simple pendulum offers a good example of this conversion. A simple
pendulum is an object, called a bob, suspended by a string or wire from a
support. The kinetic energy of the bob at the bottom of the arc is equal to
the potential energy it had at the top of the arc (Figure 3.15). Disregarding
friction, the sum of the potential energy and the kinetic energy remains
constant throughout the swing.
The potential energy lost during a fall equals the kinetic
energy gained (Figure 3.16). In other words,
PE lost = KE gained
Substituting the values from equations 3.3 and 3.4,
mgh = 1/ 2 mv 2
Canceling the m and solving for vf,
v f = √ 2 gh
EXAMPLE 3.9
A 1.0 kg book falls from a height of 1.0 m.
What is its velocity just as it hits the floor?
h = 1.0 m
g = 9.8 m / s 2
v f =√ 2 gh = √ ( 2)(9.8 m/ s 2 )(1.0 m)
= √ 2 × 9.8 × 1.0 m /s 2 = 4.4 m/s
ENERGY CONSERVATION
Law of conservation of energy:
In a closed system, energy is never created or destroyed.
Energy can be converted from one form to another, but
the total energy remains constant.
CHAPTER 4 : Electricity
4.1 CONCEPTS OF ELECTRICITY
The word electricity is also based on the Greek word for amber.
The atom is considered to have a dense centre part called a nucleus that
contains the closely situated protons and neutrons. The electrons move around
the nucleus at some relatively greater distance (Figure 4.2)
Electric Charge
Electrons and protons have a property called electric charge.
Electrons have a negative electric charge,
and protons have a positive electric charge
Electric charges interact to produce what is called
the electrical force.
like charges repel and unlike charges attract.
Ordinary atoms are usually neutral because there
is a balance between the number of positively
charged protons and the number of negatively charged electrons.
An atom that is ionized by losing electrons results in a positive ion because
it has a net positive charge. An atom that is ionized by gaining electrons results in
a negative ion because it has a net negative charge
Static Electricity
An object that acquires an excess of electrons becomes a negatively charged
body. electric charges on objects result from the gain or loss of electrons.
charge is not moving, is called an electrostatic charge ( static electricity)..
When you comb your hair with a hard rubber comb, the comb becomes
negatively charged because electrons are transferred from your hair to the
comb. Your hair becomes positively charged
Electrical Conductors and Insulators
Materials like the metal of a doorknob are good electrical conductors
because they have electrons that are free to move throughout the metal.
Materials such as plastic and wood do not have electrons that are free to
move, and they are called electrical nonconductors (electrical insulators)
Our body is a poor conductor, which is
why you become charged by friction.
Metal wires are used to conduct an
electric current from one place to another,
and rubber, glass, and plastics are used as
insulators to keep the current from going
elsewhere.
MEASURING ELECTRICAL CHARGES
The size of an electric charge is identified with the number of electrons that have
been transferred onto or away from an object. The quantity of such a charge (q)
is measured in a unit called a coulomb (C).
A coulomb unit is equivalent to the charge resulting from the transfer of 6.24 ×
1018 of the charge carried by particles such as the electron.
The coulomb is a metric unit of measure like the meter or second.
quantity of charge = (number of electrons)(electron charge)
Or
q = ne
e = q /n
equation 4.1
where q is 1.00 C, and n is 6.24 × 1018 electrons
Every electron has a charge of
–1.60 × 10–19 C,
and every proton has a charge of +1.60 × 10–19 C.
EXAMPLE 4.1
Combing your hair on a day with low humidity
results in a comb with a negative charge on
the order of 1.00 × 10–8 coulomb. How many
electrons were transferred from your hair to
the comb?
= 6.25 × 10+10 e
ELECTROSTATIC FORCES
Two objects with like charges, (–) and (–) or (+) and (+), produce a
repulsive force, and two objects with unlike charges, (–) and (+), produce
an attractive force. The size of either force depends on the amount of
charge of each object and on the distance between the objects. Th e
relationship is known as Coulomb’s law, which is
F=k
equation 4.2
where k has the value of 9.00 × 109 newton-meters2/coulomb2
(9.00 × 109 N·m2/C2).
The force between the two charged objects is repulsive if
q1 and q2 are the same charge and attractive if they are diff erent
(like charges repel, unlike charges attract). Whether the force
is attractive or repulsive, you know that both objects feel equal
forces, as described by Newton’s third law of motion. In addition,
the strength of this force decreases if the distance between
the objects is increased (a doubling of the distance reduces the
force to 1⁄4 the original value
EXAMPLE 4.2
Electrons carry a negative electric charge and
revolve about the nucleus of the atom, which
carries a positive electric charge from the
proton. The electron is held in orbit by the
force of electrical attraction at a typical
distance of 1.00 × 10–10 m. What is the force
of electrical attraction between an electron
and proton?
FORCE FIELDS
4.2 ELECTRIC CURRENT
Electric current means a flow of charge in the same way that water current
means a flow of water.
To keep an electric current going, you must maintain the separation of
charges and therefore maintain the electric field (or potential difference),
which can push the charges through a conductor.
An electric circuit contains some device, such as a battery or electric
generator, that acts as a source of energy as it gives charges a higher
potential against an electric field. The charges do work in another part of the
circuit as they light bulbs, run motors, or provide heat. The charges flow
through connecting wires to make a continuous path.
The electrical potential difference between the two connecting wires shown in
Figure 4.10 is one factor in the work done by the device that creates a higher
electrical potential (battery, for example) and the work done in some device
(lamp, for example). work done per unit of charge is joules/coulomb, or volts
(equation
The source of the electrical potential difference is therefore referred to as a
voltage source
Electrical potential difference is measured in volts, so the term voltage is often
used for it. A voltage of 120 volts means that each coulomb of charge that
moves through the circuit can do 120 joules of work in some electrical device.
A coulomb/second is called an ampere (A), or amp for short.
A 1.00 amp current is 1.00 coulomb of charge moving through a conductor
each second.
THE NATURE OF CURRENT
The conventional current describes
current as positive charges moving
from the positive to the negative
terminal of a battery.
The electron current description is in
an opposite direction to the
conventional current.
When a potential difference is applied to the wire in a circuit, an electric field
is established everywhere in the circuit. The electric field travels through the
conductor at nearly the speed of light.
(1) an electric potential difference establishes, at nearly the speed of light, an
electric field throughout a circuit,
(2) the field causes a net motion that constitutes a flow of charge, or current,
A circuit like the one described with your car battery has a current that always
moves in one direction, a direct current (dc).
Electric utilities and most of the electrical industry, on the other hand, use an
alternating current (ac). An alternating current, as the name implies, moves
the electrons alternately one way, then the other way.
ELECTRICAL RESISTANCE
Materials have a property of opposing or reducing a current, and this property is
called electrical resistance (R).
Resistance (R) is therefore a ratio between the potential difference
(V) between two points and the resulting current (I). This ratio is
The ratio of volts/amps is the unit of resistance called an ohm
(Ω) after G. S. Ohm (1789–1854), a German physicist.
Another way to show the relationship between the voltage, current,
and resistance is V = IR
which is known as Ohm’s law.
EXAMPLE 4.3
A light bulb in a 120 V circuit is switched on, and a current of 0.50 A flows
through the filament. What is the resistance of the bulb?
SOLUTION
The current (I) of 0.50 A is given with a potential difference (V) of 120 V.
The relationship to resistance (R) is given by Ohm’s law
ELECTRICAL POWER AND ELECTRICAL WORK
The work done by a voltage source (battery, electric generator) is equal to the
work done by the electric field in an electric device (lightbulb, electric motor)
plus the energy lost to resistance.
Disregarding losses to resistance, electrical work (W) to move charges (q) to
a higher potential (V) can be measured from equation
work = (potential)(charge)
mechanical power (P) was defined as work (W) per unit time (t), or
Since electrical work is W = Vq, then electrical power must be
a quantity of charge (q) per unit time (t) as a current (I), or I = q/t.
Therefore, electrical power is
RESISTANCES IN CIRCUITS
When several resistances are connected in series, the resistance (R) of
the combination is equal to the sum of the resistances of each component.
In symbols, this is written as
R total = R 1 + R 2 + R 3 + . . .
EXAMPLE 4.11
Three resistors with resistances of 12 ohms
(Ω), 8 ohms, and 24 ohms are in a series circuit
with a 12 volt battery. (A) What is the total
resistance of the resistors? (B) How much
current can move through the circuit? (C) What
is the current through each resistor?
B. Using Ohm’s law,
C. Since the same current runs through one after the other, the current is the
same in each.
In a parallel circuit, more than one path is available for the current, which
divides and passes through each resistance independently (Figure 4.38).
This lowers the overall resistance for the circuit, and the total resistance is
less than any single resistance. In symbols, the effect of wiring resisters in
parallel is
EXAMPLE 4.12
Assume the three resistances in example
4.11 are now connected in parallel. (A)
What is the combined resistance? (B)
What is the current in the overall circuit?
(C) What is the current through each
resistance?
B. I =V/R=12/4=3 A
HOUSEHOLD CIRCUITS
Adding more resistances in a parallel circuit results in three major effects
that are characteristic of all parallel circuits:
1. an increase in the current in the circuit;
2. the same voltage is maintained across each resistance; and
3. a lower total resistance of the entire circuit.
The total resistance is lowered since additional branches provide more
pathways for the current to move.
Each appliance connected to such a parallel circuit has the same voltage
available to do work, and each appliance draws current according to its
resistance. This means that as additional appliances are turned on or
plugged in, additional current flows through the circuit.
The current could reach high enough levels to cause overheating and
possibly a fi re. A fuse or circuit breaker in the circuit is used to disconnect
the circuit if it reaches the preset value, usually 15 or 20 amps.
Fuses and circuit breakers are “blown” or “tripped” by a short circuit, a new
path of lesser electrical resistance.
A three-pronged plug provides a third appliance-grounding wire through the
grounded plug. The grounding wire connects the metal housing of an
appliance directly to the ground. If there is a short circuit, the current will
take the path of least resistance—through the grounding wire—rather than
through you.
A household circuit always has two wires, one that is carrying the load and
one that is the neutral system ground. The load carrying wire is usually
black (or red), and the system ground is usually white. A third wire, usually
bare or green, serves as an appliance ground.
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