Concepts and principles of electricity

Transcription

Unit 5
Physics
Uses Concepts and Principles of Electricity
Chapter 5
Concepts and principles of
electricity
Competency
Uses concepts and principles of electricity in daily life
Competency level
Subject Content
5' 1' Produces electrical charges
and stores it.
² Static electricity
² Charging bodies by rubbing
² Positive charges
² Negative charges
² Detection of charges
² Gold leaf electroscope
² Capacitors (as a charge storage device)
5' 2' Uses the relationship
between the potential
difference and the current in
daily activities.
² Electric current ^I&
² Units of measurement of current (ampere - (A)
² Uses of ammeter
² Potential difference (V)
² Meaurement of potential difference (Volt - V &
² Uses of voltmeter
² Ohm’s Law (V = IR)
² Resistance
² Unit of Resistance ^Ohm - Ω &
5' 3' Investigates how resistance
affects the current.
² Value of resistor
² Colour code
² The equivalent resistance of series resistors
² R = R1 + R2
² The equivalent resistance of parallel resistors
² 1/R = 1/R1 + 1/R2
5' 4' Constructs simple
electrical circuits to suit the
situation.
² Circuit components
² electric cells
² switches
² resistors
² electric bulbs
For free distribution
77
Physics
5.1
Generating of electric charges and storing them
Static Electricity
Electric charge on an object is often called static electricity.
Activity 1
Rub a dry drinking straw with a cotton cloth and bring it
closer to small pieces of paper (or small pieces of rigifoam).
You’ll observe that these pieces are attracted to the straw.
Now do the same activity using a dry straw which is not
rubbed with a cloth.
Piece of
cotton cloth
Str
aw
You’ll observe that small pieces of paper are not attracted to
the straw now.
Can you explain these observations?
Fig : 5.1
Take a perspex rod and rub it with a cotton cloth. Bring it
closer to small pieces of paper. They are
attracted to it.
Pe
rsp
ex
rod
Activity 2
Piece of cotton cloth
Fig : 5.2
Activity 3
Rub a plastic rod (or comb) in your hair and bring it closer to small pieces of paper. They are
attracted to the comb.
Fig : 5.3
What did you notice in all these instances? Small pieces of paper were attracted by
the objects (plastic rod, perspex rod, comb, etc.) only if they were rubbed with piece
of cloth or hair.
78
Unit 5
Actually what has happened to these objects when they were rubbed?
These objects are commposed of atoms. As a result of rubbing against each other, they
acquire ‘static charges’. i.e. they acquire suface chages by gaining or loosing electrons.
They become ‘negatively charged and of they lise electrons. They become postibely
charged there these objects can attract small pieces of paper and other light particles.
Acceptance or loss of electrons from surfaces of objects leads to static electricity.
Activity 4
Take two a and glass rods charge them by rubbing with dry silk cloth and hang
them closer to each other. You’ll see that they are repelled. (Fig : 5.4)
P
From this activity it is very clear that both
have the same charge. (Like charges)
¬ ¬
¬ ¬
Two rods repel
each other
P
Fig : 5.4
P
¬ ¬ ¬
Fig : 5.5
Now take a glass rod rubbed with silk cloth
and a P.V.C. tube rubbed with silk and
hang them closer to each other. You’ll see
that they are attracted.
Why? They have ‘unlike charges’ i.e. the
charges present on the surface of glass rod
and P.V.C. are not the same.
Unlike charges repal
each other
• All substances are made of atoms.
• Atoms are made of electrons, protons and neutrons.
• Protons are positively charged particles.
• Electrons are negatively charged particles.
• Neutrons have no any charge. They are neutral particles.
• Protons and neutrons are found in the nucleus of the atom.
• Electrons orbit around the nucleus.
• Only the electrons can be removed from atoms.
• When electrons are removed from a surface it becomes positive.
• When electrons are added to a surface it becomes negatively charged.
79
Physics
DO YOU KNOW ?
^¬&
Electric Series
Fur
Flannel
Glass
Perspex
Cotton
Silk
Leather
Wood
Wax
Resins (Amber)
Polythene
Plastic
P.V.C
There is a series called ‘Electric series. This
enables us to know approximately which
becomes positive and which becomes
negative when rubbed against with eachother.
Look at the series given here.
When an object above in the series is rubbed
with a substance (an object) below in the
series, the substance upper in the series
becomes positive and the lower one becomes
negative.
^-&
e.g : Glass is found above silk in this series.
Therefore if glass is rubbed with silk, glass
becomes positively charged and silk become
negatively charged.
Now let us see how the objects in the activities
1 to 4 are charged.
In the activity 1; a straw was rubbed with cotton cloth. Therefore straw (made of
plastic) becames negatively charged and the cotton cloth became positively charged.
This happens because electrons are removed from the surface of cotton cloth and they
are added to the surface of the straw.
In the activity 2 ; perspex rod was rubbed with cotton cloth. Therefore perspex
becomes positive and cotton cloth becomes negative.
In the activity 3 ; comb (plastic) was rubbed with hair. Here electrons were removed
from the hair and were added to the comb (plastic). Therefore comb became negative
and hair became positive.
In the activity 4 ; glass rod was rubbed with silk. Therefore glass rod becomes positive
and silk becomes negative. P.V.C. tube was rubbed with silk. Therefore P.V.C. tube
became regative while silk become positive.
• Like charges repel each other
• Unlike charges attract each other
When a neutral object (body) is brought closer to small pieces of paper, they are not
attracted to the neutral object (body).
80
Unit 5
When a charged body is brought closer to small pieces of paper or dust, they
are attracted to the charged body. Why?
Let us conduct a small activity to find out.
Bring a charged body (positive or negative) closer
to a rigifoam ball or a ball made pith of manioc
(pith ball) which is hung from a thread. Observe what
happens.
Charged rod
Pith ball or
rigifoam ball
1) Rigifoam ball is attracted
to the charged body
2) They come in contact
3) It is repelled
Fig : 5.6
Let us assume the rod is a negatively charged one. When
the charged rod is brought closer to the pith ball (or
rigifoam ball) electric charges are induced in the pith
ball as shown in the Fig : 5.6 (a), and therefore the pith
ball gets attracted to the charged body.
Fig : 5.6 (a)
When they come in contact, positive charges on the pith ball are neutralized by the
electrons which are coming from the charged body. Now as both the rod and the pithball
are negative, repulsion takes place.
Metal disc
Metal
rod
Gold leaf electroscope
You have learned about this instrument in
grade 8.
Insulator
Charging a gold leaf electroscope by
contact method
Gold
leaf
When the metal disc of the electroscope is
made in contact with a positively charged body
the electroscope is charged positively.
Fig : 5.7
Gold leaf electroscope
81
Physics
Bringing closer a positively charged
body to an electroscope
Positively charged body
made in contact
+
+
+
+
When the charged body
is removed
+
+ +
Fig : 5.8
Identification of charges on a body using a
gold leaf electroscope
Charge an electroscope positively. Now bring
another positively charged body closer to the metal
disc of the electroscope. (Do not touch.) Gold leaves
are deflected. The reason for this is the attraction of
electrons from the gold leaves towards the metal disc
because the charged body is a positive one.
Fig : 5.9
The same thing happens when a negatively charged
body is brought closer to an electroscope which is
also charged negatively.
What happens when a negatively charged body is brought closer to an
electroscope which is charged positively?
Here what happens is electrons are repelled from the metal disc and as they reach
the gold leaves the electro positiveness is decreased. Therefore deflected gold
leaves collapse.
• When a neutral body is brought near a charged electroscope also, the gold
leaves shrink.
Applications of electro-static charges
Fig : 5.10
A capacitor
82
Capacitors
The function of capacitors is to store electric charges. They
are used in radio and TV sets and many electronic circuits.
They are also called condensers. A common capacitor is
made of two strips of metal foil seperated by a strip of
insulator. Or, two metal plates are kept parallely and the
area between these two plates is filled with an insulator or
dielectric substance.
Unit 5
dielectric
substance metal
Metal
plate
Plate
e
Charging condenser
e
e
e
Fig : 5.12
Some dielectric substances are given below
1. Air
2. Paper
3. Wax
4. Polystyrene
5. Mica
6. Al2O3 (Alumina)
When the two plates of a parallel plate condenser
are connected to the two terminals of a battery, it
gets charged. It happens as follows.
e
Fig : 5.11
Discharging condenser
Fig : 5.13
Inside a parallel plate condenser
Electrons flow from the negative terminal of the
battery to one metal plate. Therefore this plate gets
negatively charged while the other plate is
charged positively. Electrons flow from the
positively charged plate to the the positive
terminal of the battery.
When the two plates of a charged capacitor is
connected by a wire, electrons flow from the
negatively charged plate to the positively
charged plate. Therefore the plates are discharged.
When a capacitor is discharged do not forget to
connect a suitable resistor to prevent destroying the
capacitor.
Fig : 5.14
Different types of capacitors
The unit used to measure the capacity of a capacitor is ‘FARAD’ (F).
1
l
F (= 10-3 F) is one ‘milli Farad’. (i.e. 10-3 F = 1 mF)
1000
l
l
One millionth of a Farad is one ‘micro Farad’. (i.e. 1/1000000 F = 10-6 F = 1 µF)
l
One trillionth of a Farad is one ‘pico Farad’. (pico is pronounced as ‘peeco’)
(i.e. 10-12 F = 1 pF).
83
Physics
²
Factors affecting the
capacity of a capacitor
1. Area of the plates
2. The distance between the two
plates
3. Nature of the dielectric
substance
When the area of plates is increased, the
capacity is also increase. When the
distance between the plates increases
the capacity decreases. As the insulating
property of the dielectric substance
increases, the capacity increases.
Some uses of capacitors
l These are used in electronic devices
such as radios, TVs and computers.
l These are used to prevent electric
sparks in electronic circuits.
l These are used to prevent variations
of direct current.
Some other instances they are used
are given below.
w In lightning conductors.
w In photocopying machines.
w In electro static precipitators.
w Electro static ink spraying machines.
Electric current
Electric current is the “rate of flow of electric charges”. The unit used to measure the
‘charge’ is Coulomb (C).
One coulomb of negative charge is the charge of 6.24 × 1018 electrons. Therefore when
the rate of flow of electrons through a conductor becomes 6.24 × 1018 electrons in one
second, this current is called one ampere (1 A). i.e. When the rate of flow is one coulomb
per second then current is said to be one ampere. (1 C s-1 = 1 A) Therefore current of
5 A means that the rate of flow of electric charge is 5 coulomb per second.
i.e. 5 A = 5 C s-1.
The charge carriers found in solid conductors are “free electrons”. Charge carriers in the
molten state and in solutions are positively charged ions and negatively charged ions.
Free electrons are the electrons which become free from nuclear attraction.
Inner energy levels
Nucleus
Fig : 5.15
Free electrons
Flow of current through a solid conductor is due to these free electrons. When the two ends
of a conductor are connected to the two terminals of an electric supply (battery), electrons
are coming from the negative terminal and these electrons force the free electrons of the
84
Unit 5
conductor to move forwards, that is towards the positive terminal of the battery. But the
conventional current is considered to be a positive current which flows from the positive
terminal to the negative terminal (against the direction of free electrons).
A
B
B
e
e
e
Fig : 5.16
Fig : 5.17
Conventional current is represented by the letter ‘I’ and the electron current by ‘e’
i
e
A
Fig : 5.18
B
When we say that a current is flowing from A to
B, actually what happens is electrons flow from
B to A.
The instrument which is used to measure the
current is the ammeter.
Bulb
Ammeter
Fig : 5.19
The ammeter
Fig : 5.20
The ammeter is always connected in series.
Potential difference
Positive potential is there at the positive terminal of a battery and a negative potential
is found at the negative terminal.
The difference in these potentials is called the potential difference. The unit used
to measure the potential difference is “volt”. The instrument used to measure the
potential difference is called volt meter.
85
Physics
Fig : 5.22
Fig : 5.21
Volt meter
A voltmeter is always connected in
parallel.
If one joule (J) of work is done when a charge of 1 coulomb (1 C) flows from
one point to another in a circuit, the potential difference between these two
points is said to be 1 Volt (1 V).
+ X
When you want to measure both the current (I)
and the potential difference (V) at the same time,
this is how the circuit is set up.
Fig : 5.23
This represents how volt meter and ammeter are
connected.
Activity 5
A piece of nichrome wire is shown here by
‘X’. Connect a volt meter in parallel and an Fig : 5.24
ammeter is series. The battery ‘E’ is
X
connected to give the electric supply. Switch
is shown here by ‘S’ and ‘Y’ is a variable
resistor (Rheostat). Close the switch to get
one reading. i.e. the current (I) through the
nichrome coil and the potential difference
S
Y
E
(V) across it.
Switch off the circuit as soon as possible to prevent heating the resister ‘X’.
Now adjust the rheostat a bit and get another reading (V and I).
Thus get five readings by adjusting the rheostat. Now divide the volt meter reading by
the ammeter reading. i.e. divide potential difference (V) across the resister ‘X’ by the
current (I) flowing through it. You’ll see that the value obtained here to be a constant. i.e,
When the temperature of the resister is kept constant,
86
Volmeter reading
= constant
Ammeter reading
Unit 5
That is, when the temperature of a conductor is kept constant
Potential difference(V)
= constant
Current (I)
This relationship was first discovered by a scientist called ‘George Simon Ohm’.
Therefore this relationship is called Ohm’s law.
Ohm’s Law
When the temperature and all other factors are
kept constant, the current (I) through a conductor
is directly proportional to the potential difference
(V) across it (i.e. V á I).
i.e
= Constant
Current
(i.e.)
V
= I = Constant
This constant is known as the electrical
‘resistance’ (R) of the conductor.
Fig : 5.25
George Simon Ohm
i.e.
V
R= I
The resistance (R) of a conductor is the potential difference (V) that has to be supplied for
1 ampere (1 A) current to flow through it. If a potential difference of 1 V should be
supplied for 1 A current to flow through a conductor, the resistance of that
conductor is 1Ω.
If a potential difference of 6 V should be supplied for 1 A current to flo through a conductor,
the resistance of that conductor is 6 Ω.
Solved example
The following readings were obtained in an experiment which has conducted to prove
the Ohm’s law.
0
1.5 V
3.0 V
4.5 V
6.0 V
Current
0
0.5 A
1.0 A
1.5 A
2.0 A
i) What has proved from the above
readings?
ii) Calculate the resistance of the
conductor.
87
Physics
(V) Current (I)
1.5
3.0
4.5
6.0
=
=
=
0.5
1.0
1.5
2.0
=
= 3
The constant obtained is 3.
This shows that V = 3.
I
i.e. The resistance of the given connector = 3 Ω.
V =R
I
V = IR
I =V
R
Resistors
Resistance is the barrier force to the flow of current.
Resistors are used to give a resistance to a circuit.
Current or voltage (potential difference) can be
changed by connecting resistors.
Fig : 5.26
Some resistors
Some main types of resistors are,
1. Carbon resistors
2. Metal oxide resistors
3. Wire wound resistors
Out of these, mostly used type is the carbon resistors.
Ink coating is found around resistors, to get them protected by humidity, because
the resistance is changed according to it.
The external appearance of the carbon resistors is more similar to the metal oxide
resistors, but the internal structure is different.
Black
Brown
Red
Orange
Yellow
0
1
2
3
4
Green
Blue
Purple
Grey
White
Colour bands and relevant numbers
88
5
6
7
8
9
Wire wound resistors are made by
winding resistance wire arround
porcelain core.
The resistance of a resistor can be
found out by using the colour
bands found around the resistor.
Colours of the bands and numbers
relevant to them are given here.
Unit 5
Find out the first two digits of the resistance value by the first two colour bands.
Now see the value for the third colour band. Find out the ten to the power of this
value.Now multiply the number made from first two digits by the value of the ten
to the power of this number. The resistance of any resistor could be calculated like
that. The fourth band represents the tolerance value of a resistor.
Colour of the 4th band
Tolerance
Red
±2%
Gold
±5%
Silver
± 10 %
No band
± 20 %
If the fourth band is absent, it shows that
the tolerance value is ± 20%.
The colours of the 4 th band and the
relevant tolerance values are given
below.
Tolerance is the percentage of change
occurs due to temperature, humidity etc.
Worked Example
Calculate the resistance of the following resistor.
Blue - 6
Green - 5
Red - 102
Gold - ±5%
Blue
= 6
Green
= 5
No.obtained from the first two digits = 65
Resistance = 65 × 102
= 6500 Ω
Tolerance = ± 5 %
R1
R2
I
I
I
Fig : 5.27
Resistors in series
R1
i
i1
i1
i2
i2
R2
Fig : 5.28
i
Resistors in parallel
Connecting Resistors / Electronic
components in series
R1 and R2 resistors are connected in series. When
the resistors are connected in this way, the current
is not divided. The same current flows through both
resistors.
Connecting resistors in parallel
When the resistors are connected so as to divide
the current through the resistors, then they are said
to be connected in parallel.
Here, the two resistors R1 and R2 are connected in
parallel. Then the potential difference applied for
both is the same.
Current gets divided to two resistors. Here the
current is divided as i1 and i2. If the resistance are
equal than the current divided equally.
89
Physics
If ,
and ,
R1 = 12 Ω
R2 = 6 Ω
then ;
1 A will flow through 12 Ω resistor while 2 A will flow through 6 Ω resistor.
A
B
R1
C
R2
D
When resistors are connected in series, same
current flows through all the resistors.
R3
I
I
Fig : 5.29
This current is gained according to the total
resistance. Let us study the following circuit.
Equivalent resistance of resistors connected in series.
In the above circuit all the resistors R1, R2 and R3 connected in series. The total
resistance of these is called the equivalent resistance. Then what should be the
potential difference?
Potential differance between A and D ( VAD)
VAD = Current (I) × Equivalent resistance (R)
VAD = I R
Potential difference between A and B = VAB = I R1
Potential difference between B and C = VBC = I R2
Potential difference between C and D = VCD = I R3
Potensial differance between A and D =
PD between A and B + PD between B and C
+ PD between C and D
i.e. VAD = VAB + VBC + VCD
As V = I R
I R = I R1 + I R2 + I R3
When the equation is divided by I you’ll get,
R = R1 + R2 + R3
This shows that the equivalent resistance (or the total resistance) can be find out
by adding resistances of all the resistors which are connected in series.
eg. : The equivalent resistance or the total resistance of a 12 Ω resistor and 6 Ω
resistor is, 6 + 12 = 18 Ω
90
Unit 5
Connecting resistors in parallel
The following resistors R1, R2 and R3 are connected in parallel.
R
R1
i1
i2
R2
i3
I
R3
I
I
or
I
V
Fig : 5.30
Equivalant resistance when the resistors are connected in parallel
V
Here the potential difference is not divided. Same is applied for all the resistors.
Current gets divided here according to their resistance. If the resistances of all the
parallel resistors are equal then current gets divided equally.
Let the total resistance of the three resistors R1, R2 and R3 to be R. Current gained from
the electric supply is I.
V
Therefore, I = R
Also,
i1 = V
R1
i2 = V
R2
i3 = V
R3
Because I = i1 + i2 + i3
V= V +V + V
R
R1 R 2
R3
When this is divided by V,
1= 1+ 1+ 1
R R 1 R 2 R3
The receprocal of the total resistance (equivalent
resistance) is equal to the sum of the reciprocals
of resistance of all the resistors which are
connected in parallel.
• When resistors are connected in series the total resistance is the sum of resistances
of all the resistors.
• When the resistors are connected in parallel, the reciprocal of the total resistance
is equal to the sum of reciprocals of all the parallel resistors.
91
Physics
5.4 Construction of simple electric circuits to suit the needs
An electric circuit is a path through which electric charges flow.The following
components are used to make electric circuits.
• Electric cells
• Switches
• Electric bulbs
• Connecting wires
• Resistors
Activity 6
To make a circuit to light a bulb from a dry cell.
When the two terminals of the dry cell are connected to the two terminals of the electric bulb, it
lights up. But a switch also has to be connect for enable to control it to light or to prevent lighting.
Some symbols of some components which are used to make circuits.
Electric cell
(Off& Switch
Resistor
Bulb
Bulb
B
A
Switch
Electric cell
+ Circuit A - Without using a switch
Fig : 5.31
Circuit B - With using a switch
Fig : 5.32
The following diagram shows how to connect
two bulbs in series to a dry cell.
The following diagram shows how they are
connected in parallel.
Fig : 5.33
92
Fig : 5.34
Unit 5
Summary
•
•
•
•
•
•
•
•
•
•
•
Objects acquire static charges as a result of rubbing against each other.
When electrons are removed from a surface, it becomes positively charged.
When electrons are added to a surface it becomes negatively charged.
Charges can be detected by using an electroscope.
eg. : Gold leaf electroscope.
Capacitors are used to store charges.
Electric current is the rate of flow of electric charges.
Potential difference is the difference of potensials between two points
of a circuit.
When the temperature and all other factors are kept constant,
Potential differance
= Constant.
Current
There are two main methods to connect resistors.
1. Series method
2. Parallel method
When the resistors are connected in series,
R = R1 + R2 + R3
When the resistors are connected in parallel,
1 = 1 + 1 + 1 ....
R1 R2 R3
R
Excaercises
(1)
(2)
I.
A plastic rod was rubbed with a dry cloth. What is the charge resent
on the plastic rod.
II.
What charge is gained by a perspex rod when it is rubbed with a dry
cloth.
III. Draw a gold - leaf - electroscope and name its parts.
IV. How is a gold leaf electroscope charged negatively.
V.
How do you detect charges present on an object using a charged
gold leaf electroscope.
6 Ω 12 Ω 4 Ω
Three resistors 6 Ω, 12 Ω and 4 Ω are
connected in series to an electric supply of 12V.
I.
Calculate the total resistance of these
I
three resistors.
12 V
II.
What is the current gained from the
electric supply.
93
Physics
Three resistors 4 Ω, 12 Ω and 6 Ω are
connected to a supply of 12 V as shown below.
I.
Calculate the total resistance of these three
resistors.
II.
What current is gained from the electric
supply?
III. Calculate the current flowing through 6 Ω. I
IV. What is the current flowing through 12 Ω?
V.
What is the current flowing through 4 Ω?
i2
12 Ω
6Ω
i1
I
12 v
II.
Draw a diagram to show how a bulb is connected to two electric cells
which are connected in series.
Draw a circuit diagram to show how two electric bulb are connected
parellely to two cells which are also connected in parallel.
d
Find out resistance and tolerence of the following resistor.
III.
(5)
i3
I.
Bro
wn
Blu
e
Red
(4)
4Ω
Gol
(3)
Three bulls B1, B2 and B3 are connected to 12 V, supply as shown in the
following diagram.
B2
Calculate the total resistance of
6Ω
B1
the two bulls B2 and B3
(between Y and Z).
x
z
y
B3
II.
What is the total resistance
6Ω
between the two points X and Z.
3Ω
III. What is the current gained from
the electric supply.
12 V
IV. Calculate the potential difference
between X and Y.
V.
Calculate the potential difference between Y and Z.
VI. Calculate the current flowing through B2 bulb.
VII. Calculate the current through the bulb B3.
VIII. If the bulb B3 is removed, then what would be the current gain from the
electric supply.
I.
94

Concepts and principles of electricity

Transcription

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