1.
THE ELECTRIC SPARK
Q1. Choose the correct alternative and rewrite the following:
•1mA = ___________ A
1.103A
2.10 – 3 A
3.106A
4.10 – 6 A
•To increase the effective resistance in a circuit, the resistors are connected in _________
1.Series
2.Parallel
3.Both ways
4.None of these
•1 kilowatt hr = ____________ Joules.
1.4.6 x 106 Joule
2.3.6 x 106 Joule
3.30.6 x 106 Joule
4.3.6 x 105 Joule
•If a P.D. of 12 V is applied across a 3Ω resistor, then the current passing through it is _______
1.36A
2.4A
3.0.25A
4.15A
2.
Q2. State True or False. If false correct it.
•The SI unit of charge is volt.
False: The SI unit of charge is the coulomb and the SI unit of P.D. is
Volt.
•A voltmeter is always connected in parallel with the device.
False: A voltmeter is always connected in series with the device.
•The conventional direction of flow of current is from positive terminal
to negative terminal of the cell.
Ans. True
•Silver and copper are good conductors.
Ans. True
•Resistivity of pure metal is more than alloys.
False: Resistivity of pure metals is less than that of alloys.
•The electric bulb consists of the filament whose melting point is low.
False: The electric bulb consists of the filament whose melting point is
high.
3.
Q3. Match the column.
I
II
Heat generated
Is used to reduce V = IR
effective resistance in a
circuit
Resistance in parallel
Proportional to the
square of current
Gives relation between
V and I
Depends
on
the
material
of
the
conductor
Resistivity
Ohm’s law
III
4.
I
II
Heat generated
Proportional to the
square of current
Is used to reduce
effective resistance
in a circuit
Resistance in
parallel
Resistivity
Depends on the
material of the
conductor
Ohm’s law
Gives relation
between V and I
III
5.
Q4. Give scientific reasons.
•The material used for fuse has low melting point.
•A fuse is used to protect a circuit and the appliances
connected in the circuit by stopping the flow of an
excess electric current. For this, a fuse is connected in
series in the circuit.
•When the current in the circuit passes through the
fuse, its temperature increases. When the current
exceeds the specified value, the fuse must melt to
break the circuit. For this, the material used for a fuse
has low melting point.
6.
•Wood and glass are good insulators.
•When a current flows through a conductor,
the free electrons in the atoms move from
one end of the conductor to the other.
•Certain materials have less free electrons
in their atoms and the current does not
easily flow through the material.
•There are no such free electrons in wood
and glass. Hence they cannot conduct
electricity. Hence, they are good insulators.
7.
•The melting point of filament of a bulb
is very high.
•The bulb begins to glow only when the
filament is heated to a high temperature,
and it becomes incandescent (bright)
without melting.
•This happens only when the material of
the filament has a high melting point.
•Hence, the material used in the filament of
an electric bulb must have a high melting
point. For eg. Tungsten (33800C)
8.
•Connecting wires in a circuit are made of
copper and aluminium.
•Copper and aluminium offer a low resistance to
the flow of current and hence they are good
conductors of electricity.
•Copper and aluminium are highly ductile and
hence can be used for preparing the wire.
•Copper being more ductile, it is used in making
thin wires, and aluminium is used for making
thicker wires.
Thus, they are suitable for making wires used in
electrical circuit.
9.
Define
•1 volt: The potential difference between two points is said to be 1 volt if
1 joule of work is done in moving 1 coulomb of electric charge from one
point to another.
•1 ampere: 1 coulomb of charge passing through a cross – section of a
conductor in 1 second is one ampere. It is the S.I. unit of an electric
current.
•1 ohm: If one ampere current flows through the conductor, and 1 volt
potential difference is applied across it, then its resistance is 1 ohm.
•Potential: Electric potential is the electrical level.
•Resistivity: The resistivity of a conductor is defined as the resistance of
a conductor of unit length and the unit area of the cross – section.
•Electric power: Electric power is the rate at which electric energy is
consumed. It is the electrical work done per unit time.
10.
Differentiate between.
Resistances in series and parallel.
Resistance in series
1. If a number of resistances are connected
in such a way that the same current
flows through each resistance, then the
arrangement is called resistances in
series.
2. The effective resistance is a series
combination is greater than the
individual resistances.
3. This combination is used to increase
resistance in a circuit.
4. This combination decreases the current
in the circuit.
Resistance in parallel
1. If a number of resistances are connected
between two common points such that
the potential difference across each is
the same then that arrangement is called
resistances in parallel.
2. The effective resistance of the
combination is less than the individual
resistances.
3. This combination is used to decrease
resistance in the circuit.
4. This combination increases the current
in the circuit.
11.
Conductors and insulators.
Conductors
Insulators
1. Those
substances
through
which electricity can flow are
called conductors.
2. Electrical
resistances
of
conductors are very low.
3. They contain large number of
free electrons.
4. Generally
metals
are
conductors. E.g. silver, copper,
aluminium
1. Those substances through which
electricity cannot flow are called
insulators.
2. Electrical resistances of insulators
are infinitely very high.
3. They do not contain free electrons.
4. Generally non – metals are
insulators. E.g. wood, rubber,
plastic
12.
•Resistance and resistivity.
Resistance
Resistivity
1. The property of the conductor 1. The resistivity of a conductor is
due to which it opposes a flow of
the resistance of a conductor of
current through it is called
unit length and unit area of cross
resistance.
section.
2. The SI unit of resistance is Ohm 2. The SI unit of resistivity is Ohm()
metre (m)
3. The resistance of a conductor 3. The resistivity of a conductor
depends on its length and area of
does not depend on its length and
cross section.
area of cross section.
13.
High resistance and low resistance.
High resistance
Low resistance
1. A high resistance indicates a
material that hardly allows the
movement of electrons.
2. It is due to the less number of
free flowing electrons in the
outer most orbit of an element.
3. Substances with infinitely high
electrical
resistance
are
insulators.
4. High resistance provides low
conductivity.
1. A low resistance indicates a
material that readily allows the
movement of electrons.
2. It is due to large number of
electrons in the outer most orbit
of an element.
3. Substances with low electrical
resistances are good conductors.
4. Low resistance provides high
conductivity.
14.
State the laws:
Ohm’s law: Ohm’s law states that the electric current flowing
in a metallic conductor is directly proportional to the potential
difference across its terminals, provided physical conditions of
the conductor such as length, area of cross section,
temperature and material remain constant.
If I is the current and V is the potential difference across the
ends of a conductor then
(Where ‘R’ is constant)
R is the resistance which is constant for given conductor. The SI unit of resistance is ohm (
)
15.
•Joule’s law: - Joule’s law can be stated as
the quantity of heat generated (H) in a
conductor of Resistance (R), when a
current (I) flows through it for a time (t) is
directly proportional to:
•The square of the current.
•The resistance of the conductor, and
•The time for which the current flows.
Using Ohm’s Law we can write
16.
Find the expression for the resistors connected in series
•Resistors connected in series: - If the number of
resistance are connected one after another in
such a way that the same current flows through
each resistance, then the arrangement is called
resistance in series.
17.
LetR1, R2 and R3 are three resistances
connected in a series combination and
let RS be their effective resistance in the
circuit.
Let V1, V2 and V3 be the P.D. across
resistances R1, R2 and R3 respectively.
Let ‘V’ be the P.D. of the cell. Let ‘I’
be the current flow through each
resistance.
19.
Conclusion:
1. If the resistors are connected in series then.
2. In a circuit the current is the same in every part of
the circuit.
3. The resistance of the combination of resistors is
equal to the sum of the individual resistors.
4. The total voltage across the combination is equal to
the sum of the voltage drop across the separate
resistors.
5. The effective resistance in a series combination is
greater than the individual resistances.
6. This combination is used to increase resistance in a
circuit.
20.
Resistors connected in parallel: If the numbers
of resistance are connected between two
common points, such that the potential
difference across each resistance is the same,
then the arrangement is called resistance in
parallel.
21.
Three resistances R1, R2 and R3 are
connected in parallel between the points A
and B. Let Rp be the effective resistance in
the circuit.
A Cell E, Key K and the ammeters A
are also connected with resistances.
Let the current passing through R1 be
I1, R2 be I2, and R3 be I3- and that of R be I.
22.
Conclusion:
1. If the resistors are connected in parallel then:
2. The sum of reciprocals of the individual resistance is equal
to the reciprocal of equivalent resistance.
3. The current in various resistors are inversely proportional
to the resistances (higher is the resistance lower is the
current through it). However the total current is the sum of
the currents flowing in the different branches.
4. The voltage (Potential difference) across each resistors is
same.
5. The effective resistance of the parallel combination is less
than the individual resistance in the combination.
6. This combination is used to decrease resistance in the
circuit.
23.
Find the expression for resistivity of a material.
Resistance R of a conductor depends on the length ‘l’
and area of cross section ‘A’ of the conductor
Where:
(rho) is called resistivity of the conductor it is also called as specific
resistance.
If we put l = 1m and A = 1 sq. m. then R =
24.
Conclusion
Thus, resistivity of a conductor is defined
as the resistance of a conductor of unit
length and unit area of cross section. The SI
unit of resistivity is ohm – metre
Resistivity is the characteristic property of
material. It is different for different materials.
25.
P and Q are the two wires of same length
and different cross sectional areas and
made of same metal. Name the property
which is same for both the wires and that
which is different for both the wires.
Ans.
The property which is same for both the
wires is resistivity.
The property which is different for both the
wires is resistance.
26.
•Resistivity of some material is given below. State
which one will be the best conductor.
From the above table we find that of all the metals,
silver has the lowest resistivity (1.60 x 10 – 8 Ωm),
which means that silver offers the least resistance to
the flow of current through it. Thus, silver metal is the
best conductor of electricity.
Material
Resistivity
(Ωm)
Copper
Aluminium
Silver
Nickel
1.62 x 10 – 8 2.63 x 10 – 8 1.60 x 10 – 8 6.48 x 10 – 8
27.
If the resistance of wire A is
four times the resistance of
wire B, find the ratio of their
cross sectional areas.
Ans. Let resistance of wire A be
R1 and that of wire B be R2 .
28.
Two dissimilar bulbs are connected
in series, which bulb will be
brighter? (Hint: consider the
resistance of the bulb).
Ans. When two bulbs are connected in
series the first bulb will receive more
current. As the bulb has its own
resistance, less current will flow to the
next bulb. Hence the first bulb will
glow more.
29.
Electrical symbols used in electrical circuits
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