THE ELECTRIC SPARK
Q1. Choose the correct alternative
and rewrite the following:
1. 1mA = ___________ A
a.
103A
b. 10 – 3
A
c.
106A
d.
10
– 6 A
2. To increase the effective resistance
in a circuit, the resistors are connected in _________
a. Series
b.
Parallel
c.
Both
ways
d.
None
of these
3. 1 kilowatt hr = ____________ Joules.
a.
4.6
x 106 Joule
b. 3.6 x 106
Joule
c.
30.6
x 106 Joule
d.
3.6
x 105 Joule
4. If a P.D. of 12 V is applied across a
3Ω resistor, then the current passing through it is _______
a.
36A
b. 4A
c.
0.25A
d.
15A
Q2. State True or
False. If false correct it.
1. 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.
2. A voltmeter is always connected in
series with the device.
False: A voltmeter is always
connected in series with the device.
3. The conventional direction of flow of
current is from positive terminal to negative terminal of the cell.
Ans. True
4. Silver and copper are good
conductors.
True
5. Resistivity of pure meal is more than
alloys.
False: Resistivity of
pure metals is less than that of alloys.
6. 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.
Q3. Match the column.
I
|
II
|
III
|
Heat generated
|
Is used to reduce
effective resistance in a circuit
|
V = IR
|
Resistance in parallel
|
Proportional to the square
of current
|
|
Resistivity
|
Gives relation between V
and I
|
|
Ohm’s law
|
Depends on the material of
the conductor
|
|
Ans.
I
|
II
|
III
|
Heat generated
|
Proportional to the square
of current
|
|
Resistance in parallel
|
Is used to reduce
effective resistance in a circuit
|
|
Resistivity
|
Depends on the material of
the conductor
|
|
Ohm’s law
|
Gives relation between V
and I
|
|
Q4. Give scientific reasons.
1. The material used for fuse has low
melting point.
Ans.
i.
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.
ii.
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.
2. Wood and glass are good insulators.
Ans.
i.
When
a current flows through a conductor, the free electrons in the atoms move from
one end of the conductor to the other.
ii.
Certain
materials have less free electrons in their atoms and the current does not
easily flow through the material.
iii.
There
are no such free electrons in wood and glass. Hence they cannot conduct
electricity. Hence, they are good insulators.
3. The melting point of filament of a
bulb is very high.
Ans.
i.
The
bulb begins to glow only when the filament is heated to a high temperature, and
it becomes incandescent (bright) without melting.
ii.
This
happens only when the material of the filament has a high melting point.
iii.
Hence,
the material used in the filament of an electric bulb must have a high melting
point.
4. Connecting wires in a circuit are
made of copper and aluminium.
Ans.
i.
Copper
and aluminium offer a low resistance to the flow of current and hence they are
good conductors of electricity.
ii.
Copper
and aluminium are highly ductile and hence can be used for preparing this wire.
iii.
Copper
being more ductile, it is used in making thing wires, and aluminium is used for
making thicker wires.
iv.
Thus,
they are suitable for making wires used in electrical circuit.
Q5. Define
1. 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.
2. 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.
3. 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.
4. Potential: Electric potential is the
electrical level.
5. 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.
6. Electric power: Electric power is the rate at which
electric energy is consumed. It is the electrical work done per unit time.
Q6. Differentiate
between.
1. Resistances in series and parallel.
Resistance in series
|
Resistance in parallel
|
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.
|
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.
|
2. 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
|
3. Resistance and resistivity.
Resistance
|
Resistivity
|
1.
The property of
the conductor due to which it opposes flow of current through it is called
resistance.
2.
The SI unit of
resistance is Ohm (
3.
The resistance of a
conductor is inversely proportional to its area of cross section. This means
that a thick wire has less resistance.
|
1.
Resistivity of
a conductor is the resistance of a conductor of unit length and unit area of
cross section.
2.
The SI unit of
resistivity is Ohm metre (
3.
Resistivity is
independent of the shape and size of the conductor.
|
4. 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.
|
Q7. State the laws:
1. 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,
R is the resistance
which is constant for given conductor. The SI unit of resistance is ohm (W)
2. Joule’s law: - Joule’s law states that the
quantity of heat generated (H) is a conductor of Resistance (R), when a current
(I) flows through it for a time (t) is directly proportional to:
i.
The
square of the current.
ii.
The
resistance of the conductor, and
iii.
The
time for which the current flows.
Q8. Find the following.
a. Find the expression for the resistors
connected in series and parallel.
Ans.
i.
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.
1.
LetR1,
R2 and R3
are three resistances connected in a series combination and let R be
their equivalent resistance.
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.
2. According to
Ohm’s Law,
Conclusion
Therefore, equivalent resistance in series(R) is
equal to the sum of the individual resistances
ii.
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.
Three resistances R1, R2 and
R3 are connected in parallel between the points A and B. Let R be
the equivalent resistance of the parallel combination.
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.
Thus, the reciprocal of the equivalent resistance is equal to the sum of
the reciprocal of the individual resistance.
b. Find the expression for resistivity
of a material.
Ans.
Resistance R of a conductor depends on the length ‘
and area of cross section
of the conductor.
Where
(rho)
is called resistivity of the conductor it is also called as specific
resistance.
If we put
and
then
Conclusion
Thus, resistivity of a
conductor is defined as the resistance of a conductor of unit length and unit
area of cross section.
Q9. Answer the following:
1. 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.
i.
The
property which is same for both the wires is resistivity.
ii.
The
property which is different for both the wires is resistance.
2. Resistivity of some material is given
below. State which one will be the best conductor.
Material
|
Copper
|
Aluminium
|
Silver
|
Nickel
|
Resistivity (Ωm)
|
1.62 x 10 – 8
|
2.63 x 10 – 8
|
1.60 x 10 – 8
|
6.48 x 10 – 8
|
Ans. From the above table we find that of all the
metals, silver has the lowes 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.
3. 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
be
and that of wire B be
.
4. 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.