Electricity revision

1.Which of the two wires of similar dimensions, copper or nichrome,

would you use as heating element is an electrical heater.
Give reasons to justify your answer
Nichrome wire is used for electric heaters because it has a high melting
point and high resistivity.
Copper has little resistance, so it does not glow.
Copper melting point is low as compared to nichrome

2.Write a mathematical relation relating emf, terminal voltage and

potential drop.
Let V be the terminal voltage and E be the emf. Having a load R and
current flowing I and internal resistance r can be calculated as –
I=E/R+r
And the potential difference across the terminal voltage calculated as:
V=I.R
V= ER/R+r
3. Define Electro-motive force.
Electromotive force is defined as the electric potential produced by either
an electrochemical cell or by changing the magnetic field.
4. State the factors affecting emf
(i) nature of electrodes (ii) concentration of electrolyte (iii) temperature of the
cell (iv) area of electrode immersed in electrolyte (v) nature of electrolyte.
5.State two differences between the e.m.f. and terminal voltage of a cell.

Terminal voltage
E.m.f of cell
of cell
It is measured by the It is measured by
amount of work done the amount of work
per unit change in done per unit
moving a positive test charge in moving a
charge in the complete positive test charge
circuit inside and in the circuit
outside the cell. outside the cell.
It depends on the
It is the the
amount of current
characteristic of the cell,
drawn from the
i.e., it does not depend
cell. More the
on the amount of
current drawn from
current drawn from the
the cell, less is the
cell.
terminal voltage.
6.Name two factors on which the internal resistance of a cell depends and
state how does it depend on the factors stated by you.
The factors on which the internal resistance of a cell depends are —
The surface area of the electrodes — larger the surface area of electrodes,
less is the internal resistance.
The distance between the electrodes — more the distance between the
electrodes, greater is the internal resistance.

7.A given metallic wire of resistance R is doubled on itself. What will

be its new resistance?
When a given metallic wire is doubled on itself, its length is reduced to half,
but its area of cross section gets doubled. So, the resistance of the wire will
become one-fourth i.e., the new resistance of wire will be R/4
8.Define specific resistance
Specific resistance is also known as resistivity. The resistivity of a material is
the resistance of a wire of that material of unit length and unit area of cross-
section.
Its S.I. unit is ohm-metre.
9.What is meant by saying that the potential difference between two
points is 1 v?
The potential difference between two points is said to be 1 volt, if 1 joule of
work is to be done for moving charge of 1 coulomb from one point to another
10. On what factor does the resistance of a conductor depend?
: The resistance of a conductor depends upon the following factors:
(a) Length of the conductor
(b) Cross-sectional area of the conductor
(c) Material of the conductor
(d) Temperature of the conductor
11. Why are coils of electric toasters and electric irons made of an alloy
rather than a pure metal?
Coils of electric toasters and electric irons are made of an alloy due to
the following reasons:
(i.) Resistivity of an alloy is generally higher than that of pure metal.
(ii.) At high temperature, an alloy does not oxidize readily. Hence, coil of an
alloy has longer life.
12.What are the advantages of connecting electrical devices in parallel
with the battery instead of connecting them in series?
Advantage of connecting electrical devices in parallel with the battery are as
follows:
(i) Voltage across each connecting electrical device is same and device take
current as per its resistance.
(ii) Separate on/off switches can be applied across each device.
(iii) Total resistance in parallel circuit decreases, hence, a great current may
be drawn from cell.
(iv) If one electrical device is damaged; then other devices continue to work
properly.
13.Explain the following:
(a) Why is the tungsten used almost exclusively for filament of electric
lamps?
(b) Why are the conductors of electric heating devices, such as bread-
toasters and electric irons, made of an alloy rather than a pure metal?
(c) Why is the series arrangement not used for domestic circuits?
(d) How does the resistance of wire vary with its area of cross-section?
(e) Why are copper and aluminium wires usually employed for electric
transmission?
Solution :
(a) For filament of electric lamp we require a strong metal with high melting
point. Tungsten is used exclusively for filament of electric lamps because its
melting point is extremely high.

(b) Conductors of electric heating devices are made of an alloy rather than a
pure metal due to high resistivity than pure metal and high melting point to
avoid getting oxidized at high temperature.

(c) Series arrangement is not used for domestic circuits as current to all
appliances remain same in spite of different resistance and every appliance
cannot be switched on/ off independently.

(d) Resistance of a wire is inversely proportional to its cross-section area.

(e) Copper and aluminium wires are usually employed for electricity
transmission because they are good conductor with low resistivity. They are
ductile also to be drawn into thin wires.

14.State and define Ohm’s law.

Answer: Ohm’s law states that at a constant temperature, the current
flowing in a conductor is directly proportional to the potential difference
across its ends, i.e.,
if the potential difference is halved, the current is also halved and if p.d. is
doubled, the current is also doubled, i.e., V (potential difference) ∝ i
(current) Or V = iR, where R is constant, called the resistance of the
conductor. So V = iR represents Ohm’s law.
15: Define the unit ohm.
Answer: One ohm is the resistance of a conductor in which 1 ampere
current flows when the potential difference across its ends is 1 volt.

16: Define ampere and volt with respect to Ohm’s law.

Answer: Ampere: It is the S.I. unit of current. If a current flows in a
conductor of resistance of 1Ω, when the potential difference across its ends
is 1 volt, the current is said to be 1 ampere, i.e., 1 ampere = 1 volt/1 ohm
Volt:
It is the S. I. unit of potential difference. If a current of 1 ampere flows in a
conductor of resistance 1 ohm, then potential difference across its ends is
said to be 1 volt, i.e., 1 volt = 1 ampere × 1 ohm.
 17: State the limitations of Ohm’s law.
Answer: There are two main limitations: (i) Temperature, (ii) Physical
condition of the conductor. It is found that the ratio V/I is no longer
constant when the temperature is not kept constant. Generally, resistance
increases with the rise in the temperature of the conductor. Physical
conditions of any conductor mainly include: (i) Its length, (ii) Its cross-
sectional area, (iii) The kind of the material. If there is no change in any of
the above three conditions and also other condition like temperature
remains constant, then Ohm’s law holds good, i.e., the ratio V/I remains
constant.
18: What is non-ohmic resistor?
Answer: This is a resistor which does not obey Ohm’s Laws. V -1 graph for
non-ohmic resistor is not a straight line.
19.A given wire is stretched to double its length. How will its resistance
change?
Answer: When a given wire is stretched to double its length, its area of
cross-section will be halved, so the resistance of wire will become four times.