Electricity HOTS

Question 1

What are the precautions to be taken while perfoing verificationof ohm’s law

Answer 1

i) current should not be allowed to pass through the circuit continuously, that is the key should not be closed for a long time. this is because when curent passes for a long time, heat is evolved and ohm’ law is invalid.

ii)Connections should be tight
iii) posiition of voltmeter and ammeter should be correct

Question 2

What is the cause of resistance?

Answer 2

i)A conductor has a large number of free electrons.
ii) When a potential difference is maintained across two ends of a conductor, the free electrons drift from one end to tnaother.
iii) During this movement, they collide with each toher as well as with the conductor( walls of the wire).
iv)These collisions make them lose their kinetic energy which appears as heat energy.
v)When the current is passing through a resistor for a long time, a lot of heat will be given out.
vi) When the collisions suffered by the electrons increase, the resistance increses.

Question 3

define least count?

Answer 3

in any instrument the value of each divison is called least count. Instrument of minimal least count measures accurately.

Question 4

what is zero error?

Answer 4

when the needle does not rest exactly on the zero.
real value- observed value- zero error

zero error-+ve- right side needle
zero error- -ve - left side needle.

Question 5

give the resistivity of skin.

Answer 5

The resistivity of human body when skin is dry is 5 x 10⁵Ωm.
resistance becomes lower, when skin is wet.

Question 6

what can be said about resistivity of pure metals,alloys and insulators?

Answer 6

The metals and alloys
have very low resistivity in the range of 10–8 Ω m to 10–6 Ω m. They are
good conductors of electricity. Insulators like rubber and glass have
resistivity of the order of 1012 to 1017 Ω m. Both the resistance and
resistivity of a material vary with temperature
metals=10-8
alloy=10-6
insulator=10¹¹⁻¹⁷

Question 7

why do alloys often have higher resistivity?

Answer 7

Alloys have higher resistance than pure metal because alloys are a mixture of many metals. The conductance of one metal is influenced/decreased by the other.

Question 8

why are connecting wires made of cu or al

Answer 8

the connecting wires are made of cu or al because they offer very low resistivity. al and cu are less expensive and easily rather than ag so the wires are made of them.

Question 9

what are some uses of insulators?

Answer 9

insulators are used to protect ourselves from sever shocks of electric current. When we touch, handle with elecric wires we use rubber gloves.

Question 10

why is lighting element of bulbs made of tungsten?

Answer 10

filament of higher electric bulb i made of tungesten metal. Tungsten isused as an alloy. It has high resistance( the filament is coiled so as to increase the length, increase resistance,heat energy is more which is converted to light) and low resistivity.Tungsten has high melting point, low resisitivty and high resistsance and does nit burn. It also incandescent, that is it glows at high temperature. It has low thermal expansion

Question 11

which one is having higher resistance: a 60 w bulb or a 40 w bulb

Answer 11

P=V^2/ R
when V is kept constant
P inversely proprtional R.
So 40 w has higher resistance.

Question 11

write a short note on fuse

Answer 11

i) It is a safety device
ii)It is based on the heating effect of electric current
iii) The fuse wire is made of a metal that has a very low melting point such as solder( alloy of tin,lead),cu,al
iv) It is connected in series to the circuit bfr all other appliances
v)Fuse wire must be connected with the live wire
vi)the thickness of the fuse wire gives its capacity, the 15A fuse is thicker than 5A fuse.
vii)Fuse helps prevent a fire that is caused due to shortcircuiting. Whenever there is a large amount of current flowing throught he circuit, it causes greater heat due to the heating effect of current. The fuse wire with a low melting point melts and breaks circuit saves appliance
viii) We should always use the fuse with the correct fuse rating. EXcessive fuse rating, it cannot act as a safety device
ix) If fuse rating is 5A, then as soon as current in circuit reachs 5A, the fuse metls

Question 12

write a note on super conductor

Answer 12

Mercury at a very low temperature at 4.2K has 0 resistance or 100% conductance.This phenomenon of loss of electrical resistance of a substance on cooling it to an extremely low temperature is known as superconductivity.

Question 13

Why are coils of electric toasters and electric irons made of an alloy
rather than a pure metal?

Answer 13

i) alloys have more resistivity and hence more resistance to the flow of electric current. This is because the conductance of a metal is infleunced/decreased by another.
ii)They do not oxidise or burn readily even at high temp
iii)They have a high melting point and do not melt even at very high temp

Question 14

which is the best conductor of electricit

Answer 14

silver is the best conductor of electricity
ag>cu>al

Question 15

explain the importance of the alloy manganin

Answer 15

alloy of magnaese,copper and nickel.
- it is the alloy used to prepare standard resistance
-the resistance does not change with temperature
-manganin has higher resistivit coz alloy

Question 16

an electric fuse of 3A is connected to an electric iron of P=1kW which oeprates at 220V. what is the result why

Answer 16

since the iron box withdraws more current than the rating of the fuse,due to heating effect of current the fuse wire melts and breaks the circuit.

Question 17

state coulomb’s law

Answer 17

The electric force between two charges depends directly on the product of charges and inversely as the square of the distance between the charges, F ∝ q1q2 and F ∝1/r2 ⇒ F = kq1q2/r2 Where q1, q2 are the charges, r is the distance between them and k is the constant of proportionality.

Question 18

why does resistance increase with temp

Answer 18

with increase temp, the kinetic energyof electrons increase resistance increase

Question 19

why does an ammeter have 0 resistance?

Answer 19

i)An ammeter is a device that is used to measure the current flowing in the circuit.
ii)Ammeter is a low resistance device.
iii) this is because it is connected in series to the circuit.
iv)It adds to the resistance of the circuit.
v) if the ammeter has a high resistance,the total resistance of the circuit will become high and then the current will not flow through the circuit, It will minimize the current.
vi) so it has low reistance

Question 20

why does a voltmeter has infinite resistance

Answer 20

i) The ideal voltmeter has infinite rsistance.
ii) This is because it is connected in parallel to the circuit.
iii) If it has low resistance, then it will draw current and the other resistors will not flow through the other resistors,
iv) the current will flow through voltmeter, and give a fuly reading.

Question 21

P directly proportional I but also inversely proportiona to V why is tht

Answer 21

P=I²R
P∝R
this is only when I is constant. I is constant only when series connection. So this expression is used only in series connection

P=V²/R
P∝1/R
this is only used when V is constant. V is constant only when parallel connection. So this expression is used only in paralle connection.