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Question 1. A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R, then the ratio RR is:
  1.    125
  2.    15
  3.    5
  4.    25
Answer: Option D
: D

Given the wire iscut into 5 equal parts.
Let the initial resistance of the wire be R
where R=ρlARl(lengthofthewire)
then, resistance of each one of the five parts =(R5)
Equivalent resistance (R')of resistorsconnected in parallel is given by1R=1R1+1R2..1Rn
Equivalent resistance (R') of five parts connected in parallel is given by:
1R=1(R5)+1(R5)+1(R5)+1(R5)+1(R5)R=R25
Therefore, RR=25

Question 2. 1 kg piece of copper is drawn into a wire of cross-sectional area 1 mm2, and then the same wire is remoulded again into a wire of cross-sectional area 2 mm2. Find the ratio of the resistance of the first wire to the second wire.
  1.    2:1
  2.    4:1
  3.    8:1
  4.    16:1
Answer: Option B
: B

Volume of a solid is the product of length and cross-sectional area. V=AL
Since volume is same in both the cases, the length of second wire will be half of the length of 1st wire.
Accordingly, let length of 1st wire = L
area of 1st wire= A = 1mm2
So resistance R=ρLA=ρL1=ρL

Length of the second wire =L2 and
area of 2ndwireA=2mm2
So resistance of 2nd wire R=ρL2A
R=ρL2×2
R=14(ρL)
R=14(R)
RR=41
R : R' = 4 : 1
we get the resistance of the 1st wire is 4 times the value of resistance of the 2nd wire.

Question 3. The physical quantity that determines the rate at which energy is delivered by an electric current is ___.
  1.    potential difference
  2.    electric power
  3.    resistance
  4.    electrical resistivity
Answer: Option B
: B

The physical quantity that determines the rate at which energy is delivered by an electric current is electricpower. The SI unit of electricpower is watt (W). 1 W electrical poweris equivalent to 1 Joule ofelectricalenergy consumed in 1 second time. It is mathematically expressed as P=Et where E is the electrical energyand t is time.

Question 4. The resistivity of all pure metals increase with the rise in temperature.
  1.    True
  2.    False
  3.    resistance
  4.    electrical resistivity
Answer: Option A
: A

Resistivity depends on temperature. Withthe increasein temperature, the random motion of electrons increases. Asa result, the number of collisions of electrons with the positive ions increases. Hence, the resistivity of all pure metals increases with the rise in temperature and decreases with the decrease in temperature.

Question 5. Resistance of alloys decreases with the increase in temperature.
  1.    True
  2.    False
  3.    8:1
  4.    16:1
Answer: Option B
: B

In alloys,the increase in temperature results in an increase of its resistance, but unlike pure metals, the increase is relatively small and irregular.As the temperature increases, the molecules in the atom vibrate with more frequency, they collide with each other more frequently which makes it tough for the movement of free electrons. Thus the resistance increase with increasing temperature in the metal.

Question 6. For a constant resistance, the correct relation between heat produced (H) and electric current (I) flowing is:
  1.     H∝I 
  2.    H∝1I
  3.     H∝I2
  4.    H∝1I2
Answer: Option C
: C

According to Joule's law, theheat produced in a resistor is;
(i) directly proportional to the square of current for a given resistance.
(ii) directly proportional to resistance for a given current.
(iii) directly proportional to the time for which the current flows through the resistor.
That is H=I2×R×t
HenceHI2 is the right relation.