Wednesday, May 27, 2015

Physics 9702 Doubts | Help Page 154

  • Physics 9702 Doubts | Help Page 154


Question 762: [Current of Electricity]
In the circuit below, ammeter reading is I and voltmeter reading is V.

When switch is closed, which row describes what happens to I and V?
I                                   V
A         decreases                     decreases to zero
B         increases                      decreases to zero
C         increases                      stays the same
D         stays the same             increases

Reference: Past Exam Paper – June 2012 Paper 11 Q36 & Paper 13 Q35



Solution 762:
Answer: B.
When the switch is opened, current passes through both resistors in the circuit.

Let the resistance of one resistor be R. Assume they are of the same resistance.
When switch is opened, Total resistance RT in circuit = R + R = 2R

Ohm’s law: V = IR
This gives a current I (= E / RT where E is the e.m.f. of the supply). The total e.m.f. in the circuit is divided as p.d. across each of the resistor.

When the switched is closed, the resistor connected in parallel to the wire is short circuited. That is, all current would pass through the wire (of negligible resistance) instead of passing through that resistor.

When switch is closed, Total resistance RT in circuit = R + 0 = R
This gives a current I (= E / RT where E is the e.m.f. of the supply) which is now greater than before since RT is less.

Ohm’s law for the resistor: V = IR
Since the current I through the resistor (in parallel) is zero, the p.d. across it is also zero.









Question 763: [Current of Electricity]
A cell, two resistors of equal resistance and ammeter are used to construct four circuits. The resistors are the only parts of the circuits that have resistance.
In which circuit will ammeter show the greatest reading?


Reference: Past Exam Paper – November 2011 Paper 11 Q37 & Paper 13 Q36



Solution 763:
Answer: D.
Ohms’ law: V = IR
Current I in the whole circuit = V / RT where RT is the combined resistance of the resistors.

Let the resistance of 1 resistor = R. Let the e.m.f. of the supply = E
Circuit A consists of a series connection of the 2 resistors. So, RT = R + R = 2R. Current I = E / 2R. Here, the current will be the lowest.

To measure the current flowing through a resistor, the ammeter, which has a negligible resistance, must be connected in series with it. If it is connected in parallel with a component, the current would flows through it instead of the component (since the ammeter has zero resistance) and that component would be short circuited.

In circuit B, RT = R + 0 = R. Current I = E / R.

Circuit C and D both consists of a parallel combination of the 2 resistors.
Combined resistance RT in circuit = [1/R + 1/R]-1 = R / 2

Current I in circuit = E / (R/2) = 2 (E / R)
These 2 circuit contain the greatest current of all the 4 choices. However, at a junction, the current splits. So, the ammeter in circuit C gives only half the total current in the circuit.

Current flows from the positive terminal of the supply to its negative terminal. In choice D, the ammeter is connected directly to the positive terminal, so it reads the value of the current flowing in the whole circuit.










Question 764: [Dynamics > Collisions]
Diagram shows a particle X, with kinetic energy Ek, about to collide with a stationary particle Y.
Both particles have the same mass.

After colliding, X and Y travel onwards together as a single larger particle.
How much kinetic energy is lost in collision?
A 0                  B Ek / 4                        C Ek / 2                        D 3Ek / 4

Reference: Past Exam Paper – November 2014 Paper 13 Q16



Solution 764:
Answer: C.
In any closed system, momentum is always conserved.

Before collision,
Let speed of X be v. Let mass of X = mass of Y = m
Momentum of X = mu
Momentum of Y = m(0) = 0

After collision,
Both particles travel as a single larger particle.
Mass = m + m = 2m
Let the velocity after collision be v.
Total momentum = (2m)v = 2mv

From the conservation of momentum,
2mv = mu
Velocity v = u / 2


Kinetic energy before collision, Ek = ½ mu2
Kinetic energy after collision = ½ (2m) (u/2)2 = ¼ mu2

Lost in Kinetic energy = ½ mu2 – (¼ mu2) = ¼ mu2 = Ek / 2









Question 765: [Current of Electricity > Potential divider]
In circuit shown, XY is a length L of uniform resistance wire. R1 and R2 are unknown resistors. J is a sliding contact that joins the junction of R1 and R2 to points on XY through small signal lamp S.

To determine ratio V1 / V2 of the potential differences across R1 and R2, a point is found on XY at which the lamp is off. This point is at a distance x from X.
What is value of the ratio V1 / V2?
A L / x                         B x / L                         C (L – x) / x                            D x / (L – x)

Reference: Past Exam Paper – June 2011 Paper 11 Q37 & Paper 13 Q36



Solution 765:
Answer: D.
For the lamp to be off, the potential difference across it should be zero so that no current flows through it. That is, the potential at the junction of R1 and R2 should be equal to the potential at the sliding contact J

The p.d. across R1 is V1 and the p.d. across R2 is V2. So, V1 should be equal to the p.d. across XJ and V2 should be equal to the p.d. across JY.

Since point X is connected to the negative terminal of the supply, it can be considered to be at a potential of 0. Point Y is at the maximum potential, equal to the e.m.f. of the supply.

The wire XY, together with the sliding contact J, can be considered as a potential divider.

Resistance R of the wire = ρL / A
Since ρ and A are assumed to be constant, the resistance of the wire is proportional to the length L.
Ohm’s law: V = IR
The p.d. across the wire is proportional to R, which is itself proportional to L. So, the p.d. across the wire is proportional to the length of wire being considered.

A length of L corresponds to a p.d. equal to the e.m.f. E of the supply
A length of x corresponds to a p.d. equal to (E / L) × x = (x / L) × E

Note that this is a p.d. (potential difference – or in other words, a difference in potential).  Since X is at a potential of zero, point J would be at a potential of [(x / L) × E] – 0 = (x / L) × E.
V1 = (x / L) × E

Now, the part JY of the wire corresponds to a length (L – x) of the wire.
A length of (L – x) corresponds to a p.d. equal to (E / L) × (L – x) = [(L – x) / L] × E
V2 = [(L – x) / L] × E

Ratio = V1 / V2 = x / (L – x)










Question 766: [Pressure]
Atmospheric pressure at sea level has value of 100 kPa.
Density of sea water is 1020 kg m–3.
At what depth in the sea would total pressure be 110 kPa?
A 1.0 m                       B 9.8 m                       C 10 m                        D 11 m

Reference: Past Exam Paper – June 2010 Paper 11 Q17 & Paper 12 Q18 & Paper 13 Q15



Solution 766:
Answer: A.
The pressure at sea level is 100 kPa due to the atmosphere (atmospheric pressure).

The pressure only due to a liquid column of depth h and density ρ is hρg.

Total pressure at a depth h = Atmospheric pressure + hρg
110 000 = 100 000 + (h × 1020 × 9.81)
Depth h = 10 000 / (1020 × 9.81) = 0.999 = 1.0m



6 comments:

  1. Sir, by which month of this year are you planning to start with paper 5 please?

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    Replies
    1. I don't know. Have not decided yet. I plan the add the other papers first (all / most) of paper 1, 2 and 4.

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  2. Can I just say, how much this site has helped me, I can never put into words. I didn't know whom to ask the questions I couldn't solve and I came across this and it was like a huge blessing for me. I am forever grateful and indebted to you, whoever you are.

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    Replies
    1. No problem. I'm glad it's helping others. I hope the explanation can be understood easily

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