Monday, May 30, 2016

Physics 9702 Doubts | Help Page 238

  • Physics 9702 Doubts | Help Page 238



Question 1109: [Waves > Phase difference]
A displacement-time graph for a transverse wave is shown in the diagram.

The phase difference between X and Y can be expressed as nπ.
What is the value of n?
A 1.5               B 2.5               C 3.0               D 6.0

Reference: Past Exam Paper – November 2006 Paper 1 Q25



Solution 1109:
Answer: C.
The diagram consists of 2 complete waves. X is at the crest of the 1st wave and Y is at the trough of the 2nd wave.
The distance between the 2 points is λ + 0.5 λ = 1.5 λ

1 λ represents one complete wave and this corresponds to a phase of 2π
1.5 λ corresponds to a phase of (2π / 1λ) × 1.5λ = 3π











Question 1110: [Current of Electricity > Parallel connection]
A network of electrical components is connected across a battery of negligible internal resistance, as shown.

The resistance of the variable resistor is increased.
What is the effect on the readings of the ammeter and voltmeter?
ammeter          voltmeter
A         decreases         increases
B         increases          decreases
C         unchanged       decreases
D         unchanged       increases

Reference: Past Exam Paper – November 2012 Paper 11 Q36



Solution 1110:
Answer: C.
The sum of p.d. in any loop is equal to the e.m.f. in the circuit (Kirchhoff’s law). When the resistance of the variable resistor is increased, the p.d across it increases (V = IR). Thus, the voltmeter reading, which shows the p.d. across the fixed resistor in series with the variable resistor, decreases. [A and D eliminated]

The current flowing through the ammeter is the same as that flowing through the resistor in series with it. Again, from Kirchhoff’s law, the p.d. across that branch is equal to the e.m.f. in the circuit. Since the resistor has a fixed resistance, the current flowing through it will be the same as previously (I = V / R and both V and R are the same). [C is correct]

Increasing the resistance of the variable resistor causes the current flowing through the variable resistor to decrease and thus, the overall current in the circuit decreases. But the current in the right branch is still the same.











Question 1111: [Current of Electricity > Kirchhoff’s laws]
Each of Kirchhoff's two laws presumes that some quantity is conserved.
Which row states Kirchhoff's first law and names the quantity that is conserved?



Reference: Past Exam Paper – November 2009 Paper 11 Q33 & Paper 12 Q32 & June 2015 Paper 12 Q32



Solution 1111:
Answer: A.
Kirchhoff’s first law states that the algebraic sum of currents into a junction is zero. Quantity conserved: Charge (as current is the flow of charges).

Kirchhoff’s second law states that the e.m.f. in a loop is equal to the algebraic sum of the product of current and resistance round the loop. Quantity conserved: Energy.


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