# 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 1

^{st}wave and Y is at the trough of the 2^{nd}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]

__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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