• Physics 9702 Doubts | Help Page 155

Question 767: [Current of Electricity > Resistance]

Which graph best represents the way in which current I through a thermistor depends upon the potential difference V across it?

Reference: Past Exam Paper – June 2011 Paper 12 Q35

Solution 767:

Answer: A.

As the potential difference V across the thermistor increases, the temperature of the thermistor also increases, causing its resistance R to decrease.

Ohm’s law: V = IR

As the p.d. V increases, the current increases. This is represented by a positive gradient in the graph. [D is incorrect]

Resistance R = V / I

I / V = 1 / R

As the resistance R decreases, the value of (1/R) increases. So, the ratio V / I increases.

So, current I increases with the p.d V, with an increasing gradient since the resistance R is also decreasing as the p.d. V increases.

It is important to note though that the gradient only tell us the WAY the resistance is changing. We cannot use the value of the gradient to obtain the VALUE of the resistance. We need to consider the ratio of V / I of a single point.

Question 768: [Waves > Diffraction Grating]

Light of wavelength λ passes through diffraction grating with slit spacing d. A series of lines is observed on a screen.

What is angle α between the two first order lines?

A sin^-1 (λ / 2d)             B sin^-1 (λ / d)               C 2sin^-1 (λ / 2d)                       D 2sin^-1 (λ / d)

Reference: Past Exam Paper – November 2013 Paper 11 & 12 Q28

Solution 768:

Answer: D.

Diffraction grating: d sinθ = nλ

The angle θ used in the diffraction grating formula is taken from the 0^th order (along the line of incidence of the light) to the other orders. So, we need to divide the angle α shown in the diagram by 2. This can be done by drawing a line bisecting α.

d sin(½ α) = nλ 

For first order, n = 1

½ α = sin^-1(λ/d)

α = 2sin^-1 (λ / d)

Question 769: [Dynamics > Equilibrium]

Diagram shows a solid cube with weight W and sides of length L. It is supported by a frictionless spindle that passes through the centres of two opposite vertical faces. One of these faces is shaded.

Spindle is now removed and replaced at a distance L / 4 to the right of its original position.

When viewing the shaded face, what is torque of the couple that will now be needed to stop the cube from toppling?

A WL / 2 anticlockwise

B WL / 2 clockwise

C WL / 4 anticlockwise

D WL / 4 clockwise

Reference: Past Exam Paper – June 2012 Paper 12 Q15 & November 2017 Paper 12 Q12

Solution 769:

Go to
The diagram shows a solid cube with weight W and sides of length L. It is supported at rest by a frictionless spindle that passes through the centres of two opposite vertical faces.

Question 770: [Current of Electricity]

When four identical resistors are connected as shown in diagram 1, ammeter reads 1.0 A and voltmeter reads zero.

Resistors and meters are reconnected to the supply as shown in diagram 2.

What are meter readings in diagram 2?

voltmeter reading / V              ammeter reading / A

A         0                                              1.0

B         3.0                                           0.5

C         3.0                                           1.0

D         6.0                                           0

Reference: Past Exam Paper – November 2007 Paper 1 Q35

Solution 770:

Answer: A.

In diagram 1, the voltmeter reads zero because the potential difference across it is zero – that is, both of its junctions are at the same potential. Current splits at a junction. Since the resistors are identical, the current splits equally into 2. The ammeter reads a current of 1.0A, so the total current in the circuit is 1.0 + 1.0 = 2.0A.

Now, consider diagram 2. Current flows from the positive terminal of the supply and is split equally at a junction. So, the ammeter still reads the current that has been split equally into 2. This current is 2.0 / 2 = 1.0A. [B and D are incorrect]

From that junction connected to the positive terminal of the supply (the junction at the bottom), one of the current flows through 1 resistor to reach the other junction (on the right). The other current flows through the other resistor to reach the other junction (on the left).

Since the same amount of current (= 1.0A) flows through identical resistors (as described above), the potential of the left and right junctions would be the same. These are connected to the voltmeter. Thus, the potential difference across the voltmeter (difference in potential at its junctions) is zero. The voltmeter reading is zero.

Question 771: [Kinematics]

A boy throws a ball vertically upwards. It rises to maximum height, where it is momentarily at rest, and then falls back to his hands.

Which row gives acceleration of the ball at various stages in its motion? (Take vertically upwards as positive. Ignore air resistance.)

rising               at maximum height                 falling

A         –9.81 m s^–2                  0                                  +9.81 m s^–2

B         –9.81 m s^–2                  –9.81 m s^–2                  –9.81 m s^–2

C         +9.81 m s^–2                  +9.81 m s^–2                  +9.81 m s^–2

D         +9.81 m s^–2                  0                                  –9.81 m s^–2

Reference: Past Exam Paper – November 2011 Paper 11 Q8 & Paper 13 Q9

Solution 771:

Answer: B.

The acceleration of free, g is a constant and is directed towards the surface (downwards). Since the positive direction is taken as being vertically upwards, the value of g should be negative.

So, g is negative and constant at any position.