Monday, November 2, 2015

Physics 9702 Doubts | Help Page 222

  • Physics 9702 Doubts | Help Page 222


Question 1056: [Current of Electricity > Charge]
The charge that a fully-charged 12 V car battery can supply is 100 kC. The starter motor of the car requires a current of 200 A for an average period of 2.0 s. The battery does not recharge because of a fault.
What is the maximum number of times the starter motor of the car can be used?
A 21                            B 25                            C 42                            D 250

Reference: Past Exam Paper – November 2008 Paper 1 Q34



Solution 1056:
Answer: D.
The starter motor of the car requires a current of 200 A for an average period of 2.0 s.
For one use, total charge Q = It = 200 × 2.0 = 400 C

The charge that a fully-charged 12 V car battery can supply is 100 kC (= 100 000 C).
Number of times = 100 000 / 400 = 250










Question 1057: [Kinematics > Graph]
When a car driver sees a hazard ahead, she applies the brakes as soon as she can and brings the car to rest.
The graph shows how the speed v of the car varies with time t after she sees the hazard.

Which graph represents the variation with time t of the distance s travelled by the car after she has seen the hazard?

For November 2004:


For June 2009:



Reference: Past Exam Paper – November 2004 Paper 1 Q8 & June 2009 Paper 1 Q6



Solution 1057:
Answer: C.
The distance travelled is given by the area under the speed-time graph. In a distance-time graph, the speed is given by the gradient.

From time = 0 to time = t1, the speed is constant. So the graph should be a straight line in the distance-time graph for this section. [D is incorrect] The speed is not zero, so the gradient cannot be zero. [A is incorrect]

From t1 the speed decreases uniformly until t2, that is, there is a constant deceleration. Thus, the gradient of the distance-time graph (which represents speed) should decrease with time. [B is incorrect]











Question 1058: [Current of Electricity > Resistance]
The diagram shows a light-dependent resistor (LDR) and a thermistor forming a potential divider.

Under which set of conditions will the potential difference across the thermistor have the greatest value?

illumination                 temperature
A                     low                  low
B                     high                 low
C                     low                  high
D                     high                 high

Reference: Past Exam Paper – June 2006 Paper 1 Q37 & June 2014 Paper 12 Q37



Solution 1058:
Answer: B.
A potential divider is formed.
For the potential difference across the thermistor to be greatest, the resistance of the LDR should be small and the resistance of the thermistor high.

Since the resistance of the LDR decreases as light intensity increases, the illumination should be high.

When the temperature is low, the resistance of the thermistor is high.











Question 1059: [Matter > States of matter]
Which line in the table gives approximate ratios of density and molecular spacing for a substance in its solid, liquid and gas phases?


Reference: Past Exam Paper – November 2002 Paper 1 Q22



Solution 1059:
Answer: A.
Density = mass / volume

The molecular spacing can be taken as a measure of the volume of a substance.
Volume = (molecular spacing)3
From the formula for density, it can be seen that a larger molecular spacing results (in a bigger volume and thus,) in a smaller density.

The kinetic model of matter assumes that there is no intermolecular forces between gas molecules / atoms. Thus, a gas would definitely have larger molecular spacing than solids and liquids.

Now, the molecules of a liquid is less closely packed than in a solid, but by what amount?

Consider choice B. If the molecular spacing of a liquid is 10 times that of a solid, it means that solids have a density of 1000 times that of liquids since the volume of liquids would then be (103 =) 1000 times that of solids. But we know that this is not correct. So, the molecular spacing of solids and liquids mat be taken to be nearly the same.

The relative molecular spacing of a gas would be 10 times greater (1000 is too big).



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