Free IGCSE Physics Online Lesson and Q&A Session

48 Replies to “Free IGCSE Physics Online Lesson and Q&A Session”

1. Hi!

   Kindly answer the following questions:

   1- How many significant figures should the final answers be rounded up to?

   2- (From Paper 43, Question 4cii, May/June, 2018, Syllabus Code- 0625)
   “State and explain the effect on the range of the thermometer of using a smaller bulb that contains less mercury”. Kindly explain how the range will increase (as mentioned in the marking scheme).

   • Hello Sahlarasul,

     The common practice is to report the answer to the nearest whole number unless it is stated in the question. The range of a thermometer is the difference between the maximum and minimum temperatures that the thermometer can read. Using a thermometer with a smaller bulb, it contains less liquid and therefore, absorbs heat in a shorter time. This would mean that the temperature range of the thermometer will have to increase to show the correct temperature.

     Hope this helps! We are here for you.

     Best Regards,
     Physics Department

     • Thank you so much for your help!

       • Dear Sahlarasul,

         You’re welcome.

         Best Regards,
         Physics Department

2. Hi! In Physics Paper 41 Oct/Nov 2020, the answer provided by the mark scheme for Q3 is a bit unclear: the answer in 3(b)(i) is 100 N which is calculated by excluding the atmospheric pressure; could you please explain to me why it has to be excluded? Also, surprisingly, the next question[3(b)(ii)] doesn’t use the answer from (b)(i) (100N) but uses the answer in which the atmospheric pressure is included (2600 N) to calculate the work done — which I also do not understand. Is there a possibility that the answer from the mark scheme is incorrect? Or is there something that I’ve misunderstood?
   Thank you and I look forward to your reply.

   • Hello Kyla,

     For this particular question, you should take note that force is different from pressure. For 3(b)(i), the question is the resultant force acting on the piston so we should consider the gauge pressure exerted by the gas inside the piston. For 3(b)(ii), once the gas was able to move the piston at a certain distance, we need to consider that the piston is open to the atmosphere (as stated in Fig 3.3), and the force exerted by the gas must be based on the absolute pressure, that is atmospheric pressure and gauge pressure.

     We hope this will help and we are here for you.

     Best regards,
     Physics Department

3. Hi, I have doubts about these two questions in paper 0625/42/M/J/20:
   About question 2(d), why is the answer ‘less air resistance/drag’? Is it because when speed decreases, air resistance on the train decreases as well, so the resultant force (backwards) is smaller hence more time is needed (according to F=m(v-u)/t)? But isn’t braking force being applied too?

   About question 4(c), in 0625/42/F/M/19 it says to increase the range of the thermometer it has to have a smaller bulb, but in this paper (0625/42/M/J/20) however, it says we’ll need a larger glass bulb to increase the range of the thermometer. Which one is correct?

   • Hello Lea,

     For question 2(d), you are correct that the same braking force is applied on the train which is the case in a common train system. What we need to consider here is the decreasing air resistance on the train as it comes to a halt which affects the motion of the train. It would take more time to make the train stop than to decrease its velocity.
     For the second question, I believe you are pertaining to the mark schemes for these previous papers. Mark schemes show the proposed basis on which examiners would award marks and it is different from the Model Answers that we have so there will always be some difference. What I can share with you about thermometer sensitivity and thermometer range is that they are somewhat opposite. To increase the sensitivity of a liquid-in-glass thermometer, you can use a narrower bore but if you want to increase the range you can use a wider bore. You can also use a liquid that expands more to increase the sensitivity of the thermometer and use a liquid that expands less to increase the range of the thermometer.

     We hope this will help and we are here for you.

     Best regards,
     Physics Department

     • Really sorry, but for question 2(d) I still don’t get it. Aren’t both air resistance and the braking force being applied backwards? So why would air resistance have any effect? In fact, I wrote ‘more impulse is needed to overcome the inertia’, is that acceptable?

       Thank you and I appreciate your quick response.

       • Hello Leah,

         Yes, you are correct in terms of the direction of air resistance and braking force. However, air resistance is dependent on the speed of the train. The slower the speed, the less air resistance will be on the train, and there will be a lesser force to stop it from slowing down.
         We hope this helps.

         Best regards,
         Physics Department

4. Hi! I would like to ask a tip on the graph questions in Physics 0625 Paper 6 (Alternative to practical). When do we generally draw a smooth curve and when do we draw the line of best fit?

   • Hello Agamjit,

     A line of best fit is a straight line that is the best approximation of the given set of data and is used to study the relation between two variables. A line of best fit can be roughly determined by drawing a straight line on a scatter plot so that the number of points above the line and below the line is approximately equal. Drawing curved graphs is no different from drawing straight line graphs, except that a good curve is a bit more difficult to draw well. A curved graph means that the slope is changing with respect to the independent variable and the relationship is non-linear. you may start off by drawing the curve lightly, and then make it firmer when you are satisfied. We hope this will help and remember that we are here for you.

     Best regards,

     Physics Department

5. Hi,could you please solve this and give me a brief about the concept about half life as i have had difficulty in all the questions regarding radioactivity half life.
   Plutonium-239 has a half life of of 24 000 years
   2 kg of plutonium 239 is sealed in a lead container
   calculate the mass of plutonium 239 remaining after 120 000 years
   ans-0.0625kg

   • Hello Saanvi,

     The half-life of a radioactive material is the time taken for half of the total nuclei in the material to decay. For this particular question, half-life of plutonium 239 is given which is 24 000 years and the original amount is 2 kg. If you would take a series of half-life until the time is equal to 120 000 years and taking half of the amount at each time step, you will arrive at that answer. We hope this will help and we are here for you.

     Best regards,
     Physics Department