• Physics 9702 Doubts | Help Page 45

Question 271: [Circular motion]

(a) Define the radian

(b) Stone of weight 3.0 N is fixed, using glue, to one end P of rigid rod CP, as shown.

The rod is rotated about end C so that the stone moves in vertical circle of radius 85 cm.

Angular speed ω of rod and stone is gradually increased from zero until the glue snaps. The glue fixing the stone snaps when tension in it is 18 N.

For position of the stone at which the glue snaps,

(i) on dotted circle of Fig, mark with the letter S the position of the stone

(ii) calculate angular speed ω of the stone.

Reference: Past Exam Paper – June 2010 Paper 41 Q1



Solution 271:

Go to
A stone of weight 3.0 N is fixed, using glue, to one end P of a rigid rod CP, as shown  in Fig. 1.1.

Question 272: [Current of Electricity > Capacitance]

Rectified output of sinusoidal signal generator is connected across a resistor R of resistance 1.5 kΩ, as shown.

Variation with time t of potential difference V across R is shown.

(a) State how rectification shown in Fig may be achieved.

(b) A capacitor is now connected in parallel with resistor R. Resulting variation with time t of potential difference V across R is shown.

(i) Using Fig, determine

1. mean potential difference across resistor R

2. mean current in resistor

3. time in each cycle during which capacitor discharges through the resistor.

(ii) Using answers in (i), calculate

1. charge passing through the resistor during one discharge of the capacitor

2. capacitance of capacitor

(c) Second capacitor is now connected in parallel with resistor R and the first capacitor. On Fig, draw a line to show variation with time t of the potential difference V across the resistor.

Reference: Past Exam Paper – November 2003 Paper 4 Q4



Solution 272:

(a) By connecting a single diode EITHER in series with resistor R OR in series with an a.c. supply

(b)

(i)

1.

{Maximum V = 6.0V. Minimum V = 4.8V. Mean potential difference = (6.0 + 4.8) / 2 = 5.4V}

(From graph,) 5.4 V

2.

Ohm’s law: V = IR

Current I = 5.4 / (1.5x10³) = 3.6x10^-3A

3.

{As V falls, the capacitor discharges by supplying charge. So, the cycle during which the capacitor discharges through the resistor corresponds to the each region where V decreases.}

Time = 0.027s

(ii)

1.

Charge Q = IT = (3.6x10^-3) x 0.027 = 9.72x10^-5C

2.

{Since the CHANGE in the charge has been calculated, we need to substitute the change in potential difference into the formula for capacitance. Change in V = 6.0 – 4.0 = 1.2V}

Capacitance C = ΔQ / ΔV = (9.72x10^-5) / 1.2 = 8.1x10^-5F

(c)

{If no capacitor was present (capacitance = 0), then a graph similar to part (a) is obtained.  So, by increasing the capacitance, the decrease in V is less – we have the smoothing effect. So, a greater value of capacitance causes a greater smoothing effect. Connecting capacitors in parallel causes the total capacitance in the circuit to increase. Thus, in the graph, the ripple should be less.}

The line should have a reasonable shape with less ripple

Question 273: [Current of Electricity > Resistance]

What is the equivalent resistance of this circuit?

Solution 273:

First label the different junctions present in the circuit. Note the junction A and B are the same and E and F are the same.

The 1Ω resistor is between B and C but considering the upper part, junction B is the same as D (no component is connected in the upper wire connecting B and D). This means that the 1Ω resistor (currently between B and C) can also be drawn between C and D. So the circuit can also be drawn as:

Similarly, considering the bottom part between C and E (they are the same as no component is connected between them), the circuit becomes

So, the equivalent resistance = [1/1 + ½ + 1/3]^-1 = 6/11

Question 274: [Current of Electricity > Resistance]

Circuit contains three similar lamps A, B and C. Circuit also contains three switches, S₁, S₂ and S₃, as shown.

One of the lamps is faulty. In order to detect fault, an ohm-meter (meter that measures resistance) is connected between terminals X and Y. When measuring resistance, ohmmeter causes negligible current in the circuit.

Fig shows readings of the ohm-meter for different switch positions.

(a) Identify faulty lamp, and nature of the fault

(b) Suggest why it is advisable to test the circuit using an ohm-meter that causes negligible current rather than with power supply.

(c) Determine resistance of one of the non-faulty lamps, as measured using the ohmmeter.

(d) Each lamp is marked 6.0 V, 0.20 A.

Calculate, for one of the lamps operating at normal brightness,

(i) its resistance

(ii) its power dissipation

(e) Comment on answers to (c) and (d)(i).

 

Reference: Past Exam Paper – June 2006 Paper 2 Q7

Solution 274:

(a)

{When all the switches are open, the meter reads infinity. This is obvious since the circuit is shorted.

In the third test: S₁: open, S₂: closed and S₃: open, the meter read a resistance of 30Ω. This is because test 2, where only S₁ is closed, proves that lamp A is NOT faulty. So, the third test proves that lamp B is also NOT faulty.

In the last test, where only S₁ is opened, the resistance read is 15Ω. This proves that lamp C is shorted – that is current passes as if there was no lamp. In this case, it’s as if a wire of negligible resistance is connected in parallel with lamp B.

Current is larger when the resistance is smaller. [From Ohm’s law, current I is inversely proportional to resistance R] So, all current would flow in that wire instead of the lamp B (since lamp B has some resistance). So, the only resistance encountered by the current is due to lamp A only. So, the resistance read by the meter is 15Ω. 

If S₁ and S₂ were closed, and S₃ was opened, the meter would again read 15Ω since all the current would pass through S₁ (which has no resistance) instead of passing through lamp B. The current would then flow through lamp A (which is the only lamp that causes a resistance in this case).} 

Faulty lamp: Lamp C

Nature of fault: The lamp is shorted

(b) The shorted lamp A would cause damage to the supply/lamps / blow fuse in the supply

(c) Resistance = 15Ω

(d)

(i)

V = IR

Resistance R (= V / R = 6.0 / 0.20) = 30Ω

(ii)

Power P = VI  or I²R  or V²/R

Power P = 1.2W

(e)

The filament is cold when measuring with the ohm-meter in (b). The resistance of the filament rises as the temperature rises.

• O-Level Physics (5054) | Skills to be tested?

Examinations for the Cambridge O-Level Physics (5054) syllabus test you on the following skills:

1. your knowledge (what you remember) and understanding (how you use what you know and apply it to unfamiliar situations)
2. how you handle information and solve problems
3. your experimental skills

The theory papers (paper 1 and paper 2) test skills 1 and 2 (listed above). The practical papers (paper 3 and paper 4) test skill 3.

Candidates should note that 65% of the marks are for A and of these only 30% are for simple recall.

Let’s now look at the 3 skills mentioned in more details and the range of the skills candidates should try to develop.

Skill 1: Knowledge with understanding

What this means: remembering facts and applying these facts to new situations

What you need to be able to do:

• Use scientific ideas, facts and laws
• Know the meaning of scientific terms (e.g. centre of mass)
• Know equations and definitions
• Use simple equations (e.g. speed = distance/time)
• Know about apparatus and how it works
• Know about symbols, quantities (e.g. mass and weight) and units (e.g. kg and N)

Skill 2: Handling information and solving problems

What this means: how you extract information and rearrange it in a sensible pattern and how you carry out calculations and make predictions

What you need to be able to do:

• Select and organise information from graphs, tables and written text
• Change information from one form to another (e.g. draw graphs)
• Arrange data and carry out calculations
• Identify patterns from information given and draw conclusions
• Explain scientific relationships (e.g. use the moving (kinetic) particle theory to explain ideas about solids, liquids and gases.)
• Make predictions and develop scientific ideas
• Solve problems

Skill 3: Experimental skills and investigations

What this means: planning and carrying out experiments, recording and analysing information

What you need to be able to do:

• Follow instructions to set up and use apparatus safely
• Make observations and measurements and record them with regard to accuracy
• Analyse experimental results
• Plan and carry out an experiment describing any problems and suggesting improvements

Previous: O-Level Physics (5054) | Exam Tips
Next: O-Level Physics (5054) | Details of Topics

• O-Level Physics (5054) | Exam Tips

This post contains tips and advices that highlight some common mistakes made by past candidates. It is intended that these tips will help candidates in their preparation and even when undertaking their upcoming O-Level Physics (5054) exams.

GENERAL ADVICE

• There is no escaping it; the best way to prepare for a physics examination is thorough and careful revision.
• Just reading your notes over and over again is not an efficient way to revise. Make your revision productive by making it interesting and fun. Make notes, revision cards or mind maps. Revision should be an active process, i.e. you should be ‘doing things’, not just sitting and reading a book.
• Don’t try to learn it all in one go! Not understanding a particular concept at one time does not mean you will never understand it. Do not give up easily! Take regular breaks and review what you have learnt regularly.
• Learning equations is essential; put them on small pieces of paper and stick them somewhere you will see them every morning.
• Revise with a friend so you can test each other or try explaining the physics of a topic to a friend – as if you were a teacher! Share your tips with your friend for topics you understand very well and do not hesitate to ask for help from your friends on topics you are less familiar. Do not try to learn on your own.
• Working through past paper questions is the best way to complete your revision. This helps you to know the type and style of questions to expect in the examination. This also helps you develop your problem solving skills, which are essential for physics.
• Try timed questions so you can learn to answer quickly.
• Get your answers checked so you know you are correct!

Spelling

The spelling of technical terms is important, so make sure your writing is legible as well as spelt correctly. If your writings cannot be read properly, you will lose marks. Some words are very similar, such as reflection and refraction, fission and fusion. Check the spellings of technical terms carefully.

General tips

In Cambridge O Level Physics (5054) examinations candidates have to be able to complete a variety of tasks; always read the question carefully to make sure you have understood what you are expected to do.

Descriptive answers

In descriptive answers, candidates should:

• Check the number of marks available and make sure you give sufficient points.
• Plan your answer first so that you don’t repeat yourself or contradict yourself.
• Read your answer through carefully afterwards to check you have not missed out important words.
• Read the question again to check you have answered the question asked.
• Use sketches and diagrams wherever you can to help your explanation.
• Add labels when referring to a diagram, e.g. point X, so that you can refer to it easily in your explanation. This can save many words and much confusion.

Numerical answers

In numerical answers, candidates should:

• Quote any formulae you are going to use and show clearly all the steps in your working. It may be tempting to use your calculator and just write down the answer, but if you write down one figure wrongly then you may lose all the marks for the calculation. If the examiners can see the formula and the numbers you have used then you will lose only a little credit. Some questions ask for a formula to be quoted; even if you get the right answer, failure to quote the formula will lose you a mark.
• Check the units are consistent, e.g. if the distance is given in km and the speed in m/s, then you must convert the km to m.
• Be careful when you are converting minutes and seconds: 1 minute 30 seconds is not 1.3 minutes and 150 seconds is not 1.5 minutes. These are common mistakes, so always double check any conversion of units of time.
• State the answer clearly at the end.
• Give your answer as a decimal to an appropriate number of significant figures. Don’t leave your answer as a fraction unless specifically asked to do so.
• Check that you have given the unit of your final answer.
• Look at your final answer and see that it is reasonable. If you have calculated the cost of using an electrical appliance such as a kettle for a few minutes and found it to be hundreds of dollars, then check the powers of ten in your calculation.

Graphs

Plotting graphs can be tested in Papers 2, 3 or 4 (reading from graphs may be present in Paper 1 too).

When drawing graphs, candidates should:

• Remember to label the axes with both quantity (e.g. distance or d) and unit (e.g. metres or m). Then write it as distance / metres or even just d / m.
• Make sure the axes are the correct way round. You are usually told, for example, to plot distance on the x-axis, so make sure you know that x is the horizontal axis!
• Make the scales go up in sensible amounts, i.e. 0, 5, 10… or 0, 2, 4… but not 0, 3, 6… or 0, 7, 14…
• Make sure that the plotted points fill at least half the graph paper. This means if you can double the scale and still plot all the points then you should double the scale.
• Check if you have been told to start the scales from the origin. If not, then think carefully about where to start the axes.
• When you are told to start the axes from a certain point (e.g. x=1, y=20) you must do so. You will lose a mark if you use a different point (e.g. the origin).
• Use a sharp pencil to plot the points and draw the line.
• Plot the points carefully. It is best to use small neat crosses. Every point will be checked by the examiner, and you will lose the mark if any are wrongly plotted.
• Draw either a straight line or a smooth curve. In physics we never join the dots!
• Your line may not go through all the points – especially in the practical papers.
• Remember that a best fit line (curve or straight) should have some points above and some points below the line.

When taking readings from a graph, candidates should:

• Draw a large triangle when measuring the gradient of a line. It must be at least half the length of the line. Top tip: draw a triangle the full size of the graph! It is best to show the numbers on the sides of the triangle when finding the gradient.
• Always use points on the line, not your plotted points, when calculating the gradient.
• Draw a tangent to find the gradient of a curve. Make sure it is at the right place on the curve. Again, use a large triangle.
• Make sure you read the scales correctly when reading a value from a graph. It may be that they are in mA rather than A or km rather than m.

When describing the shape of a graph, remember that:

• directly proportional means a straight line through the origin.

There are two ways to check if quantities are directly proportional:

– doubling one quantity will cause the other to double

– dividing one by the other will give the same result

i.e. if two quantities F and L are directly proportional then if you find several values of F/L they should be the same.

• if the straight line does not go through the origin, then it is just called a linear graph.

• inverse relationship means increasing one quantity will cause the other to decrease.

• if doubling one quantity causes the other to halve, then they are inversely proportional.

This can also be checked by:

– multiplying the two quantities together will give the same result

i.e. if two quantities F and L are inversely proportional then if you find several values of F × L they should be the same.

Previous: O-Level Physics (5054) | Exam Structure
Next: O-Level Physics (5054) | Skills to be tested?