Electromagnetic Spectrum – updated

One amendment done – Mobile communication should be using microwaves.

Article: ‘Li-fi 100 times faster than wi-fi’

‘Li-fi 100 times faster than wi-fi’

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Flyte – Levitating Light Bulb

Check out the site: http://www.flyte.se/

Light – Reflection – Furthest distance Peter can walk before Joe loses sight of his image

Solutions: Option D

Answering electromagnetism questions

Heating and Cooling Curve

 

Finding effective resistance when a wire is bent into different shapes

The concepts needed to solve such questions

1. Length increases, resistance increases and vice versa.
2. Finding resistors in parallel and in series.

Fluid pressure in a capillary tube

A uniform capillary tube closed at one end, contained air trapped by a thread of mercury 85 mm long. When the tube was held horizontal, the length of the air column was 50 mm. When it was held vertically with the closed end downwards, the length was 45 mm. Find the atmospheric pressure in Pa. (Density of mercury = 14 x 10³ kg/m³ )

Solutions:

Waves Summary – Amended

Amendments to the Sound information – added info when sound travels from less dense (air) to denser medium (water or solid)

Demonstration of Reflection, Refraction and TIR using light source with a slit

Reflection of light using a mirror

Refraction of light using various shaped optical mediums

Total Internal Reflection (TIR) using a semi-circular perspex

For Total Internal Reflection (TIR) to occur:
1) angle of incidence i is greater than the critical angle c
2) the light is travelling from an optically denser medium towards a less dense medium

Click on this post for the explanation of TIR

Ratio of resistiviities of wires X and Y – effect of diameter on cross-sectional area

The resistance of two wires X and Y are in the ratio of 2 : 1, their lengths are in the ratio 1 : 2 and their diameters are also in the ratio of 1 : 2.

What is the ratio of the resistivities of wires X and Y?

A     1:2     B     1:1     C     2:1     D     4:1

Solutions: Option B

Note that when the diameter of the wire doubles, the cross-sectional area increases by 4 times.

Comparing the resistances of both X and Y as shown below, the ratio of resistivities is 1:1.

Wave – Displacement-time graph of a particle on a wave

Solutions: Option C

Consider positive displacement if the particle P is above the undisturbed position, and negative displacement if the particle is below the undisturbed position.

The next instance, the particle P will be moving vertically downwards, i.e. moving nearer to its undisturbed position. So the displacement decreases to zero before it moves below the undisturbed position (negative displacement).

Manometer – mercury levels difference changes with different density of liquid used

The pressure of a gas is measured using a manometer as shown in the diagram.

The mercury in the manometer is replaced with a liquid which is less dense. How does the value of h change?

A It becomes zero.

B It decreases, but not to zero.

C It stays the same.

D It increases.

Solutions: Option D

The pressure to be measured remains constant. Since P = pgh, where p is the density of the liquid used in the manometer. If a liquid of lower density is used, height h of the liquid (level difference) will be greater. The gravitational field strength g remains constant.

SP N2007 P1 Q16 – Which events will cause the fuse to blow?

An electric cable contains three wires live, neutral and earth. The cable is correctly wired to a plug which contains a 3A fuse. The insulation becomes damaged and bare metal wires show.

Five possible events can occur.

• A person touches the earth wire.
• A person touches the neutral wire.
• A person touches the live wire.
• The live wire touches the neutral wire.
• The live wire touches the earth wire.

How many of these five events cause the fuse in the plug to blow?

A   1          B   2        C   3         D   4

Solutions: Option B

Consider the five events:

• A person touches the earth wire – As the person is at 0V, same as the earth wire, there will be no current flowing through the person. So current through the circuit will not be affected, which is lower than the 3A fuse rating. Fuse will not blow.
• A person touches the neutral wire.-  As the person is at 0V, same as the neutral wire, there will be no current flowing through the person. So current through the circuit will not be affected, which is lower than the 3A fuse rating. Fuse will not blow.
• A person touches the live wire. – The live wire is at high potential of 240 V. The person will get an electric shock. But a common misconception is that if a person gets an electric shock, the current flowing through him is very large, which is wrong. In fact, the current is very small, much smaller than the fuse rating. Assuming the average body resistance of the person is 100 000 ohms, and the potential difference in Singapore is 240 V, since I = V/R = 240/100 000 = 0.0024 A, which is lower than 3A fuse rating. Hence the fuse will not blow.
• The live wire touches the neutral wire. – This will create a short circuit as a large current which exceeds the fuse rating will from the live (240 V) to the neutral wire (0V) as that path has very low resistance. The fuse will blow.
• The live wire touches the earth wire. – This will create a short circuit as a large current which exceeds the fuse rating will from the live (240 V) to the earth wire (0V) as that path has very low resistance. The fuse will blow.

OLevel SP P1 Q12 2010 focal length of lens using distant object

Which distance is equal to the focal length of a lens?

(A) the distance between a distant object and the image

(B) the distance between the image of a close object and the centre of the lens

(C) the distance between the image of a distant object and the centre of the lens

(D) the distance between two principal foci

Solutions: Option C

Focal length f is the distance between the focal point and the centre of the lens (optical centre).

Note that only when parallel rays of light enter a converging lens, the rays will converge to a point. That point is considered to be focal point F (principal focus). The distance between focal point F and the optical centre is the focal length f. Refer to below.

As none of the options is similar to the above definition, you have to consider that the rays from a distant (far away) object are considered parallel. Hence the sharp image formed on the screen is considered the forcal point F of the lens and the distance between the image and the optical centre is the focal length f.

If it is a close object, the rays entering are not considered parallel. Hence even if the rays converged to a point, that point is NOT focal point F and the distance between this converged point and the optical centre is NOT focal length f.

 

Click the following posts for other lens concepts:

https://evantoh23.wordpress.com/2017/08/19/different-lens-ray-diagram-questions/

https://evantoh23.wordpress.com/2010/11/09/20101109converging-lens-important-concepts/

Magnus Effect

What is Total Internal Reflection (TIR)?

Overview of Total Internal Reflection (TIR)

Explanation of the various stages:

Click here to view example of TIR question 01

Click here to view example of TIR question 02

Total Internal Reflection (TIR) example 02

Solutions:

a) 38.8o      b) 41.8o

c) 51.2o . The ray does not emerge as total internal reflection has occurred. The angle of incidence is greater than critical angle and the light is traveling from a denser towards a less dense medium.

Click here to view the video tutorial for working and explanation for (a) and (b).

Click here to view the video tutorial for working and explanation for (c) and (d).

Total Internal Reflection (TIR) example 01

Solutions:

a) Refraction     b) 48.6o     c) c = 41.8o     e) 70o

d) Total internal reflection has occured, as the angle of incidence is greater than critical angle and the light is traveling from a denser medium (glass) towards a less dense medium (air)

Click on the video tutorial for explanation and working for part (a) and (b).

Click on the video tutorial for explanation and working for part (c) to (e).

Will the whole lake be frozen during winter? At what temperature is water at its max density?

During summer, though the surrounding is hot, the water in the lake will not reach very high temperature mainly due to its high specific heat capacity. In general, cool water sinks as it is denser and warm water rises as it is less dense, the bottom of the lake will always be lower temperature compared to near the surface. The bottom of the lake may not necessary be 4 degree Celsius.

Why ice floats on water?

Remembering Mr Lee Kuan Yew

Thank you Mr Lee, for what you have done for Singapore. You will always be in our heart.