Senior Physics Electricity

Here's a great set of videos to learn basic concepts of static electricity (video 1 to 3) and current of electricity (video 4 to 6).

Video 1: Conductors and Insulators

Video 2: Charging & Discharging

Video 3 - Charging by Induction

Video 4 - Current Electricity

Video 5 - Potential Difference

Video 6 - Resistance

Thin Converging Lens Challenge

A thin converging lens produces a real image. The graph shows how the image distance varies with the object distance for the lens. 

(a) Determine the focal length of the lens.

A person takes a picture of a waterweed in a fishbowl using a camera with the thin converging lens.The fishbowl is filled with water. When the film, lens, and waterweed are positioned as shown in the figure below, a clear image of the waterweed is recorded on the film. 

(b) Determine the refractive index of water.

Ans: (a) 5 cm; 1.33

Acknowledgement: Part (b) is modified from IJSO 2015 MCQ Question 16.

The Blue Planet - The Deep

Physics SPA
From 01:40 to 02:22: What is the relationship between pressure of gas and volume of gas (i.e. Boyle's law)?

Lower Secondary Science

From 09:41 to 10:23 and 16:00 to 17:15: How we see colors of objects in colored lights?

Question on Thermocouple

This questions appeared in Cambridge International O-level but I think that there's something wrong with the question. Anyone shares the same sentiment?

Hint: Do the values provided in (a)(ii) make sense to you?

Notes for Stability

•  Stability is a measure of an object’s ability to maintain its original position and can be explained by the position of its centre of gravity and moment of force due to its weight.

•  Centre of gravity (CG) of any object is defined as the point through which it whole weight appears to act.
• An external agent exerts a force on the object and the object is tilted through an angle away from the original position due to the force. The original position of the object is the position that the object takes before the action of the external agent. The part of the object which is still in contact with the ground/fixed point when the object is tilted is known as the contact point.
•  Most objects topple when they are tilted through a sufficiently large angle away from the original position.
•  An object can be in stable, unstable or neutral equilibrium.
•  An object is said to be in stable equilibrium if when slightly tilted (small angle away from the original position), it will return to its original position.
•  When slightly tilted, the CG of an object in stable equilibrium will rise. The line of action through its weight (i.e., the vertical line through the CG) still lies within its base. A moment due to its weight about the contact point (pivot) will cause the object to return to its original position.
•  An object is said to be in unstable equilibrium if when slightly tilted, it will topple over.
•  When slightly tilted, the CG of an object in unstable equilibrium will lower. The line of action through its weight will lie outside its base. A moment due to its weight about the contact point will cause the object to move away from the original position.
•  An object is said to be in neutral equilibrium if when slightly tilted, it remains in the new displaced position. Only objects with symmetrical forms (e.g. sphere) can be in neutral equilibrium.
•  When slightly tilted, the CG of an object in neutral equilibrium will remain at the same height. The line of action through its weight will lie within its base. No net moment is generated and it will remain in its new displaced position.
•  The more stable an object is, the larger the angle it can be tilted away from the original position before the line of action through its weight lie outside its base.
•  An object can be made more stable by lowering its CG or increasing its base area. Since the CG of an object is usually located near the region that has more mass, having more mass at the bottom can lower the CG of the object.

Manometer with two liquids

GCE O-level 5076 Science (Physics) 2015 November Paper 2 Question 4(c)

Question:

How would you find the density of liquid B? I believe most of you would find the pressure at point X and then equate it with the pressure at point Y. Have you ever wonder why this method works? At least I do.

Let us investigate by considering the pressure at points P and Q. Point P is at the junction between air and liquid A. Point Q is at the same horizontal level in liquid B.

Pressure at P = Atmospheric pressure

Pressure at Q = Atmospheric pressure + Pressure due to the column of liquid above Q

In other words, the pressure at A and at B are different. Hence, we cannot solve the question by equating the pressure at P and pressure at Q. This begs the following question: What's so special about points X and Y?

Explanation:

As the liquids are at rest (i.e. in equilibrium), the pressure at the bottom of the manometer due to the left column must be equal to the pressure due to the right column.

Pressure due to left liquid column = Pressure due to right liquid column.

For points X and Y,

Pressure at X + Pressure due to water column below X = Pressure at Y + Pressure due to water column below Y.

Since Pressure due to water column below X = Pressure due to water column below Y, Pressure at X = Pressure at Y.

For point A and B,
Pressure at P + Pressure due liquid column between P and X + Pressure due to water column below X = Pressure at Q + Pressure due liquid column between Q and Y + Pressure due to water column below Y.

Since Pressure due to water column below X = Pressure due to water column below Y,
Pressure at P + Pressure due liquid column between P and X = Pressure at Q + Pressure due liquid column between Q and Y.

Since liquid A and liquid B have different densities, pressure due liquid column between P and X is NOT equal to pressure due liquid column between Q and Y even though both liquid columns have the same vertical height (i.e. PX = QY). Hence, pressure at P is NOT equal to pressure at Q.

Isolated System for Principle of Conservation of Energy

According to Physics Matters GEO 'O' Level Physics (4th edition) by C. Chew, S.F. Chow and B.T. Ho:

The Principle of Conservation of Energy states that the energy cannot be created or destroyed, but can be converted from one form to another. The total energy in an isolated system is constant.

But what's an isolated system? According to Fundamentals of Physics (10th edition) by D. Halliday, R. Resnick and J. Jewett:

… the system is isolated from its environment; that is, no external force from an object outside the system causes energy changes inside the system.

Let us now consider a free-falling object. We often use it to illustrate the principle of conservation of energy (e.g. KE + GPE = constant). But are we "allowed" to do so? After all, we would not consider a free-falling object to be an isolated system because weight is an external force acting on the object. But why does the principle of conservation of energy holds for a free-falling object?

Hint: Redefine the system you consider to include …

Common Orders of Magnitude

Questions on the order of magnitude have been quite popular in O-level Physics examinations in recent years. Here's a list to help my readers:

Length / m
Diameter of proton ~ 10^-15
Radius of typical atom ~ 10^-10

Diameter of red blood cell ~ 10^-5

Thickness of hair/paper  ~ 10^-4

Length of soccer field ~ 10^2

Radius of Earth ~ 10^6

Diameter of Earth ~ 10^7

Mass / kg

Mass of electron ~ 10^-31

Mass of proton~ 10^-27

Mass of typical atom ~ 10^-25

Mass of 5 cent coin ~ 10^-3

Mass of apple ~ 10^-1

Mass of car ~ 10^4

Mass of bus ~ 10^5

Mass of Earth ~ 10^24

Time / s

Length of a day ~ 10^5

Anyway, here's a website where you can find out the order of magnitude for other physical quantities (http://hypertextbook.com/facts/index-topics.shtml) and here's a book on how order of magnitude can help you to make good estimates (https://www.bookdepository.com/Guesstimation-Lawrence-Weinstein/9780691129495).

Do you have a burning O-level Physics homework question to ask?

If you have a burning O-level or A-level Physics homework question to ask, please pose it on https://www.facebook.com/groups/367251610136022/ (O-level) or https://www.facebook.com/groups/1878911782343369 (A-level). I will try to come back to you with the answers as soon as possible (with 95% confidence).

It is free of charge because I'm interested to find out what are some Physics questions that students/ teachers have difficulty in. In case you are wondering, I am not giving any tuition.

The World's Blackest Material - An Inside Look At Vantablack

Notes on Pressure

•  Pressure is an effect which occurs when a force is applied on a surface.
•  Pressure is defined as the force acting perpendicularly on per unit area.
•  Pressure is a scalar quantity.
•  The SI unit of pressure is newton per square metre (N/m^2), also known as the pascal (Pa).

Fun Facts related to Electromagnetic Spectrum

Black Bulb

• —The black bulb emits UV radiation.
• —A dark filter material (partially) blocks the visible light emitted by the filament.
• —The filter material absorbs the visible light produced by the filament, causing the bulb to become very hot after use.

Producing X-rays at Home

O-level Science (Physics) Suggested Answers for Electromagnetic Spectrum

N05/2/3
(a) ultra-violet radiation [1], radio waves [1]
(b) number of complete waves produced per second [1]
(c) speed of wave = 3.0 × 10^8 m/s [1]
(d) transverse wave OR can travel through a vacuum [1]

N06/2/5a

different frequencies or wavelengths [1]

N08/2/2

(a) infra-red radiation [1]
(b) microwaves [1]
(c) gamma rays [1]

Satellite Communication

Microwaves are more suitable for satellite communication (e.g. Global Positioning System, a.k.a. GPS) than radio waves because microwaves have higher frequencies than radio waves, which allows microwaves to pass through the ionosphere (part of the Earth’s atmosphere).

Reference: GCSE Bitesize

O-level Science (Physics) N09/2/10a

There are two different kettles. The bodies of the kettles are made from different materials. Kettle A is made from shiny silver-coloured metal. Kettle B is made from white plastic. Discuss the suitability of each material in reducing heat losses from the kettles when they are used to boil water.

GCE O-level 5116 Science (Physics) 2009 November Paper 2 Question 10(a)

Answer: Both shiny surfaces (Kettle A) and white surfaces (Kettle B) are poor emitters of thermal radiation and so they help to reduce the thermal energy loss through radiation from the surfaces of the kettles to the surroundings. However, plastic surfaces (Kettle B) are poorer thermal conductors than metal surfaces (Kettle A) and so plastic surfaces are less effective than metal surfaces in transferring thermal energy, which is then lost through radiation, through conduction from the contents of the kettle to the surface of the kettle. Hence, the surface of Kettle B is more suitable in reducing heat loss when it is used to boil water.

Should velocity be defined as the rate of change of displacement?

What is displacement?

According to Fundamentals of Physics (10th edition) by D. Halliday, R. Resnick and J. Jewett:

The displacement of a particle is the change in its position.

Mathematically, displacement, x = final position - initial position and it is a vector quantity.

What is velocity?

According to Fundamentals of Physics (10th edition) by D. Halliday, R. Resnick and J. Jewett,

Velocity is the derivative of x (position) with respect to t (time).

In other words, velocity is the time rate of change of  position and it is also a vector quantity. Mathematically, velocity = dx/dt.

However, according to Physics Matters GEO 'O' Level Physics (4th edition) by C. Chew, S.F. Chow and B.T. Ho:

Velocity is the rate of change of displacement.

In other words, Physics Matters GEO 'O' Level Physics is saying that velocity is the rate of change of change of position. In my humble opinion, the definition might be technically incorrect.

Since I am on the topic of displacement and velocity, let me end off by posing a challenge to you: For motions in 1 dimension, are you able to determine the speed-time (distance-time) graph from its velocity-time graph (displacement-time) graph? How about velocity-time graph (displacement-time) from its speed-time (distance-time) graph?

O-level Science (Physics) N04/2/4bii

An athlete runs a 100 m race in 12.5 s. Explain why, for part of the race, the athlete must have been running faster than the speed calculated in (a).

GCE O-level 5052 Science (Physics) 2004 November Paper 2 Question 1(b)

Answer: The runner accelerates from rest.

Proof: Consider the area under an arbitrary speed-time graph that starts from rest and the area under another speed-time graph that is travelling at a constant speed, which is equal to the maximum speed of the first graph. Both graphs stop at time t. Thus, the area under the first speed-time graph  = average speed x t while the area under the second speed-time graph = maximum speed x t. Since first area < second area, hence average speed < maximum speed.

Is sound transmitted as a transverse or longitudinal wave in solid?

Is sound transmitted as a transverse or longitudinal wave in solid?


Answer: Both. But for O-level Physics, you can safely assume that sound is transmitted as a longitudinal wave in solid, liquid or gas.

According to Physics Matters GEO 'O' Level Physics (4th edition) by C. Chew, S.F. Chow and B.T. Ho:

Sound waves travel parallel to the direction of vibration of a medium, and hence are longitudinal waves.

But according to University Physics with Modern Physics (6th edition) by H. Young and R. Freedman:

Sound waves in fluids are longitudinal.

Longitudinal waves depend on the compressibility of the fluid for their propagation. Transverse waves require a restoring force in response to sheer strain. Fluids do not have the underlying structure to supply such a force. A fluid cannot support static sheer. A viscous fluid can temporarily be put under sheer, but the higher its viscosity the more quickly it converts input work into internal energy. A local vibration imposed on it is strongly damped, and not a source of wave propagation.