In general, assuming temperature of a conductor is constant, the resistance of a conductor is affected by
1) length (L) – [L increases, R increases]
2) cross-sectional area (A) – [A increases, R decreases]
3) resistivity (p) = [p increases, R increases]
As R is directly proportional to L and p, and is inversely proportional to A,
R = pL/A
There are times when using the formula is more appropriate.
Below are the questions.
2003PPq1q7 (0 min to 2.42 min) Ans: Option A
2023PPp1q4 (2.47 min 3.54 min) Ans: Option C
2018PPp1q2 (3.45 min to 6.22 min) Ans: Option D
2011PPp1q6, 2016PPp1q6 (6.22 min) Ans: Option A
When two components are connected in series, where one is a fixed resistor and the resistance of the other can vary (e.g. thermistor, light dependent diode, variable resistor etc), you always make use of the fixed resistor to explain the its p.d. as resistance is constant.
Then use the concept of sum of p.d. across both components is equal to the e.m.f of the circuit to explain how the p.d. across the other component varies.
Though thermistor is not in SciPhy syllabus, it can still be tested as long as the information on thermistor is given. Refer to the videos below where 2 are from Sciphy and the other is from Pure.
Instead of P = IV to explain, it might be easier to explain using P = I^2R when the components are connected in series where current I is constant. Hence power P is directly proportional to R. The bigger the R, the more power it uses.
Likewise, if the components are connected in parallel, it will be easier to use P = V^2/R, as the potential difference is constant for the components connected in parallel. Hence, power P is inversely proportional to R. The smaller the R, the more power it uses.
SP2021Q8
PP2017Q11
In general, the plotting of graph for Physics practical is usually 4 marks.
The break down of the 4 marks are as below:
Go through the video to have a better idea on how to plot the graph and get the marks.
Video 01: The various physical quantities in kinematics
Video 02: Acceleration
Video 03: Graphical Analysis of Motion (Displacement (distance)-Time Graph and Velocity (speed)-Time Graph)
Video 04: Describing the motion of the car – practice
Video 05: Free-fall, acceleration due to gravity
Video 06: Motion of 2 objects
Refer to the Graphs of Sound Waves here
The ‘D.C.’ here mean direct current. It means that the current flows in one direction in the circuit.
The direction of the direct current refers to the conventional current, and it flows out of the positive (+) terminal of the cell / battery, around the circuit and flows back into the negative (-) terminal.
[Note: the direction of the electron flow is opposite to the conventional current]
To understand DC circuit, it will be useful to relate to a river system. Refer to the 3 videos below to learn more about 01: DC circuit similar to river system, 02: Series Circuit 03: Parallel Circuit and 04: Combined Circuit.
01: How is DC circuit related to a river system
02: Series Circuit
03: Parallel Circuit
04: Combined Circuit (Series and Parallel)
Now, after you have the basic concepts and rules for the circuits, let’s look at some simple example to reinforce you understanding.
Example 01: Series Circuit
Example 02: Parallel Circuit
Example 03: Combined Circuit
Example 04: PP2012P2Q7 DC circuit : pd across 2 points on two separate branches.
Hope these series of videos help in you understanding of DC circuit.
The video include 3 different light experiments which involve glass block and pins.
Most students faced difficulties in locating the pins through the glass block. It is important to close one eye and use your master eye to view.
Hope this video helps!
In experiments like this, the sources of error include:
– the holes created by the pins are too big, hence the line drawn using the two holes may not be very precise, hence affecting the distance/angle measured (depends on experiments)
– When putting back the glass block back on the paper, the position may be not exactly the same, hence affecting the angle/distance measured.
As a precaution, it is good to ensure that the distance between the two pins are more than 4 – 5 cm. With the holes further apart, the line drawn will be more accurate.
1) Newton’s first law states that an object will remain at rest or in uniform motion (constant speed) in a straight line unless an external force acts on the body.
In other words, when a body is at rest or moving at constant speed in a straight line (constant velocity), straight away you should know it is Newton’s first law. Next you must know these 3 basics concepts about 1st law:
– forces acting on the body are balanced
– net force / resultant force acting on the body is zero
– there is no acceleration.
2) Newton’s second law states that when a net force (resultant force) acts on a body, it will cause an acceleration on the body (accelerating or decelerating).
Basically F = ma where F is the net or resultant force in N,
m is the mass in kg
a is the acceleration in ms-2
In other words, when a body is moving faster or slower (or going round a bend), you should know its Newton’s second law. Next you must know these 3 basic concepts about 2nd law:
– forces acting on the body are not balanced
– there is a net force / resultant force acting on the body
– there is an acceleration (accelerating of net force is in the direction of motion, or decelerating if the net force is opposite to the direction of motion)
The video below shows a typical lens experiment. (reference to O-Level SciPhy 2015). I will briefly go through the set-up, main steps and how to get the 1st set of readings.
In the next video, it highlights the various types of lens practical which you might have in the school lab. e.g. different kind of crossed-wire, a beaker of water as a converging lens and different kind of images formed.
Key points:
1) Make sure the object (illuminated crossed-wire), lens and screen are aligned properly.
2) Source of Error: identifying the sharpest image
Improvement: (i) Repeat the experiment a few times for the same
independent variable to identify the sharpest image.
(ii) Move the lens (or screen) forward and backward about
the sharp image, until the sharpest image is determined.
3) In general, the focal lens of the lens used in the lab is usually 10 cm or 15 cm. Most lens experiment requires you to find the focal length. There is also a easy way to quickly determine the focal length before starting the experiment.
Refer to this post for another lens experiment which is different and more challenging.
In a typical parachute jump, there are various distinct stages/sections as you can see from the graph in the video below.
AB = constant acceleration, free fall, a = 10 ms-2
BC = decreasing acceleration
CD = constant speed, zero acceleration
at D = the time where the parachute is fully opened
DE = constant deceleration
EF = lower constant speed, zero acceleration
FG = constant deceleration
After you have learned Dynamics, you should be able to explain each stage using forces acting on the skydiver, namely the weight and air resistance.
Refer to the video below to understand the motion at various stages and how to explain in terms of forces, esp the part on why the acceleration is decreasing during BC.
You can refer to the detailed explanation in words in the comics below. Hope this post helps you to understand better!
Ticker tape timer is a device which punches dots on a tape at specific time interval. The tape can be attached to a moving object. With information of the frequency of the ticker timer and the spacing of the dots, we are able to analyse the motion of the moving object.
View the following videos to understand more about the ticker tape timer and also some typical questions on ticker tape timer.
Explanation of Ticker Tape Timer
Questions on ticker tape timer
Refer to this post for another example of finding acceleration
https://evantoh23.wordpress.com/2010/10/26/20101026ticker-tape-timer-finding-acceleration/
A common mistake is to state that the reason for switch to be on the live wire is “so that the switch can turn on/off the appliance”. This explanation is wrong.
Refer to the view for the reason why switch and fuse must be on the live wire.
In general, in the event where the fuse is blown or the switch is open, it disconnect the appliance from the high potential of the live wire. So the appliance will no become ‘live’ and it is save to touch even though there is a fault.
Simply put, electric appliance can be very simple with just a live wire bring in the current and a neutral wire to bring the current out. The appliance can just work like this forever (without the hassle of having earth wire, fuse etc), provided there is no electric fault developed.
Both the earth wire and the fuse are safety features to protect the user and the appliance respectively. Take a look at the video below to understand the rationale.
Recall these notes given:
Click here for other related posts
Prefixes are used to simplify the writing of very big or very small numbers.
For instance, instead of having to write 12 500 000 m every time, you can simplify it by using the prefix mega (M), hence it can also be written as 12.5 Mm. Similarly, 0.00054 s can be written as 0.54 ms.
If you are unsure or confused with the unit conversions, you can refer to the following videos.
1) Prefixes can be used for different physical quantities units.
2) Unit conversion with examples
3) Unit conversion for units of area and volume
4) Unit conversion for units of speed and density
There were some errors in the labelling of the coil. It has been updated.
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Three processes of thermal transfer: conduction, convection and radiation.
Conduction vs Convection:
Similarity:
Differences:
Radiation vs Conduction & Convection
Light and sound are both waves. So both carry energy from one place to another.
Light, which is part of the electromagnetic spectrum, is a transverse wave, It can travel through a vacuum at speed 3.0 x 108 m/s. As the light travels from an optically less dense medium (air) to an optically denser medium (liquid or glass), the light undergoes refraction and bends towards the normal due to a decrease in speed.
Light: Optically less dense medium to denser medium:Â
– speed decreases
– wavelength shorter
– frequency remains constant
Sound is a longitudinal wave. It requires a medium to pass through and it cannot pass through a vacuum. Opposite to light, as the sound travels from a less dense medium (air) into a denser medium (water or solid), the speed increases.
Sound: Less dense medium to denser medium:
– speed increases
– wavelength longer
– frequency remains constant
Refers to the image below to understand how the waves behave in different mediums.
Click here to revise on the calculation of refractive index for light
1. Converging lens (convex lens)
Converging lens, also known as convex lens, is thicker at the centre. Below shows some examples.
In O-level, we learned about symmetrical converging lens. i.e. the curvature of the lens are the same on both sides. As light rays pass through the converging lens, the rays come closer together.
Take note that the bending of light, refraction, takes place on the air-glass boundaries on both sides of the lens (as shown above). But for easy drawing, we draw the bending at the imaginary centre vertical which passes through the optical centre as shown below.
2. The 3 Rays
The following 3 rays are important for us to construct the ray diagram and locate the image. We always draw these 3 rays as they have rules to follow, hence guiding us in our drawing.
Refer to the video below for better understanding of the 3 rays.
3. The 4 Key Scenarios
Depending on the distance of the object to the centre of the lens (object distance u), the kind of image you get varies.
Refer to the video below for the better understanding of how the various images are formed.
3. The Pattern
Besides knowing the 4 key scenarios, it is important to know how the image behaves as the object is moved towards the lens.
In general, as the object (starting from a distance of >2f) moves closer to the lens, the image will move further away from the lens and the size of the image becomes bigger.
But when the object is within a focal length, as it moves closer to the lens, the virtual image moves closer to the lens and it becomes smaller compared to the image previously. But the virtual image is always bigger than the object.
Refer to the video for better visualisation and understanding.
4) Other posts on converting lens:
What is focal length and how to identify
Finding focal length f of the lens (using a mirror and pin practical experiment)
Different converging lens ray diagram questions (must know)
Different ways to have a sharp image formed on the screen
Which distance is the focal lens of the converging lens? Olevel question
When temperature is constant (for o-level), when a fixed mass of gas (fixed number of air molecules) is compressed in a closed system (e.g. piston), the volume V decreases and pressure P increases, and vice versa.
But when you multiply pressure and volume, PV, it is always a constant.
PV = constant
Hence we can always equate the PV of the first scenario = to the PV of the second scenario, provided there is no addition or removal of air molecules from the system.
Hence, you have P1V1 = P2V2
The followings are 4 different questions which require this concept to solve. Do revise them.
Solutions: Option D (refer to the worked solutions below)
Solutions: Option D
Solutions: A
4)
Solutions:
5)Â 
Solution: Option C
Evan's Space 