# Evan's Space

## Physical Quantities Videos

The smallest to the biggest thing in the universe.
This video shows the various prefixes used to denote very small or big numbers.

Is Usain Bolt faster than a typical car on the road with a constant speed of 60 km/h?
Calculate the average speed of Usain Bolt for the 100 m world record of 9.58 s in m/s. Then convert it to km/h to do a comparison.

Wanna watch his 200 m race?

How to use a vernier caliper?
The precision of vernier caliper is 0.01 cm (0.001 m).

How to use a micrometer screw gauge?
The precision of the micrometer is 0.01 mm

Factors that affect the period of the pendulum bob
Only length and gravitational field strength (g) affect the period.
– Length of pendulum increases, period (T) is longer
– Gravitational field strength (g) decreases, period (T) is longer (i.e. on the moon)

Mass of pendulum bob and angle of release (5 to 10 degree) will not affect the period (T).

## Identifying what lens, focal length and image from 2 rays – PP2010P1Q23 and SP2014P1Q11

These 2 questions are actually the same. Q23 is from 2010 Pure Physics P1 while Q11 is from 2014 Sci Physics P1. Take a look at these 2 questions. If you are not sure, view the video below for the explanation.

If you do not know how to answer these 2 questions, view this video and also refer to the lens summary below.

## Finding unknown resistor R and setting up the electrical circuit

To find the unknown resistor R, the following apparatus are setup.

Refer to the video below for the setting up of the apparatus.

Why do you need a variable resistor (rheostat)?

• Without the variable resistor, you will have only one set of current I and potential difference V readings. Using the formula R = V/I, you are able to find the unknown resistor. But this method is not so accurate.

• Hence, to make it more accurate, we include a variable resistor to control the size of the current through the circuit. Thus having different readings of the potential difference V across the unknown resistor.
• Instead of just one set of readings of I and V, we now have about 5 sets.
• This allows us to plot a graph of V against I.
• By finding the gradient of the best fit line, we are able to find the resistance more accurately. [gradient = V / I = R, hence the gradient of V-I graph represents resistance R]

For pure metallic conductor, like the fixed resistor R, it obeys the Ohm’s Law, hence it is an ohmic conductor.

From the graph, the current I flowing the conductor is directly proportional to potential difference V across the conductor, provided physical conditions like temperature remains constant. [the graph is a straight line with constant gradient, and passes through the origin]

## Different Lens Ray Diagram questions

Drawing ray diagrams for converging lens come in many forms. But the basic concepts needed are the 3 rays (which have rules to follow) to locate the image. The following ray diagrams for the 4 scenarios must be learned well, together with the respective image characteristics and applications.

With the basic concepts learned, when questions are asked in different ways, you should be able to draw the ray diagrams. Refer to some different ray diagram questions below and their video tutorials.

Click the followings for other lens ray diagram questions. https://evantoh23.wordpress.com/2010/11/09/20101109converging-lens-important-concepts/

https://evantoh23.wordpress.com/2015/08/27/olevel-sp-p1-focal-length-of-lens-using-distant-object/

## Magnetic field around a magnet and an electromagnet

Electromagnet is like a ‘temporary magnet’. When a current flows through the solenoid, it will be magnetised. When the current is switched off, the eletromagnet loses its magnetism.

To make the electromagnet stronger,

1) Increase the number of turns in the solenoid.
2) Increase the size of the current.
3) Insert a soft iron core in the solenoid.
– iron is a soft magnetic material
– it can be magnetised easily
– it can be demagnetised easily