# Evan's Space

## Am I paying a lot on electrical bill for charging my phone?

With new advances in mobile devices, the battery capacity of the lithium-ion battery increases with each new model.

For instance, the flagships phone like iPhone 14 max Pro has an capacity of 4323 mAh, Samsung S22 Ultra 4855 mAh and the Z fold 4 4400 mAh. We tend to think that the constant charging of our phones will use a lot electrical energy, hence increases our electrical bill.

But is it true?

## Which V I readings are for torch lamp?

Pure Physics 2019 P1Q33 and 2007P1Q28

Torch lamp or bulb is a non-ohmic conductor. The V-I graph has an increasing gradient indicating resistance increases. As the current through the filament of bulb increases, temperature increases and resistance increases.

For an ohmic conductor, the graph of V against I is a straight line with constant gradient and passes through the origin.

What if there is an option E where
V increases from 0, 1, 2, 3
I increases from 0, 6, 10, 12 respectively

The graph will be a typical ohmic graph, similar shape to option B, but will you choose E?

Refer to the 2nd part of the video from 3min 40s

## Direction of friction on the wheel

There have been a few questions in Olevel asking students the direction of the friction on the wheel, It is important to consider if the wheel is connected to the engine or not.

Below is an illustration on this concept.

You can view the explanation plus the 4 examples from Olevel p1 in the video below.

Examples

Refer to the video above for the answers

## Isn’t tyre with tread better in providing grip/traction/friction on the road in all weather?

Most of us know that when the treads of our typical car tyres are worn out, we need to change as to provide grip, hence our car will not skid easily.

But actually, the treads of our typical car serve a more important role during rainy days. They allow the water to be displaced (quickly letting the water to move into the treads) so that the tyres can still maintain contact with the road to provide the grip. Of course, if you still go too fast on a wet road, that layer of water may not be able to be displaced in time, hence grip on the road is greatly reduced and the car is likely to skid.

Especially in the recent F1 race in Singapore, due to the wet weather, the cars had to changed to hard wet tyres. Many may be puzzled why F1 cars uses smooth slick tyres during dry weather, contrary to our common sense for our typical cars. The graphic below hopes to provide some basic explanation to the wet and dry tyre and the function of the tread on tyre. Hope it helps.

The following video gives a good overview of how a tyre works. There is a lot of science behind them.

## Turning Effect of Forces – Moment – Basic Concepts

Once you know the basic concepts, try applying the POM in other application questions. Refer to the two videos below.

## How to mark 2 points (at a distance apart) at the same level? (water finds its own level)

The video below demonstrates that water will find its own level.

An olevel question requires this concept:

Solutions: 100J

Water will always find its own level. Hence with or without the pump, the water level in the pipe in the building will be of the same level as the reservoir. Hence, the pump is only needed to pump water up a height of 10m only (instead of the 50m).

Energy, E = mgh = 1 kg x 10 N/kg x 10m = 100 J

## Hand Boiler

Hand boiler toy is a demonstration of thermal transfer and how the 3 states of matter interact.

The coloured liquid inside is usually ethyl alcohol, which is a volatile liquid (i.e. a liquid with low boiling point). Similar to the alcohol swab you applied on hand during injection. Such liquid evaporates easily in room temperature or when heat is applied to it.

Thermal energy flows from a region of higher temperature to a region of lower temperature. When we hold the boiler in our hand, the thermal energy from our hand (body temp about 37oC) is transferred to the lower bulb of the boiler. The ethyl alcohol evaporates and also due to heated air above the liquid, the air expands and the pressure increases. The air pushes the liquid down, hence pushing the liquid up the shaft into the top bulb.

The bubbling of the liquid in the top bulb is not due to ‘boiling’ of the liquid. Rather, the gas in the lower bulb continues to expand and it goes to the top bulb, hence giving the illusion that the liquid is ‘boiling’.

## Direction of Frictional Force on Tyre or Wheel (updated)

For these type of questions, it is important to identify which tyre/wheel is connected/powered by the engine or the person.

On each tyre or wheel, there are actually two types of frictional force.

1. Frictional force on wheel

Examples:

## Oscillating or Vibrating Object in a Wave Motion

When an object is oscillating or vibrating in a wave motion, the speed of the object varies along the path.

In this video, there are 3 examples of vibrating object.

1) Mass vibrating vertically from a spring

2) Pendulum bob oscillating

3) A particle vibrating up and down on a transverse wave

In general, when the object is at the extreme ends of the oscillation or vibration, it is momentarily at rest. Hence its KE at these points is minimum or 0 J.

And in the middle that is where the object is travelling the fastest, hence the KE is the maximum.

## Determine the motion of the particles on transverse wave

Instead of imaging how the particles will move and guess their motion, there is a technique which can help you to determine that.

In addition, when particles are in phase, it means that both particles on the wave have the same velocity and same displacement. It means both have the same speed in the same direction and same distance and direction away from the rest position.

Particles that are out of phase means both are having the same speed but in opposite direction.

Refer to the video below to find out more.

Question 01: 2005/2008 PP P1 Q20/Q18

Question 02: 2018 PP P1 Q24

## Revision on Kinematics

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

## Graphs of Transverse Wave – E.g. Water waves

Refer to the Graphs of Sound Waves here

## Determine focal length of lens using mirror with object and image aligned

Click on the link below to view another similar question.

## Conservation of energy on the complete driver golf swing

Slinky coil can be used to show transverse or longitudinal wave. But do not quote slinky coil as an example of either wave.

But this guy is really pro with a slinky coil!!