Kinetic Model of Matter O level Physics

Posted on Wednesday, October 5, 2016 by

KINETIC MODEL OF MATTER!

Introduction:
Hey again everyone! Hope you are all fine. Today the topic we are going to study is the Kinetic Model of Matter. We have all studied it in the previous classes, easy wasn’t it? Let’s get started!

States of Matter:
Okay so matter can exist in three states as we all know namely the solids, liquids and gases. These are called the states of matter. As shown below, water (liquid) can exist in ice (solid) and water vapour (gas).

So what is the kinetic model of matter? It states that:
1. All matter is made of tiny particles which exist as atoms, molecules and ions.
2. The particles are always in continuous, random motion and hence, possess kinetic energy. The kinetic energy of a particle increases with temperature and pressure and at fixed temperature, lighter particles move faster.

The three states differ in arrangement and movement.
Here’s a simple diagrammatic representation of the states:

Now lets get into detail about these.
Solids
●         Solid particles of are tightly packed,
●         Solid particles vibrate about a fixed position,
●         Solids have a definite shape and  a definite volume,
●         Solids cannot be easily compressed because of very less free space between particles and
●         Solids do not flow because the particles cannot move or slide over one another.
Liquids
●         Liquids have a definite volume, but do not have a definite shape since because of less tightly packed liquid particles as compared with solids,
●         Liquid particles are far enough apart to slide over one another and therefore, liquids flow easily and
●         Liquids cannot compress easily since there is less free space between particles as compared with gases.
Gases
●         Particles of gases are much far apart and move freely and therefore, gases do not have a definite shape or a definite volume,
●         Because of more free space between particles, gases can be easily compressed and
●         Because of the random and faster movement of the particles as compared with solids or liquids, gases can flow very easily.

Now if you relate the properties with one another, they’ll be easy to memorize and will seem pretty much logical. Like for instance, why can’t solids be compressed while gases can be? That difficult? Oh no, it’s not! Gases move at very high speed, we know, which causes them to have a  lot of space between their particles. So, they can be compressed and brought closer but just think, can you bring the already so close particles of solid more closer?  Of course not. So just try to make sense out of it, it’s not that difficult.

Pressure in Gases:
First of all, what is pressure? We have already studies in the tutorial on Pressure that is the force acting per unit area.
p-T relationship:
How are pressure and temperature related. Do you know that if you heat a container containing gas, the gas particles will gain kinetic energy and move faster? When they’ll move faster, they’ll hit the walls of container more often and more force will be exerted on the walls per unit area. This increases the pressure. Hence:
p α T (pressure is directly proportional to temperature)
V-T relationship:
When a gas is heated, it’s temperature rises, causing the molecules to move at higher speeds, we just discussed. The pressure increases, that is, the particles collide with the walls of container more frequently. When this happens, the gas will expand in order to keep the pressure constant. So it will occupy more volume if it can. Logical, right? Therefore, we can conclude:
V α T (volume is directly proportional to temperature)
p-V relationship:
Okay so now, what will happen to pressure if volume of gas is increased or decreased? Take a look at the two syringes below:

The volume in 2 is decreased by pushing the same syringe further which has caused the volume to halve the original one in syringe 1. When we halve the volume, the number of molecules of air per unit volume will be doubles, this will double the frequency of collisions with the wall of the syringe and consequently, will double the pressure. Similarly, if you double the volume, the frequency of collisions per unit volume will decrease and the pressure will decrease to half.
So to conclude:
p α V (pressure is directly proportional to volume)





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Kinematics O level Physics

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KINEMATICS!
Introduction:
Welcome back! Hope you all are good. Today we are going to learn an important topic, that is kinematics. The big question: what is kinematics? It is the study of speed, velocity, time and acceleration. We’ll study these quantities in more detail in this tutorial.

Definition:
Okay so starting with speed and velocity, speed is defined as the distance travelled by an object in a given time, or more simply as distance travelled per unit time. While on the other hand velocity is displacement per unit time. So then, what is the difference between the two? The difference is the same as that between distance and displacement; Speed is a scalar quantity while velocity is a vector. Now, what is scalar and what is vector? Well, scalar quantities are those which have a magnitude only, while vector quantities have both a direction and a magnitude. Just to further clear it out, consider a car going at a speed of 40 m/s. We are not told whether the car is going northwards, down the hill or whatever. So what is 40m/s? It’s speed. A bit easier now?

How to calculate average speed?
Average speed can be calculated using the formula:
Average Speed = Total Distance Travelled / Total Time Taken

Q1. The distance b/w town A and town B is 25 km . A car travels from town A to town B and then returns to town A . The total time taken is 5 hours. What was the average speed of the journey?

Uniform acceleration:
Acceleration is the rate of change of velocity. Simple as that!
Acceleration = (Final Velocity - Initial Velocity) / Time

Q2. A car starts from rest and travels in a straight path. It reaches a speed of 40 m/s in 8 seconds. What is it’s acceleration, assuming that it accelerates uniformly?

If we know that the acceleration of an object is constant, we can find its average velocity by a different formula, which is as follows:
Average Velocity = (Final Velocity + Initial Velocity) / 2
Also if an object is travelling with constant acceleration, then these equations of motions are applicable.

●        2as = v2 - u2
●        s = ut + 1/2 at2
●        v = u + at
●        2s = (v + u)t

Key:
a = acceleration , s = displacement, v = final velocity, u = initial velocity, t = time
These equations are very helpful in finding the unknown.                       

Non-uniform acceleration:
This form of acceleration changes with time. In such case, the equations of motions cannot be used.

Graphs:
Every situation that can be considered can be represented graphically. It helps us in various calculations and is easy to interpret. In such graphs, time is always taken on the x-axis while distance on the y-axis.

Distance-Time Graphs:

Okay so here are the distance-time graphs of three objects in motion. As we know the gradient of distance-time graph represents speed. Object A is moving with increasing speed a it’s gradient is increasing. Object B is moving with uniform speed as it’s gradient is uniform. And similarly, as the gradient of C is decreasing, it’s moving with a decreasing speed. Nothing too scientific, right?

Okay so moving on to the graph of a stationary object, it should be covering any distance, right? So that explains it all! The graph of a stationary will look like this:




As we can see, the object is stuck on the same distance. Let’s say 40 m, and it’s on 40 m throughout. So it’s stationary, not moving, at rest!

Speed-Time Graphs:
So here’s a speed-time graph for an object at rest:


Since the slope is equal to zero, there is no acceleration. And secondly, the velocity is zero, so object is at rest.

And a speed time graph for an object moving at constant velocity:

Since the slope is zero, the acceleration is obviously zero, right? And as there is no acceleration, that is, change in velocity, the velocity is therefore constant. Get it?
Okay, and here’s a new concept: the area under a speed time graph gives the distance moved by the object.

 The shaded area in this case also gives the distance moved by the object:

Graph for non uniform acceleration:

Okay so as we can that the slope of the graph is increasing, and we know that the slope in a velocity time graph represents acceleration. Therefore, the acceleration in this case is increasing. Easy, no? And again, the shaded area in this case represents the distance travelled by the object.

So to sum up:
●        The gradient of a distance-time graph represents speed
●        The gradient of a speed-time graph represents acceleration
●        The area under a speed-time graph represents distance
Motion under free-fall:
The acceleration due to the gravitational pull of the earth is always constant and its value is 9.81 m/s2 . However, when a body falls from the sky, it doesn't fall with constant acceleration. This is due to the resistance provided by air which is present. As soon as the body accelerates the air resistance acting on the body also increases. Very soon, the air resistance reaches the point where it balances the weight of the body which means that the acceleration of the body becomes zero, as the resultant force acting on it is also zero (we will deal with forces in the next section). This causes the body to fall with a uniform velocity; This velocity is known as terminal velocity. A typical graph for motion under free-fall would look like this:


Task: Okay so finally, your assignment is to google questions of kinematics and assess yourself to see if you have grabbed the basic concepts.

Answers:
Q1: 10 m/s
Q2: 5m/s2



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Forces

Posted on Monday, October 3, 2016 by

FORCES!
Introduction:
Hello everyone! Hope you all are having a good day. Today we are going to start a new chapter and that is “forces”. We all are familiar with what a force is, right? Okay, let’s say your car’s wheel got punctured in the middle of..nowhere. What will you do? You will push the car to make it move somehow. In this case, you are applying a force to make the car move. Here: we defined a force!

Definition:
So simply, a force is a push or pull that an object exerts on the other. It produces or tends to produce motion (let’s say you push a wall, you are applying force but the wall does not move) or else, it stops or tends to stop motion. The SI unit of force is Newton (N). A force of one newton is roughly the amount of force with which the Earth’s gravity pulls an object of 0.1 kg i.e. 100g. Force is a vector quantity, which means it has a direction too. (We have already discussed scalar and vector quantities in detail in the tutorial on kinematics). We’ll discuss more about forces in this tutorial.

Addition of Vectors:
When we add two vectors, we have to consider the direction, unlike in scalars. As for scalar quantities we simply add up the magnitudes e.g. a weight of 50g and 20g gives a total of 70g. When we add up two vectors, we are actually trying to find a single vector which will produce the same effect as the two vectors added together. This single is called a resultant vector.
1. Addition of parallel vectors:

Okay in the above case, two parallel forces are acting on an object. How will we find the resultant? It’s easy. First see if the forces are acting in the same direction or in opposite direction. We can see the forces are in opposite direction: the 40N force is acting upwards while the 25N force is acting downwards. So the resultant force in this case is 15N upwards: 40N - 25N. (Remember to state the direction while mentioning a vector quantity).
Observe the following summation of two parallel forces:

It makes it more clear, right? Good then, let’s move on!

2. Addition of non-parallel vectors:
Now what to do with such questions?

We have all studies the Pythagoras’ Theorem in math, which goes like this:

Here’s how we’ll solve the big problems:

Now, does this make it clear? We’ll simply form a right angled triangle, apply the Pythagoras’s theorem and we have the resultant force! But how will we state the direction of the resultant in a written form?



 Here! Now we can easily write down the direction: the resultant is 15.6 N acting 45° anticlockwise. Not that of a big question, is it? Wait, are you all familiar with the trigonometric ratios: sin, cos, tan? Hmm, if not, here we go:


Okay, so you might be asked to find the resultant by drawing a scaled vector diagram. Not difficult at all. Just follow the steps!


Here, we have formed a parallelogram and found the resultant. We have to take a scale first, let’s say 1 cm on paper represents an actual of 10N. We’ll measure the length of the resultant and using the scale, will determine the actual magnitude. Easy as that.

Forces and Zero Acceleration:
Let’s say a car is moving with a constant velocity. It means that the acceleration is zero. However, even though acceleration is zero, it does not mean that there are no forces acting on the car, it means, in fact, that the forces are balanced. So we can conclude that for an object with zero acceleration, the different forces acting on it are balanced or add up to zero-i.e. the resultant net force is zero.

Newton’s First Law of Motion:
The situation we just discussed brings us to Newton’s first law of notion which states:


Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.


Newton’s Second Law of Motion:
When a resultant force acts on an object of constant mass, the object will accelerate and move in the direction of the resultant force.

The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors; in this law the direction of the force vector is the same as the direction of the acceleration vector.


Try solving this and see if you can:
Q1: Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s2. How much force is Mike applying to the car?

The formula F = ma can also be written as W = mg, because W i.e. weight is a force with SI unit Newtons. And g is acceleration due to gravity.

Newtons Third Law of Motion:
For every action, there is an equal and opposite reaction.

Okay so let’s study how a rocket works to understand the Newton’s third law!


The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force. Easier now?








Answers:
Q1: 50N



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Energy, Work and Power

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ENERGY, WORK AND POWER!

Introduction:
Hey everyone! Hope you all are good. Today we are going to study an important chapter that is work, energy and power. First tell me can you study, or play, or do anything if you haven’t eaten anything the entire day? Probably you can’t. The reason is simple: because you do not have the energy to do it!

Definition:
So what is energy then? We just said we can’t do work if we don’t have energy. We have already defined it, in less scientific terms though. Energy is basically the capacity to do work. The SI unit of energy is the Joule (J) and it is a scalar quantity as no direction is involved in it.
And now, what is work? Work done by a constant force on an object is given by the product of the force and the distance moved by the object in the direction of the force.



Okay, so in the above diagram, a force F is applied on the object and the object moved a distance d in the direction of the applied force. So the work done is given by:
Work Done = Force x distance moved in the direction of force
W = F x d
where force F is in Newtons (N) and distance d is in metres (m). The SI unit of Work is Joule (J). As you have noticed the SI unit of work and energy is the same. This is because, as we have already discussed, work can only be done when there is energy. Hence both should have the same unit - joule.
We’ll discuss Power separately later.

Types of Energy:
1. Kinetic Energy: The most obvious form of energy is in movement: we walk, life things etc, moving objects possess kinetic energy. Can you think of examples having kinetic energy? Umm..wind? Or waves of sea? Or your flying football? These all possess kinetic energy.
2. Potential Energy: The stored energy is called potential energy. It exists in different forms such as chemical potential energy which is there in the food we eat and is converted into kinetic energy when we move our bodies. Another form is the gravitational potential energy which is due to the height of the object. For example if a ball is thrown up, it gains gravitational potential energy and when it comes back, it’s potential energy is changed to kinetic energy. Not that difficult or is it?

Principle of Conservation of Energy:
This principle states that energy is neither created nor destroyed. It just changes from one form to another or transfers from one body to another but the total amount remains constant.
Like we just discussed, the food we eat contains chemical energy which is converted from solar energy of sun by the process of photosynthesis. When we move, it is converted into kinetic energy and when we climb up a flight of stairs, it is converted into gravitational potential energy due to height..and so on, but it never destroyed!

Efficiency:
Let us consider us a machine for example a power station in which fossil fuels are burnt to generate electricity. We know by the principle of conservation of energy that the energy input must equal the energy output. But we also know that energy output is less than energy input as energy is dissipated in the process, usually into sound and heat energy. To summarize this:
Chemical Energy = Electric Energy + Heat and Sound Energy
(Chemical energy in fossil fuels, Electric energy of electricity generated and heat and sound energy are dissipated in the process)
We can see that the total energy output and input are same. Though the useful energy is electric energy and the rest is wasted. So we form a general equation:
Energy Input = Useful Energy Output + Wasted Energy Output

So now we are able to measure the efficiency of a machine by the ratio:
Efficiency = ( useful energy output / energy input ) x 100 %

Task: Google different forms of energy and the different processes which may involve these energies.

Work Done:
We have already defined Work done. Let’s make it clearer: One joule is defined as the work done by a force of one newton which moves an object through a distance of one metre in the direction of force. So you know how much work is one joule, right? Let’s study some formulas which related energy and work.
The kinetic energy (Ek) of a body of mass m and speed v can be found by:
Ek = ½ (mv2)
And the gravitational potential energy (Ep) of an object of mass m at height h and gravitational pull g acting on it can be found by:
Ek = mgh
Q1. A boy drops a marble of mass 10 g from a height of 65 m. What is speed of marble when it hits the ground? Air resistance can be neglected and g = 10 N kg-1.

Hint: Since air resistance is neglected, the gravitational potential energy is entirely converted into kinetic energy. Therefore, we can assume that
Ek = Ek
½ (mv2) = mgh
Power:
Consider two boys of equal mass, A takes 40 seconds to run 20 m while B takes 80 seconds to run the same distance. A has more power although both did the same amount of work (i.e. running 20 metres).
Now consider that boy A has a greater mass and both cover the 20 m distance in same time, who has more power now? Boy A has to carry a larger mass, he has done more work in running than did boy B. Since he is able to do more work in the same time, he has more power. It’s just a bit conceptual, once you get it.

So now we can define power: it is defined as the rate of work done or the rate of energy conversion.
It is given by:
P = W / t  or  P = E / t
where W and E are work done and Energy converted respectively while t is time. Energy and Work are in Joule (J) while time is in second (s). The SI unit of Power is Watt (W).
So from the equation, 1 W = 1 J s-1.
Q2. A boy runs a fight of stairs of 2 m in 5 s. What is his body power is his weight is 450 N?

Task: Find more questions on the Internet regarding work, energy and power and try solving them to see if you can.















Answers:
Q1. 36 m/s
Q2. 180 W


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Electromagnetism- O Level Physics

Posted on Tuesday, September 27, 2016 by

ELECTROMAGNETISM!

Magnetic effect of current:
Remember that a current-carrying conductor produces a magnetic field around it. If the current direction is reversed, the magnetic field direction will be reversed too, as shown in the diagram above.


A dot in the wire, as shown above, shows the current coming out of the plane whereas a cross in the wire shows the current moving into the plane.
Now how to determine the direction, shown above, of the magnetic field?

The right hand grip rule! Grip the wire with your right hand in such a way that the thumb is pointing to the direction of the current. The curl of your fingers, in turn, will show the direction of the magnetic field. Easy?

The strength of the magnetic field in a long, straight current carrying wire depends on
●        the magnitude of the current. A larger current will produce a stronger magnetic field around the wire.
●        the distance from the wire. The strength of the field decreases as you move further out.

Magnetic field pattern around a flat coil:

The direction of the field can be determined by the Right Hand Grip Rule. Grip the wire at one side of the coil with your right hand, with thumb pointing along the direction of the current. Your other fingers will be pointing in the direction of the field. Then do the same with the other side of the coil.
Plane view of the flat coil:

In a flat coil the strength of the magnetic field is closer in the centre of the wire as you can see in the diagram above (closer field lines). To increase the strength,
●        increase the current ,
●        increase the number of turns of the coil.

Magnetic field pattern of a solenoid:
A solenoid is a long coil made up of a numbers of turns of wire. The magnetic field of a solenoid resembles that of the long bar magnet, and it behaves as if it has a North Pole at one end and a South Pole at the other.

You would have noticed that it’s equal to increasing the turns of wire of flat coil
To determine the poles in this case, again, apply right hand rule as shown below. The fingers point towards the current direction while the thumb will show North pole direction, as shown:



The strength of the magnetic field in this case can be increased by
●        Increasing the current,
●        Increasing the number of turns per unit length of the solenoid,
●        Using a soft-iron core within the solenoid.


Force on current carrying conductor:
Now if you place the same current carrying wire in a magnetic field, the wire will experience a force. The force is experienced due to the interactions between the two magnetic fields (Which two? The one which we learned is produced by the current carrying wire itself, and the one in which it is placed) which produces a force on the conductor.
The direction of the force should be known, and this can be found using the Fleming’s left hand rule as shown in the diagram:

Notice that the fore finger, middle finger and the thumb are perpendicularly to each other. The forefinger points along the direction of the magnetic field, middle finger points in the current direction and the thumb points along the direction of the force.
The strength of the force can be increased by
1.       Increase the current
2.       Using a stronger magnet.
And the force direction can be controlled easily. It can be reversed by reversing the direction of the current or the magnetic field.
 Now let’s see how exactly a force is experienced on the current-carrying conductor when placed in a magnetic field. The interaction between the two magnetic fields is as shown below:

We know that the field lines will always be from North to South. The direction of the current is shown by the cross, which is into the plane. Now apply the left hand rule and you’ll get the direction of the force which is 1 in the diagram.

Force on a moving charge in magnetic field:
Now if we consider a charge entering a magnetic field with direction into the plane, the direction of current will be the convectional current direction i.e. from positive to negative (the direction of current is shown). Now if you apply left hand rule, you’ll get the direction of force it will experience, which will be upwards.


Forces between two parallel current-carrying conductors:
If two parallel wires are placed together, we know they will generate a magnetic field around them, right? The wires will experience a force. Why? Let’s see!




If you right hand rule on the current carrying wires, you’ll get the direction of the fields around them. Now if the direction of the current in both wires is the same, they will attract. If, however, the direction is opposite, they will repel. This is because the field direction in the wires with same current direction will be the same. See the diagram carefully and draw it yourself, you’ll understand how this happens.

Force on current-carrying rectangular coil in a Magnetic field:

If a current carrying coil is placed in a magnetic field (As shown in diagram above), a pair of forces will be produced on the coil. This is due to the interaction of the magnetic field of the permanent magnet and the magnetic filed of the current carrying coil.

The direction of the force can be determined by Fleming's left hand rule. Since the current in both sides of the coil flow in opposite direction, the forces produced are also in opposite direction. The 2 forces in opposite direction constitute a couple which produces a turning effect to make the coil rotate.

This phenomenon is used in a d.c. (direct current) motor. What does an electric motor do? It simple converts electric energy to kinetic energy. The d.c. motor consists a rectangular coil of wire placed between 2 permanent magnets. The coil are soldered to a copper split ring known as commutator. 2 carbon brushes are held against the commutator.
This is shown below:

The function of the brush is to conduct electricity from the external circuit to the coil and allow the commutator to rotate continuously (since the brush is of carbon, it conducts electricity).
The function of the commutator is to change the direction of the current in the coil and hence change the direction of the couple (the 2 forces in opposite direction) in every half revolution. This is to make sure that the coil can rotate continuously.





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