IGCSE PHYSICS Topic : Forces and Acceleration - F=ma

If you push something it moves, and if you push it harder it moves faster. If you keep pushing it it will be faster and faster.. it will accelerate in fact, and the acceleration will be proportional to the force you apply.

The explanation above is very simplistic. It ignores all other forces apart from the applied force. In fact there will be other forces acting, most commonly friction, and we must find the net force F before we use F=ma to calculate the acceleration.The ball above is falling. The net force F is W-R. If m=5 Kg and R=20N then applying F=ma:

The block above is being pulled along the ground. The resultant force F is P-R. If P=30N, R=10N and m=2 Kg then applying F=ma:

IGCSE PHYSICS Topic : Using Cathode Ray Oscilloscopes

A cathode ray oscilloscopes is used to display electric voltages and waveforms. It does this using the x – axis as the time axis and the y – axis as the voltage axis: Not each square is 1 cm by 1 cm.

When it is switched on it displays just a horizontal line:

Because time is moving forward, we have to display the voltage by letting a pulse on the screen move along the x – axis. In the diagram below no input voltage is applied. If we now speed up the pulse we will have a continuous line on the screen, still reading no volts.

If we increase the sensitivity of the time base sufficiently we will have a continuous line.

Now we connect the input to a voltage source.

If the voltage is from an alternatiing source as above, it will display the actual waveform. The peak voltage is about 2..2 squares. The actual voltage will depend on the y scale measured in volta/cm. If the scale in the diagram above is 2V/cm the actual peak voltage will be To find the practical or rms voltage we use We may also have to find the period or frequency. This will depend on the scale of the x – axis. Suppose the scale is 20ms or milliseconds per division or ms/cm. Then 1 period is 4*0.02=0.08 seconds and the frequency is

IGCSE PHYSICS Topic : Snell's Law, Refraction and Total Internal Reflection

When light passes from one material to another the direction of the light usually changes. This is because of a property called the refractive index of a material, labelled by

The light changes direction according to Snell's Law, given above. This is illustrated in the diagram for light passing out of water into air.
The refractive index of air is taken to be 1. If the refractive index of water is 1.33, then if we can measurewe can findby calculation. Suppose we measureto be 40^o .


^o
Total Internal Reflection
From Snell's Law, there is a certain vale ofcalled the critical angle, for whichThis value is labelled and is illustrated below.

Fortotal internal reflection occurs. No light passes through the interface between the two materials.
Forthe angle of refraction is 90^o. All the light passes along the interface between the two materials.
Forthe light is refracted as it passes from the first material into the second. This is shown in the first diagram above.
Fibre Optic Cable

Light strikes the interface between the fibre optic thread and the cladding repeatedly but the angle of incidence is always less than the critcal angle so total internal reflaction repeatedly takes place and the light can pass for long distances along the cable.

IGCSE PHYSICS Topic : Identifying Series and Parallel Circuits

IGCSE PHYSICS Topic : Transistors

Invented in 1948, the transistor is basically an electronic switch. They are used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. Some transistors are packaged individually but most are found in integrated circuits. They can be made so tiny that millions can now be packed onto the compact electronic chips that are the brains of modern computers. Before the transistor, the only comparable device was the large, unreliable and power hungry vacuum tube.



There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN type because this is the easiest of the two to make.

The leads are labelled base (B), collector (C) and emitter (E).

For each type, a small current between the base and emitter can control a much larger current between the collector and emitter terminals.
If there is no current through the base of the transistor, it shuts off like an open switch and prevents current through the collector. If there is a base current, then the transistor turns on like a closed switch and allows a current through the collector.
SUMMARY:

• Bipolar transistors are so named because the controlled current must go through two types of semiconductor material: P and N. The current consists of both electron and hole flow, in different parts of the transistor.
  
• Bipolar transistors consist of either a P-N-P or an N-P-N semiconductor "sandwich" structure.
  
• The three leads of a bipolar transistor are called the Emitter, Base, and Collector.
  
• Transistors function as current regulators by allowing a small current to control a larger current. The amount of current allowed between collector and emitter is primarily determined by the amount of current moving between base and emitter.
  
• In order for a transistor to properly function as a current regulator, the controlling (base) current and the controlled (collector) currents must be going in the proper directions: meshing additively at the emitter and going against the emitter arrow symbol.
  

IGCSE PHYSIC Topic : Logic Gates

IGCSE PHYSICS Topic : Acceleration, Free Fall and Air Resistance

In the absence of air resistance any body falling freely under gravity falls with a constant acceleration ofA graph of speed against time is shown below for the situation where there is no air resistance. The velocity increases byfor every second that passes.

The acceleration above is equal to the gradient of the graph =The only force acting on the object whose motion is shown above is gravity. In reality gravity is not the only force acting. Air resistance is a force acting on any falling body – in fact any body moving through the air – and the force of air resistance increases as the velocity increases. The diagram below shows a parachutist falling.

1. The parachutist jumps from the aircraft. He parachute is closed and the is no air resistance acting vertically.
   
2. As the parachutist starts to accelerate the force of air resistance starts to increase. His speed is increasing but his acceleration is slowing.
   
3. As the parachutist falls faster the air resistance increases more. The acceleration is decreasing to zero.
   
4. The force of gravity equals the force of air resistance. There is no resultant force and the parachutist is not accelerating.
   
5. The parachutist opens his parachute. The force of air resistance shoots up but the force of gravity stays the same. The parachutist starts to decelerate rapidly.
   
6. As the parachutist slows down the force of air resistance decreases. The resultant force decreases, hence the gradient of the graph and the acceleration both decrease.
   
7. The force of air resistance now equals the force of gravity. The parachutist is falling with a constant speed – he has reached his terminal velocity.
   
8. The parachutist falls with a constant speed until he reaches the ground.
   

IGCSE PHYSICS Topic : Fleming's Left Hand Rule and the Right Hand Grip Rule

The Left Hand Rule

It is well know that a current carrying conductor in a magnetic field experiences a force, and that if a conducting loop of wire moves relative to a magnetic field, a current is generated. Fleming's Left Hand Rule enables us to calculate the direction of the force in the first case and the direction of the field in the second case. This is illustrated below.

In this diagram the two fingers are horizontal at right angles and the tunmb is vertical and at right angles to both. In fact, first finger, second finger and thumb are all at right angles to each other.

The Right Hand Grip Rule

A current also produces a magnetic field. The magnetic field will loop around the conductor. We can find the sense in which it loops by gripping the conductor carrying the current in our right hand. The fingers will point in the direction of the field. This shown below to the left. We can also apply it to solenoids, in a sense, backwards. If the fingers point in the direction of the current, the thumb will point AGAINST the magnetic field.

IGCSE PHYSIC Topic : Static Electricity

If you turn a television on and touch the screen, the screen feels as though it has a film of some sort over it. The tv screen is charged. It has an excess of electron. Many materials accumulate charge in this way. If for example you rub a balloon on your clothes, you may cause electrons to pass from one to the other. If you then hold the balloon close to running tap water, you may cause it to deflect in the way shown below.

Static electricity occurs in nature. Static electricity may build up between the ground and rainclouds. This happens usually when low air pressure causes high winds, causing the same effect as in the example above between your clothes and the balloon. The result is lightning.

Static electricity can be dangerous and sometimes precautions must be taken against it. This is especially so at airports when planes are being refuelled. Static electricity may build up between the tanker and the plane for example as the fuel is pumped. With the air full of fuel vapour, any tiny spark could cause an explosion. Hence tanker and plane are earthed, which disallows any build up of static electricity. Closer to home, you may build up static electricity on yourself while driving, if you move about in your seat. When you get out, at a petrol station, the charge you have built up causes a spark and BANG!