Nuclear Reactor

1. A nuclear reactor gives massive amount of energy through nuclear fission.


Source: http://avasiyamillai.blogspot.com/2011/07/nuclear-power-plant.html

2. The energy freed from the fusion of nuclear fuel heats the water in the surrounding.

3. Consequently, this produces steam which  drives the turbines. The turbines then drive the electrical generators.

4. The table below summarises the main functions of each components.

Component	Function / Explanation
Graphite Moderator	Fast moving neutrons are slowed down by collisions with nuclei in the moderator so that they can cause further fissions. In some nuclear power plant, the moderator is water.
Uranium rod (Fuel)	Fission reactions take place in the uranium rod to create nuclear energy. The uranium used is often ‘enriched’ by increasing the proportion of the isotope uranium-235 above the natural value of 0.7% to 3%.
Control Rod	The rate of the fission reaction is controlled by inserting or withdrawing these rods. The nuclei in the rods absorb neutrons without undergoing any reaction. Sometimes the rod is made of cadmium.
Coolant	To take away heat from the nuclear reactor. Substances with high specific heat capacity such as ‘heavy’ water and carbon dioxide are used.
Thick Concrete Wall	To avoid the run off of harmful radiations.
Steam generator	Water in the generator is heated and changed into steam. The steam then drives the turbines.
Turbines	To revolve the dynamo in the electrical generator to generate electricity

5. Nuclear reactors are used in the production of:

a) High-intensity neutron beams for research
b) Artificial Radioactive Isotopes for medical research
c) Fissionable transuranic elements such as plutonium from uranium-238

Reasons for the use of Nuclear Energy

1. Production of nuclear energy from nuclear fuels involves a decreased cost. A small amount of nuclear fuel can provide a large amount of energy.
2. Nuclear reactors are relatively safe especially with the sophisticated technology constantly developed and improved.
3. The decreasing supply of fossil fuels make it essential for the use of alternative sources of energy.
4. The use of nuclear energy does not release greenhouse gases such as carbon dioxide.

Reasons against the use of Nuclear Energy

1. Radioactive residues from nuclear stations have quite a long half-lives.
2. There is a chance of leakage in the radioactive waste containers placed underground or underwater.
3. High cost of constructing a nuclear power station.
4. Accidents could happen due to human error no matter how sophisticated the technology is and this should be put into consideration.

Transmission of Electricity

Most power stations are located far away from populated areas. Electricity is transmitted from the power station to consumers through power lines.

Unfortunately, some electrical energy is always lost as heat when it travels through wires.

Since the heat dissipated depends on the magnitude of the current, it is more efficient to transmit electrical energy at very low currents.

To produce this low current, the voltage has to be increased. Step-up transformers are used for this purpose to increase the voltage at the power plant. 

Step-down transformers are used to decrease the voltage before being delivered to the consumers.

In addition, the long thick cables used as transmission lines are made of copper or aluminium because they have low resistance and thus less energy will be lost when current flows through them.

Example 1:

A power station supplies a factory with 1.0 MW of electrical power at a potential difference of 2 kV. The resistance of the cable between the power station and the factory is 5 Ohm. Find the power loss in the cable.

Current to factory : from equation P = IV, I = P / V (P divided by V)

= 1 X 10^6 (ten to the power of six OR ten exponent six) / 2 X 10^3 = 500 A.

Power loss in the cable = I^2R ( I squared times R)

= (500)^2 X 5

=1.25 X 10^6 W (Watt)

Energy Losses In a Transformer

When we use the equation VpIp = VsIs we are assuming that the transformer is an ideal transformer. An ideal transformer is one which is 100% efficient. In practice, the efficiency of a transformer is less than 100%.

A transformer is designed so that as little energy as possible is lost. There are many ways that a transformer can lose energy.

(a) Power losses occur because the changing magnetic field will also induce currents in the iron core. These induced currents are known as eddy currents. Eddy currents will generate heat and reduce the transformer's efficiency. In order to reduce the formation of eddy currents, a laminated core is used.

(b) Current flowing through th primary and secondary coils will generate heat. Low resistance copper wires is used to reduce this effect.

(c) The core is magnetised and demagnetised alternately when AC current flows through the primary coil. Energy is lost during this process. This is known as Hysterisis. This effect is reduced by using a soft iron core.

(d) There may be a leakage of magnetic flux in the primary coil. A special core design is used in a transformer to ensure that all the primary flux is linked with the secondary coil.

Semiconductor Diodes

Semiconductor Diodes

1. A semiconductor diode is called the p-n junction diode.
2. it consists of a p-type semiconductor in contact with n-type semiconductor.
3. Regions of P-type is called ANODE.
4. Regions of N-type is called CATHODE.
5. A p-type material meets an n-type material across a bounding region called the depletion layer (p-n junction).
6. In order for current to flow through the diode, the voltage applied across the diode must exceed the junction voltage.
7. Junction voltage is the potential difference that is caused by the movement of the holes and free electron.






FUNCTION OF DIODES


Diode as rectifiers

1. A diode is said to be in a forward-biased arrangement if it only allow the current to flow from the anode to cathode. It is acting as a VALVE
2. A diode can CONVERT alternating current into direct current. This is known as RECTIFICATION. Therefore a diode can act as a RECTIFIER.
3. A RECTIFIER is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. RECTIFIERS have many uses including as components of power supplies and as detectors of radio signals
4. There are TWO ways to convert an alternating current into a direct current.
a. Half-wave rectification
b. Full-wave rectification

Half-wave rectification

1. The current can only flow in the forward direction through the diode.
2. The process of rectification using a diode which ALLOWS CURRENT TO FLOW IN THE HALF-CYCLE is known as half wave rectification

Half Wave Rectification

Full-wave rectification

1. The arrangement of diode in Full-wave rectification is called a bridge rectifier.
2. The process of rectification using four diodes to allow current to flow in a complete cycle and in the same direction is called full-wave rectification. 
3. A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient.


Full Wave Rectification


Smoothing


1. The output from a rectifier can be SMOOTHED by connecting a CAPACITOR across the load.
2. During the forward peaks (positive half-cycles), the capacitor is charged up. Energy is stored in the capacitor.
3. In between the forward peaks (negative half-cycles), the capacitor releases its charge (discharge). It discharges partly through the load. The energy stored in the capacitor acts as a reservoir and maintains the potential difference across the load.
4. A capacitor with greater capacitance produces a smoother current. This is because the capacitor can store more charge.

Understanding Total Internal Reflection of Light

1. If the angle of incidence is allowed to exceed the critical angle, it is found that light rays are not refracted. This is because all of the light rays are reflected back.

2.This phenomenon is called total internal reflection.

3. Total Internal Reflection occurs when:
   a. Light rays travel from a denser medium to a less dense medium.
   b. The angle of incidence is greater than the critical angle.

Light ray which travels from a denser medium to a less dense medium will be refracted away from the normal.

Here are some Q and A session:

Q: What happens when light passes from a transparent medium into air?

A: When light passes from a transparent medium into air, it bends away from the normal. It is refracted.

Q: Why the angle of refraction becomes 90° and not more? What do we call the angle of incidence at this limit?

A: This is the limit the light ray can be refracted in air because the angle in air cannot be larger than 90°. The angle of incidence in the denser medium at this limit is called the critical angle, c.



Q: What happens when the angle of incidence is more than the critical angle?

A: When the angle of incidence is greater than the critical angle, all the light undergoes reflection.

Later we will study the Relationship between Critical angle and Refractive Index

Reflection of Light on a Curved Surface: Method to draw ray diagrams

1. There are two main types of curved mirrors, namely:

(a) Convex Mirror
(b) Concave Mirror

2. On a Concave mirror, the rays that are parallel and close to the main axis (small opening) converge to a point F (main or principal focus) and the distance FP is known as the focal distance of the concave mirror. (P is the surface of the mirror)



More notes can be found here:
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/class/refln/u13l3d.html

3. On a Convex mirror, parallel rays that are close to the main axis, diverge from the surface of reflection. The rays are seen to diverge from a poinf F (main focus) behind the mirror. The distance FP is known as the focal length of the mirror.


 More notes can be found here:
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/class/refln/u13l4a.html

Characteristics of Image formed by a plane mirror

Characteristics of image formed in a plane mirror.

(a) It is virtual
(b) Has the same size as the object
(c) Is laterally inverted (i.e. inverted sideways)
(d) The distance of the object from the mirror is equal to the distance of the image form the mirror.

Understanding the Reflection of Light: Law of Reflection of Light

1. The reflection of light can be studied by using light ray(s) and a plane of mirror which is placed on a piece of white paper.

image from: http://www.hsphys.com/pmirrb.jpg

2. When the ray of light is incident onto the surface of a plane mirror, the light ray does not pass through the mirror but is reflected back by the plane mirror.

3. The phenomena of ths experiment shows the phenomena of reflected light.

The Law of Reflection of Light States that:

1. The incident Ray, the reflected ray and the normal all lie in the same plane.

2. The angle of incidence is equal to the angle of reflection.


Image courtesy:
http://www.curriki.org/xwiki/bin/download/Coll_Athabasca/Unit3-Lesson2TheMovementofLight/reflection.jpg

More information at:

www.hsphys.com/ light_and_optics.html

Characteristics of Image that is formed on a plane mirror

1) It is upright
2) It is virtual
3) The distance form the object to the mirror is the same as the distance from the image to the mirror.
4) It is the same size as the object
5) It is laterally inverted

All the best!

Understanding the Gas Laws: Gas Laws and Kinetic Theory of Gases

Gas theory can be explained by way of the kinetic energy.

When gas molecules hit the walls of the container and bounce back, a change in momentum occurs in a split second. This is obviously a very very fast action.

The end result of the above momentum is that the walls of the container experience a force.

Pressure is defined as the force that acts on a unit surface area. Therefore, all surfaces that are knocked by air will experience a pressure. In order for this to take effect all of the gases molecules in the container or free surface must be moving swiftly in a very short time and hit the surface repeatedly.

This pressure is called gas pressure.

Kinetic Theory of Gases

The basic assumption for the kinetic theory of gas is as follows:

Gas is composed of molecules.

Gas molecules are continually in random and independent motion in all directions at high and different speed.

The motion of gas molecules follows all of the Newton Laws of Motion.

All collisions between the gas molecules (i.e. one with another) and the walls of the container are assumed to be perfectly elastic. Therefore, momentum and kinetic energy are conserved during collision.

 The volume of the molecules can be conserved compared to the volume occupied by the gas.

The force among the gas molecules can be neglected except during collision.

The time period of a collision can be neglected when compared with the time interval between two collisions.

Application of Specific Heat capacity

As we have read (supposedly) about the concept of heat capacity and specific heat capacity, we will discuss briefly about the application of Specific Heat capacity in daily situations.

1. Substances having a small specific heat capacity can be quickly heated up, it also experience a big change in temperature even though only small amount of heat is supplied.

2. Substances having a small specific heat capacity, are very useful as material in cooking instruments such as frying pans, pots, kettles and so on, because, they can be quickly heated up even when small amount oh heat is supplied.

3. Sensitive thermometers also must be made from materials with small specific heat capacity so that it can detect  and show a change of temperature rapidly and accurately.

4. Substances that have a high specific heat capacity is suitable as a material for constructing kettle handlers, insulators and oven covers, because, a high amount of heat will cause only a small change in temperature aka the material won't get hot too fast!

5. Heat storage instruments are very useful and they are usually made of substances with a high specific heat capacity.

6. Water as a cooling agent acts excellent as a cooling agent in engines. Water is also used in houses in cold climate countries because as it is heated up (boiled) it tends to retain heat and warm the house due to its high specific heat capacity.

Specific Heat Capacity

Specific Heat Capacity

1. Specific heat capacity, c, of a body is the heat that is needed to increase the heat of a unit of mass or the substance by 1°C or 1K.
2. The unit of specific heat capacity is Jkg-1°C-1.
3. For example, the specific heat capacity of water is 4200 Jkg-1°C-1 . This means that 4200J of heat is needed to increase the temperature of 1 Kg of water by 1°C.
4. Therefore, when a body of a mass m and specific heat capacity, c, absorbs a quantity of Heat, H, then its heat will increase by θ.
5. Therefore H = mc θ.
6. On the contrary, when the heat of a body falls by θ, the quantity of heat that disappears (lost) is also H = mc θ.
7. The specific heat capacity is dependent upon the type of substances. Different substances have different specific heat capacity.
8. By knowing the specific heat capacity, we can determine the mass and also the change of temperature of a body if we know the amount of heat that is transferred.
9. Total heat transferred H = mc θ.
10. Generally, liquid has more specific heat capacity than solids. This means that liquids need more heat energy than solids for the same rise in temperature.

Types of Thermometer

There are several types of thermometer, here, I explain only a few of the possibly many types of thermometer.

Mercury thermometer

1. The physical quantity that is used to determine the temperature of a body by means of a mercury thermometer is the length of the thread mercury, or to be more exact, the volume of mercury.

2. When the temperature increases, the volume of the mercury increases too.

3. The sensitivity of a mercury thermometer can be increased by

a. reducing the diameter of the capillary tube.
b. increasing the size of the bulb.
c. using a thinner-walled glass bulb.

4. Normally mercury is used in a thermometer because it:

a. Expands uniformly.
b. has a higher boiling limit.
c. is opaque and therefore it is easier to read off the temperature.
d. is a good conductor of heat.
e. does not stick to the glass.

5. One weakness of the mercury thermometer in the measurement of an accurate temperature is that the glass of the capillary tube also expands when the temperature expands.

In addition to that, it is extremely dangerous if the glass tube breaks because mercury is very poisonous.

Mercury thermometer is suitable to measure temperature between -30 degree celsius to 300 degree celcius.

Resistance thermometer

1. Thermometers which use liquids inside the glass are not suitable to be used for measuring a wide range of temperature. e.g temperature ranging from -250 degree celcius to about 700 degree celsius.

2. A suitable thermometer which is used for the above range of temperatures is a resistance thermometer.

3. A resistance thermometer uses the property of the change in the platinum wire with a change in temperature.

4. The current flowing in the wire experiences more resistance when the wire becomes hot.

5. The change in the resistance of the wire is directly proportional to the change in temperature.

6. A milliammeter can and should be calibrated before hand to measure the temperature.

7. Its calibration of the melting limit of water and the boiling point of water at a pressure of 1 atmosphere is able to convert the milliameter scale to a temperature scale in degree celsius.

8. Therefore, this thermometer is very accurate.


Thermocouple thermometer

1. An electromotive force (e.m.f) will be produced in a thermocouple when there is a temperature difference between the hot junction and the cold junction. Once this happens, a current will flow.

2. This thermometer is very sensitive and responds towards slight change in temperature.

3. Since the physical quantity which is used to measure the temperature is the e.m.f, this thermometer can be connected to other electrical circuits to control or record the surrounding temperature.

4. A thermocouple thermometer is a very sensitive thermometer which is suitable for measuring temperatures ranging from -250 degree celsius to 1600 degree celsius.

Thermometers and calibration of Thermometers

The definition of temperature as a physical quantity is based on the principle of thermal equilibrium.

Let say there are Thermometer A, Liquid B and Liquid C.

We put thermometer A into liquid B and then after thermal equilibrium is achieved we record the value.

We put thermometer A again into liquid C and after thermal equilibrium is achieved we record the value of reading in the thermometer.

If the temperature in both cases are the same, then liquid B and liquid C are in thermal equilibrium with one another. Eventhough, the two liquids (B and C) are not in thermal contact, they are in thermal equilibrium because their temperatures are the same.

Therefore Temperature is a physical quantity which determines whether or not two objects are in thermal equilibrium.

We measure temperature using a thermometer. 

Thermometers must be calibrated before they can be used to measure temperatures.

The calibration of an instrument refers to the process of marking-up a scale on the instrument to be used as measurement.

To produce a scale on a thermometer, two fixed points must be determined first. Then the two points must be the temperatures which can easily and correctly reproduced in any part of the world.

On the Celsius scale, the two fixed points are the ice point (0°C) and the steam/boiling point (100°C).

The ice point (0°C), or lower fixed point is the melting temperature of pure ice at standard atmospheric pressure (760 mm Hg).

The steam point (100°C), or upper fixed point is the temperature of steam at standard atmospheric pressure (760 mm Hg).

After obtaining, the highest point and the lowest point. We divide the length between them to equal parts / scale.