UNDERSTAND AIR COMBUSTION, RUSTING AND FIRE FIGHTING

AIR COMBUSTION, RUSTING AND FIRE FIGHTING

AIR

This is a homogeneous colorless mixture of different gases in the atmosphere.

    Composition of gases in air

             The gases include: oxygen, nitrogen, carbon dioxide, and noble gas. Air contains water vapor and dust particles. However, they are not usually considered part of air.

The table below shows the percentage composition of gases.

Gas	% composition
Nitrogen	78%
Oxygen	21%
Noble gases	0.94%
Carbon dioxide	0.03%
Water vapor	0-4%

NB:The percentage composition of water vapor is generally caused by;
                                                                                                        •Weather

                                                                                                        •Geographical location

Test for gases in air.

    -It is possible to test for the presence of some of the gases that are found in air.

Carbon dioxide test

    -Lime water on the watch glass turns milky after a few days when left exposed. The conversion of the lime water from colourless to milky color indicates that there is carbondioxide present in the air.

Oxygen test

    - It relights a glowing splint i.e when oxygen is collected in a gas jar and a piece of wood that has been lighted with fire is also placed within the jar, its going to be relighted because oxygen supports burning. Or copper turns to black on heating because it reacts with oxygen.

Water

    -White anhydrous copper (ii) sulphate on watch glass turns blue when left exposed.This is due the presence of water in air.

Combustion

    -This is a chemical reaction that involves the burning of a substance in the presence of oxygen then produces energy and light.

Combustible
    -Is a material that catch fire and burn easily, combustion takes place in the open place such as fire or in a closed system such as a car engine.

Application of combustion in real life

Areas where combustion is applied;

Area	Application
Industries	In  engines In large boats Incinerators for burning waste
Domestic	Cooking Heating Burning
Laboratory	Sterilization during experiment.

FIRE FIGHTING

Fire - is the process in which ignited material combine with oxygen to give light, heat and a flame.

Fire fighting - is the extinguishing of harmful fire.

Classification of fire

Fire can be classified according to the types of materials burning hence each type of fire will need appropriate fighting technique and equipment.

The table below shows classification of fires and the appropriate extinguishers

Components needed to start fire

For a fire to burn the following components are required in suitable proportions;

1. Source of ignition heat
2. Air (oxygen)
3. Fuel

-The above three components are called fire triangles. If any of them is missing no fire will continue burning.

NB: during the fire fighting at least one component is eliminated to cut off the fire especially air.
.


FIRE EXTINGUISHERS;
                               Is an active fire protection device used to extinguisher control small fire often in emergency situation.

                             > Portable fire extinguisher is an extinguisher that can be easily moved from one place to another.

                        
                                              TYPES OF FIRE EXTINGUISHERS

There are four common types of portable fire extinguishers namely:

1. Dry powder extinguishers;

                               Contain fire sodium bicarbonate powder pressurized by nitrogen.Extinguish the fire by separating the fuel from the oxygen element or by  removing the heat element  of the fire triangle. 

2. Foam extinguishers; 

                             Contains proteins and fluoroproteins.extinguisher  fire by taking a way the heat element of the fire triangle .From agents also separate the oxygen element from           the other  element.

3. Water extinguisher;

                               Contain ordinary tap water pressurized air, extinguish fire by taking away the heat element of the fire triangle.

4. Carbon dioxide extinguisher;

                                              Contain  CO₂, a non flammable gas, and are highly pressurized air. Extinguish fire by taking a way the oxygen element of the fire triangle and also be removing the heat  with very cold discharge.

Using fire extinguishes

The fire extinguishers should be used based on the following; "PASS"

1. PULL - The safety pin from the handle. The pin is located at the top of the fire extinguisher. Once removed, it releases the locking mechanism, allowing you to discharge the extinguisher.
2. AIM - The extinguisher nozzle at the base of the fire. As explained this removes the sources or fuel of the fire. Keep yourself low.
3. SQUEEZE - The handle slowly to discharge the agent. Letting go of the handle will stop the discharge, so keep it held down.
4. SWEEP - Sweep side to side over the fire until expended. The sweeping motion helps to extinguish the fire. Stand few meters back from the fire.

                                                    RUSTING

Rust: is the reddish brown oxide of iron formed by the action of moisture and oxygen on the mental surface. It consists mainly of hydrated iron (III) oxide (Fe₂O₃.H₂O) and iron (III) hydroxide (Fe(OH)₃) 
      RUSTING: Is the process in which iron turns into iron oxide    
   Rusting process ; steps are the follows
       (i) Fe →Fe²⁺ + 2e-

         ^{(ii) 2e- +½O}₂ ^+H₂^{O →2OH}

      ^{>The hydroxide ion react with  iron (Fe²+)and more dissolved oxygen to from iron (iii) oxide usually  in hydrated form .
        (III) Fe}₂^{+ + 2OH-→ Fe (OH)} ₂
              ^{2Fe (OH)} ₂ ^+1/2O₂ ^{+ XH2O Fe}₂^O₃^.XH₂^O+2H₂^O

       

This is a chemical process that occurs in iron or steel forming a red brown coating on the metal.

Conditions for rusting

There are three conditions necessary for rusting and these are;

1. Iron steel
2. Water moisture
3. Air oxygen

Methods of preventing rusting

•Painting-Is the coating of items with a special pigment point that are made of iron and usually painted to make the last long.

•Oiling-Is the coating of iron with oil .Some machine parts cannot be protected with painting so they use oil.

•Tin plating- Is the coating of iron with tin. Tin cans are actually of steel, but inside of them can be coated with a thin layer of tin to make them suitable for carrying foods.

•Galvanization- Is the coating of iron or steel with zinc.

•Use of silica gel- This is a substance in the form of grate to absorb moisture in a small bag of silica gel is put inside carrying the instrument so as to absorb at moisture  that may be there.

•Use of plastics - Parts of some machines or instruments are coated with plastic to ensure that they do not rust.


       
                                  REVIEW QUESTION

        1. Define the following
                                   
                                   a/ (i) Rusting
                                       (ii) Rust
                                       (iii) Air
                                       (iv) Combustion
                                       (v) Fire
                                       (vi) Fire fighting

                                  b/ Mention the components of air and their percentage

       2.(a) Mention classes of fire ,their burning materials and appropriate fire extinguisher.
 
          (b) What are the components needed to start fire .
       
          (c) What are the conditions necessary for ion to rust.

      3.(a) Classify the types of extinguisher according to the chemical they contain.

         (b) Explain with equation  how rusting occur .
        
         (c) Mention the methods of preventing rusting.

---

Read More

Current electricity

CURRENT ELECTRICITY

CURRENT ELECTRICITY

Current electricity is the rate at which an electricity source will make charges to flow or pass a certain point in a conductor or in an electric circuit.

This means that, when electrical devices are joined in an electric circuit, electrons flow in a continuous path. It is the flow rate of which is referred to as current electricity or electric current.

The sources of electricity are of different nature which include;

1. Generators
2. Charged capacitors
3. Dry cells
4. Dynamo
5. Solar panels

The device in which energy transformation occurs and is maintained is called electrical source.

Load - is the device used for transforming the electrical energy into any of the sensible forms of energy.for example; heat, light, mechanical or sound energy etc.

The quantity of electricity is measured in coulombs denoted by 'C'.

It follows that, if the quantity of charge of 6 coulombs is passed at a certain point of a conductor in 3 seconds, then;

Electric current = 2 coulombs/second or 2C/s

SIMPLE ELECTRIC CIRCUITS

Electric circuit - is a continuous path formed by connecting electrical devices. Such as Battery, switch, socket etc along which electrons can flow.

A simple electric circuit may consist of;

1. Cell or any source of current electricity.
2. Switch or control to allow or restrict the flow of current electricity.
3. Conductor to transmit the current electricity.
4. Load to consume the supplied power delivered by the current electricity.

The figure below shows the simple electric circuit containing a sound (cell), a control (switch) and the load (bulb);

The electric devices used in a circuit are called circuit, components or elements.

            USES OF COMPONENTS OF ELECTRIC CIRCUIT

1. Cell or battery – Is a source of electric current.
2. Switch – Is a device used to switch on an electric current in order to allow the flow of an electric current or off an electric current thus to stop the flow of the electric current respectively.
3. Resistor – Is a component included in an electric circuit because of its resistance to current electricity flow. There are variable value resistors and fixed value resistors, all are made of resistance wire or carbon.
4. Ammeter – Is an instrument with low internal resistance used for measuring electric current.
5. Connecting wire – Is a material used to provide a direct path which allows the flow of current between two points in a circuit and are used to connect circuit components.
6. Capacitor – Is an electric conductor or a system of electrical conductors which can store electric charge.
7. Voltmeter – Is an instrument with high internal resistance used for measuring potential difference of any two points in an electric circuit.
8. Socket and plugs – Are devices that connect electrical appliances to the power supply so that electric power can flow through them.

CONCEPT OF CURRENT, VOLTAGE AND RESISTANCE IN AN ELECTRIC CIRCUIT

 The electric current flows from a point with low potential to the point with high potential.

The S.I unit of electric current is Ampere denoted by ‘A’. The electric current (I)encounters resistance (R) along its path, which will result in voltage drop (V) in a circuit.

The S.I unit of resistance is Ohms denoted as Ω.

The potential difference (P.d) which causes electric current to flow is defined as “the energy per coulomb consumed when electricity moves from one point to another”.

The S.I unit of potential difference is volt (V). This voltage is given as the product of current flowing between two points and resistance offered between the two points

     I.e. Voltage = Current In amperes X Resistance in Ohms

            V = I x R         

            V = IR 

   OHM’S LAW

-It states that “the voltage across the conductor is directly proportional to the electric current flowing if temperature is constant”

The resultants shows that the resistance of a wire (P.d) is proportional to the current flowing through the conductor

I.e. Vα I

Introducing proportionality constant ‘K’

V = KI

The constant of proportionality is called the resistance ‘R’ thus,

V = IR

COMBINATION OF RESISTORS

Resistors can be connected either in series or parallel depending on the magnitude of effective resistance required. Series connection gives a bigger value of effective resistance and the parallel connection gives small value of effective resistance.

RESISTORS IN SERIES

By connecting resistors in series, when the switch 'S' is closed, the current 'I' which flows through the circuit flows through each resistor.

Total resistance between points A and B which is commonly referred to as equivalent resistance (R_eq) will produce a potential difference in the circuit given by ohm’s law as;

V=IR_eq

The voltage across each resistor in the circuit is given by V₁ =IR₁ and V₂ =IR₂

The sum of the voltage drops equal to the potential difference in the circuit (i.e. potential difference between (A and B)

Total voltage = V₁ + V₂

.^.. V_T = V₁ + V₂

Total voltage = V₁ + V₂

Since V = IR, V₁ = IR₁ and V₂ = IR₂

IR_T = IR₁ + IR₂

IR_T =I (R₁ + R₂ )

R_T = R₁ + R₂

RESISTORS IN PARALLEL

In the figure below I is the current in the main circuit. On the other hand I₁ and I₂ are current through individual resistors R₁ and R_2.

The sum of all currents through the resistors which are connected in parallel gives the value of current equal to the main circuit.
.

Therefore, I_T = I₁ + I₂

If R_T is the equivalent resistance of the main circuit between A and B, then by Ohm’s law the current is given by;

From I_T = I₁ + I₂


On diving both sides by V

Cross multiplication

1(R₁R₂) = R_T (R₁ + R₂)           

For two resistors connected in parallel.

EXAMPLES

1.Given that R₁= 4Ω and R₂= 6Ω, find the equivalent resistance when the resistors are connected.

1. In parallel

     2.   In series

Solution

1.Series

R_T = R₁+R₂

R_T = 4Ω + 6Ω

R_T = 10Ω

2.Parallel

= 2.4Ω

2.Two conductors of resistance 4Ω and 5Ω are connected in series across a 60V supply. Find;

1. The total resistance
2. The current in the circuit
3. The potential difference across each resistor

   



 R_T = R₁+R₂

         = 4Ω + 5Ω

         = 9Ω
the total resistance = 9Ω

I = 6.7A

Potential difference across R₁

V₁ = IR₁

V₁ = 6.7 x 4 = 26.8v

Potential difference across R₂

V₂ = IR₂

V₂ = 6.7 x 5 = 33.5v

Total current = 26.8 + 33.5 = 60A

3.Consider the circuit shown below. What will be the reading on the Ammeter?

Solution

V = 12V

R_T =2Ω

I = 6A

EXERCISE

1. In a circuit, the amount of charges passing through a point is 9 coulombs in 4.5 seconds. What is the electric current passing at that point?

Solution

Quantity of charges = 9 coulombs

Time = 4.5 sec

Electric current =?

Electric current = 2coulombs/sec

2. The two resistances 15Ω and 5Ω are connected in series across 20v supply, find;

1. Total resistance
2. The total current in the circuit
3. The current through each resistor

Solution

Data given

R₁ = 15Ω

R₂ = 5Ω

Voltage = 20v

The total resistance

R_T = R₁ + R₂

    = 15Ω + 5Ω

    = 20Ω

The total current in a circuit (I)

 From V = IR 

 But v = 20v, R = 20Ω

        
 I = 1A

The current through each resistor

          

But V = 20V, R₁ =15Ω

= 1.3A

   

but V = 20 V, R = 5Ω

   I₂ = 4A

---

Read More