My Pbl Project

Introduction

—My experiment is to see what is the cause of bits of cotton inside a small tube to ignite when you quickly compress the air inside the tube.

What causes bits of cotton inside a small tube to ignite when you quickly compress the air inside the tube?

The compression of the air when the piston is quickly rammed into the cylinder causes the interior temperature to rise sharply to 500°F .This is hot enough for the tinder on or in the piston face to ignite with a visible flash that can be seen if the cylinder is made of translucent or transparent material like cotton.

Materials

The materials that I need for the experiments are Pasco Compression igniter, loading rod, and cotton balls.

Procedure

1. Turn the removable base counterclockwise to unscrew it from the bottom of the igniter. remove the base and set it aside.

2. Insert a small piece (-1cm²) of cotton fiber into the bottom of the glass tube.

3. Use the loading rob to push the material up into the lower part of the glass tube.Carefully remove the loading rob without pulling pulling the material out.

4. Insert the piston into the top of the igniter and push it in far enough that the end of the piston lines up with or is slightly below the "piston starting position" label on the outer tube 2.5 cm below the top.

5. Replace the removable base by screwing it clockwise into the bottom.

6. Place the base of the igniter on a sturdy horizontal surface and securely hold the tube with one hand.Make sure that the combustible material in the bottom of the glass tube is visible.

7. Watch the piece of combustible material at the bottom of the glass cylinder.

   1. With the palm of your other hand, quickly slap down on the knob of the piston so the end of the piston goes down into the cylinder rapidly.

Scientific Principle

The compression of air inside a hollow cylinder is an adiabatic process since the compression is so fast that there is no time for thermal energy to leave or enter the cylinder. For an adiabatic compression of gas, where P is the pressure of the gas, V is the volume of the gas is the ratio of specific heats of the gas (specific heat at constant pressure divided by specific heat at constant volume) Therefore, when the gas in the cylinder is at its original volume V0 and pressure P0 and then is compressed to a small volume V and pressure P, With the compression igniter, it is possible to compress the gas in the cylinder by a factor of about 15, so V0 = 15V. With this assumption, the final pressure at maximum compression can be calculated in terms of the initial (atmospheric) pressure:The quantity that is of interest is the final temperature: Is it high enough to ignite paper? To arrive at the temperature, assume that the gas is air and that it is an ideal gas. For an ideal gas,

The ignition temperature of paper is about 451 °F. Fahrenheit 451.

P₀V₀ = nRT₀ and

PV = nRT

where n is the number of moles of gas, R is the gas constant, and T is the absolute (thermodynamic) temperature. Solving the initial equation for nR and substituting the

expression into the final equation gives:Solving for the final temperature, T, and substituting the final pressure, P, derived from the adiabatic equation and the final volume, V, which is 1/15 the initial volume

gives:Simplifying this expression gives T = 15^{(y - 1)}T₀. For air, which is mainly diatomic, y=1.4, so this gives T = 15^0.4T₀. Assuming that the initial temperature is about 24 °C

(297 K), then T = 150.4(297 K) = 877 K. Converting to celsius, T = 604 °C. Since 604 °C = 1120 °F, the temperature is high enough to ignite paper.