Unit+2

= Measurement - Unit 2 =

Do you realize how often we measure things in every day life? When we buy foods like butter, potatoes, and flour, we are measuring weight. When we buy some cloth or determine how far we have traveled, we are measuring length. When we record the number of hours we have worked, tell our age, or keep an appointment, we are measuring time. Every measurement has a unit and a number.toc

We measure a dimension or a property of an object by comparing it with something we have agreed to accept as standard unit. The National Bureau of Standards in Washington D.C. is a special agency maintain by our government to prepare and maintain standards for all scientific and commercial measurements. The National Bureau of Standards was established by Congress in 1901. Its name was changed to the [|National Institute of Standards and Technology]in 1988 as part of the Omnibus Trade and Competitiveness Act. It does not matter what we select as standard units as long as we agree upon them. Each standard unit should have two important characteristics. It should remain constant so that it will give very nearly the same result whenever we use it to repeat the same measurement. It should be readily duplicated so that it can be available to all who need to use it.

Throughout history, many different systems of measurements have developed in different countries. This led to confusion not only in international trade but also in the exchange of scientific knowledge among countries with different measuring systems. There was clear need for a single measuring system that would be used the world over.

A great step forward in this direction was taken in 1791 by a commission of French scientist who created the metric system of measurement. Designed so that it is simple, precise, and practical, the metric system was long ago adopted by the scientific world.

=**Brief History**= The first standardized system of measurement, based on the decimal was proposed in France about 1670. However, it was not until 1791 that such a system was developed. The International System is called the SI, using the initials of its French name System International.

SI - The International System of Units

The International System of Units (SI) is a modernized version of the metric system established by international agreement. The metric system of measurement was developed during the French Revolution and was first promoted in the U.S. by Thomas Jefferson. Its use was legalized in the U.S. in 1866. In 1902, proposed congressional legislation requiring the U.S. Government to use the metric system exclusively was defeated by a single vote.

SI provides a logical and interconnected framework for all measurements in science, industry, and commerce. The metric system is much simpler to use than the existing English system since all its units of measurement are divisible by 10. The SI is maintained by a small agency in Paris, the International Bureau of Weights and Measures (BIPM), and it is updated every few years by an international conference, the General Conference on Weights and Measures (CGPM), attended by representatives of all the industrial countries and international scientific and engineering organizations.

It was called the "metric" system, based on the French word for measure. The driving force was the growing importance of weights in the sciences, especially chemistry. At that time, every country had their own system of weights and measures. England had three different systems just within its own borders!!

For the first 50 years of its existence the metric system remained undeniably French; it was adopted only in France and a few other countries bordering on France or linked to France in some other way. Around 1850, though, a strong movement began among scientists, engineers, and businessmen in favor of a international system of weights and measures. The scientific and technical revolution was well underway and a global economy was developing. The need for uniformity in measurement was becoming obvious. Furthermore, the metric system was the only real choice available. The only possible competitor, the British Imperial system, was so closely tied to the British Empire it was not even acceptable to the Americans, let alone to non-English speakers.

Between 1850 and 1900 the metric system made rapid progress. It was adopted throughout Europe (except in Britain), in Latin America, and in many countries elsewhere. It became firmly established as a key part of the language of science. Furthermore, the French made a key decision to turn control of the system over to an international body. In 1875, most of the leading industrialized countries (including the United State, but not Britain) signed the Treaty of the Meter.

The treaty established the International Bureau of Weights and Measures, which has presided ever since over what we now call the International System of Weights and Measures. It also provided for distribution of copies of the metric standards throughout the world and set up a framework for continuing consultation and periodic revision and improvement of the system.

Since 1875 the eventual triumph of the metric system in science and international commerce has been assured, despite continuing popular opposition in Britain and the United States. In fact, the metric system has met popular opposition in every country around the time of its adoption. People don't want to change their customary units, which are part of how they see and control the world. It is naturally disturbing to do so. This opposition has been overcome everywhere, except in the U.S., by economic necessity: the need to participate fully in the global economic system. Even in the U.S., economic needs assure the continued creeping adoption of the system in one area and then another.

Those Americans opposing adoption of metric units often argue that the metric system is abstract and intellectual. This is not true. The metric system has been the customary measurement system in France for two centuries, in the rest of continental Europe for at least one century, and in the rest of the world for a least a generation or two. Most people in the world know exactly how long a kilometer is, how large is liter is, and how much a kilogram weighs, because they use these units every day of their lives in the same way Americans use miles, gallons, and pounds. There are three major parts to the metric system: the seven base units, the prefixes and units built up from the base units. Here is a list of the base units which make up the metric system:
 * **Physical Quantity** || **Name of SI unit** || **Symbol for SI unit** || **Tool Used** ||
 * length || meter || m || ruler ||
 * mass || gram || g || balance ||
 * time || second || s || clock ||
 * temperature || Celsius || C || thermometer ||
 * volume || liter || L || graduated cylinder ||

=Prefixes= Prefixes were also agreed on in 1791. The set from kilo- down to milli- was developed then. For the multipliers (prefixes greater than 10), Greek was used and for the fractions (prefixes less than 1), Latin was used.

In 1958, the International Committee on [|Weights and Measures] added Mega-, Giga-, and Tera- to the multiplies and micro-, nano-, and pico- to the fractions. In 1960, at the 11th General Conference on Weights and Measures, everything was officially adopted.

Since that time, additional prefixes have been added as the need arose. Typically, as scientific instruments get better and better, smaller and smaller quantities can be detected. So, new fractional prefixes need to be added. When they are, new multipliers are added also, to keep the system symmetrical. In SI, prefixes are used to make the base units larger or smaller by multiples of ten. Instead of memorizing many different unit, all you have to remember are the meanings of the main prefixes.
 * = **Important SI Prefixes** ||
 * = **Prefix** ||= **Symbol** ||= **Multiplying factor** ||
 * = kilo ||= k ||= 1000 ||
 * = deci ||= d ||= 0.1 ||
 * = centi ||= c ||= 0.01 ||
 * = milli ||= m ||= 0.001 ||

=Conversions= The use of the SI system makes it easy to change from one unit to another. Once you learn the metric prefixes, changing units involves only moving the decimal point. Using SI is as simple as the US money system. The metric system is a decimal-based system of measurement units. Like our monetary system, units for a given quantity (e.g. length or volume) are related by factors of 10. Calculations involve the simple process of moving the decimal point to the right or to the left.

//**SI Major Prefixes**// SI works by combining prefixes with base units. Each base unit can be used with different prefixes to define smaller or larger quantities. Each prefix stands for a number that the base unit is multiplied. To change from one unit to another, you multiply or divide by multiples of ten. A quick way to do this is to move the decimal point to the right or left. Move the decimal point as many places as there are zeros in the multiple of ten you are using. Going larger --- move decimal to the left Going smaller --- move decimal to the right To help you remember the order of the prefixes, you can use the mnemonic phrase "**K**ing **H**enry **D**oesn't **B**elieve You **D**rink **C**hocolate **M**ilk."
 * **Kilo (k)** || **Hecto (h)** || **Deka (da)** || **Base Unit** || **Deci (d)** || **Centi (c)** || **Milli (m)** ||
 * 1000 || 100 || 10 || 1 || Dime || Penny || Coupon ||
 * 1000 || 100 || 10 || 1 || .1 || .01 || .001 ||

=Uncertainty in Measurement= There is no such thing as a perfect measurement. Each measurement contains a degree of uncertainty due to the limits of instruments and the people using them. In laboratory exercises, students are expected to follow the same procedure that scientists follow when they make measurements. Each measurement should be reported with some digits that are certain plus one digit with a value that has been estimated. For example, if a student is reading the level of water in a graduated cylinder that has lines to mark each milliliter of water, then he or she should report the volume of the water to the tenth place (i.e. 18.5 ml.) This would show that the 18 mLs are certain and the student estimated the final digit because the water level was about half way between the 18 and 19 mark.

Two concepts that have to do with measurements are accuracy and precision. The **accuracy** of the measurement refers to how close the measured value is to the true or accepted value. For example, if you used a balance to find the mass of a known standard 100.00 g mass, and you got a reading of 78.55 g, your measurement would not be very accurate. One important distinction between accuracy and precision is that accuracy can be determined by only one measurement, while precision can only be determined with multiple measurements.


 * Precision** refers to how close together a group of measurements actually are to each other. Precision has nothing to do with the true or accepted value of a measurement, so it is quite possible to be very precise and totally inaccurate. In many cases, when precision is high and accuracy is low, the fault can lie with the instrument. If a balance or a thermometer is not working correctly, they might consistently give inaccurate answers, resulting in high precision and low accuracy.

A dartboard analogy is often used to help students understand the difference between accuracy and precision. Imagine a person throwing darts, trying to hit the bull's-eye. The closer the dart hits to the bull's-eye, the more accurate his or her tosses are. If the person misses the dartboard with every throw, but all of their shots land close together, they can still be very precise.

You must strive for both accuracy and precision in all of your laboratory activities this year. Make sure that you understand the workings of each instrument, take each measurement carefully, and recheck to make sure that you have precision. Without accurate and precise measurement your calculations, even if done correctly, are quite useless.

=Measurement Tools=

**Rulers**
Metric rulers are fairly easy to read. They deal with centimeters and millimeters only. You wont have to worry much about fractions. Take a look at the following Metric Ruler. The larger lines with numbers are centimeters, and the smallest lines are millimeters. Since millimeters are 1/10th of a centimeter, if you measure 7 marks after a centimeter, it is 1.7 centimeters long. If it is directly on the seventh line the object is 1.70 cm long.

**Balances**
Mass is defined as how much matter there is in an object. Mass is measured in the metric unit grams. We often use a triple-beam or equal arm balance to measure mass. A triple-beam balance gets its name because it has three beams that allow you to move known masses along the beam. An equal arm balance has two pans.

**How to Use a Balance:**
The large riders must always sit in a notch. The single gram sliding rider, does not have notches and can sit anywhere. To correctly use a balance: When massing crystals or powders, use a piece of filter paper. First, mass the paper; then add the crystals or powders and re-mass. The actual mass is the total minus the mass of the paper. When massing liquids, first mass the empty container, then mass the liquid and the container. Finally, subtract the mass of the container from the mass of the liquid and container to get the mass of the liquid. To get the balance balanced, the pointer should swing an equal distance above and below the zero point. The pointer does not need to come to a complete stop, it only has to swing as far to the right as it does to the left. If you do not do this, your measurements will be completely off.
 * 1) Make sure the balance is on a level surface. Place all the riders at zero. The pointer should be at zero. Use the adjustment screw found under the pan to make any necessary adjustments.
 * 2) Place the object you wish to mass on the pan. CAUTION: Never place hot objects or chemicals directly on the balance pan, because they can damage the surface.
 * 3) Move the largest rider along the beam to the right until the pointer dips below zero. Then move the rider back one notch. Continue this procedure with the remaining two riders, moving from the back to the front.
 * 4) Determine the readings on all beams and add them together to determine the mass of the object.
 * How to Zero out the Balance**

There are three riders on the Triple Beam Balance; the last sliding one which measures how many hundred/fifty (50...100...150...)grams, the middle sliding one which measures how many ten grams (10...20...30...), and the front one which measures how many single grams (1...2...3...).
 * Triple Beam Balance**

There are two riders on the equal arm balance; the bottom sliding one which measures how many ten grams (10...20...30...), and the front one which measures how many single grams (1...2...3...).
 * Equal Arm Balance**

**Precision in Your Measurement**
You must always estimate the last digit of your measurement. Therefore, there will always be two decimal places in your answer when measuring with our lab balances.

For example: In the single gram sliding rider, you must judge if the rider is exactly on the number or in between two numbers. For instance, if both the ten and hundred gram weights are in their zero notches, and the single gram is between 4 and 5, between the 3 tenths and 4 tenths mark, your answer would be 4.35. If it was exactly on the 3 tenths mark, it would be 4.30.

Graduated Cylinders
Volume is defined as the amount of space that an object occupies. Volume is measured in the metric unit liters (L) for liquids and cm3 for solids. The scientific tools used to measure volume is the graduated cylinder for liquids and a ruler for solids. 1 mL = 1 cm3. A solid object is considered to be either regular shaped or irregular shaped. A regular shaped object is either a square or a rectangle. To find its volume use a ruler to find the measurements of the objects length, height, and width. Calculate the volume using the formula V= L x W x H. Use the water displacement method to find the volume of an irregular (not rectangular) shaped object. When trying to measure the volume of an irregularly shaped object, the "water displacement method" is the most commonly used technique. The procedure for the water displacement method is listed below. 1) Find a graduated cylinder that will be large enough to fit the object being measured. 2) Fill this graduated cylinder enough so that when placed in the graduated cylinder, the object in question will be fully submerged in the water. Also be careful not to put in so much water that the water level will rise past the graduated cylinder's markings when the object are placed in the graduated cylinder. You must use your own judgment to determine what amount of water meets these requirements. 3) After filling the graduated cylinder to a satisfactory level, record the volume of the water as (a) in your data. Make sure to read the meniscus when determining volume. 4) After the water's volume has been recorded, carefully place the object in the graduated cylinder and record this volume as (b) in your data. 5) In order to calculate the volume of the irregularly shaped object, subtract the volume of the water alone from the volume of the water and object [(b) - (a)].
 * To read a graduated cylinder:**
 * 1) Examine the graduate and note how the scale is marked. the units are usually in mL. A milliliter is equal to a cubic centimeter. Note carefully how many milliliters are represented by each scale on the graduate.
 * 2) Pour some liquid into the cylinder and set the cylinder on a level surface. Notice that the upper surface of the liquid is flat in the center and curved at the edges. This curve is called the **meniscus** and may be either upward or downward. In reading the volume, you must ignore the curvature and read the scale at the flat part of the surface.
 * 3) Bring your eye to the level of the surface and read the scale at the meniscus.
 * To find the volume of a solid:**

Thermometers
Temperature is measured using two different scales: Celsius and Fahrenheit. Almost all the countries of the world use the Celsius scale. The United States is one of the only countries that use the Fahrenheit scale. Thermometers are commonly made from a glass bulb connected to a tube of glass with a numbered scale written on the outside. Inside the glass tube is a liquid like mercury or colored alcohol that rises and falls in the tube as the temperature around it warms or cools. When the temperature rises, the liquid in the glass tube warms up and molecules expand, which in turn takes up more space in the tube.

=Density= Some objects tend to be heavy, while other objects seem light. But unless you are comparing the same volume of each object, these descriptions have little value. And that is where the concept of density comes in. for density refers to how much mass an object has in a particular volume. Scientifically, density is described as mass per unit volume, or //density = mass/volume.//

Because mass is measured in grams (g), and volume is measured in cubic centimeters (cm3) for solids and milliliters (mL) for liquids, the unit for density is grams per cubic centimeter (g/cm3) or grams per milliliter (g/mL). Density is a property of a substance that is often helpful in identifying the substance. This is because the density of a substance remains the same at a given temperature and pressure. [ [|Back to the Top] ]