Presentation on theme: "Measuring, Precision vs. Accuracy, and Intro to Significant Figures"— Presentation transcript:
1 Measuring, Precision vs. Accuracy, and Intro to Significant Figures
2 What do we measure?Volume – the amount of space taken up by a substanceMass – the amount of matter in an objectTemperature – measure of how much thermal energy a substance has compared to a reference pointLength – straight line distance between 2 points
4 Units of MeasurementYou cannot just record a number for a measurement. A unit must also be indicated.For example, a measurement is taken for the length of the laboratory table and recorded as 183 is not correct. A correct recording would be 183 centimeters (cm).All measurements should be made in metric units, or converted to metric.
5 Measuring Liquids – Volume! When you measure the volume of a liquid it is important to make sure that you use the appropriate kind and size of instrument.When you put a sample of a liquid into one of these instruments you will notice a curve at the surface of the liquid. This curve, which is called a meniscus.Adjust your body and head so that you are eye level with the instrument and the meniscus. Read the volume of the liquid at the bottom of the meniscus.
6 The triple beam balance Measuring MassThe triple beam balanceCarrying the BalanceBe sure all movable masses are back to the zero point.Place one hand under the balance and the other hand on the support (arm) to carry the balance.Using the BalanceZero the balance before you determine the mass of any substance. a) Slide all of the movablemasses back to the zeropoint. b) Check to see that thepointer swings freely alongthe scale.
7 Mass cont. Never put a hot object directly on the balance pan. Any dry chemical that is to be massed should be placed on paper or in a container.Never pour chemicals directly on the balance pan.Remember to mass the paper or container before adding the substance.Once you have placed the object to be massed on the pan, move the riders along the beams beginning with the largest mass first. If the beams are notched, make sure all riders are in a notch before you take a reading.
10 Measuring Temperature We will use degrees Celsius in this class!Never stir with a thermometer.Never read a temperature with a thermometer leaning against a side of a beaker.NO WAY! ->
11 Measuring LengthMeter sticks are carefully calibrated in centimeters with a millimeter least count. That is, the millimeter is the smallest subdivision on the meter stick.This means the millimeter is the unit of the smallest reading that can be made without estimating.
12 Figure 2. An example of a meter stick reading Figure 2. An example of a meter stick reading. The object's length is measured to be cm. (See text for description.)Figure 1. This meter stick is calibrated in centimeters (shown as the numbered major divisions) with a least count of millimeters.
13 Reading instruments to significant figures The least count of this meter stick is one millimeter, so we know with absolute certainty that the object is between 41.6 cm and 41.7 cm.We then estimate the object's length to the fractional part (doubtful figure) of the least count subdivision… cm.
16 Sig FigsSignificant digits or significant figures are digits read from the measuring instrument plus one doubtful digit estimated by the observer.Examples:203.4 cm: least count of ruler was 1 cm, tenth of centimeter was estimated.4.07 cm: least count was 0.1 cm. hundredth of centimeter was estimated.
17 Rules for Sig Figs Digits from 1-9 are always significant. Zeros between non-zeros are always significantZeros at the beginning of a number are never significant, they merely indicate the position of the decimal pointZeros that fall both at the end of the number AND after the decimal point are always significantWhen a number ends in zeros but contains NO decimal point the zeros may or may not be significant….if this is the case, we use scientific notation!
18 Examples EXAMPLES # OF SIG. DIG. COMMENT 453 kg 3 All non-zero digits are always significant.2004 L4Zeros between 2 sig. dig. are significant.5.00Additional zeros to the right of decimal and a sig. dig. are significant.0.0071Placeholders are not
19 Practice problemsWhat is the number of sig figs in each of the following:1282 kgs8.070 mmLx 10-4 m
20 Accuracy and Precision Accuracy-the closeness of a measurement is to the true value of what is measuredPrecision- a gauge of how exact a measurement isWhich is more precise…a glug ruler or a 0.01 glug ruler? Why?With which ruler can you report more significant figures?