Physical Quantities And Units

Question 1

What is a physical quantity ?

Answer 1

A physical quantity is a feature that can be measured and consists of a numerical magnitude AND a unit.

NOTE:

1. Physical quantities are defined In terms of physical quantities only.
2. Units are defined in terms of units only.

Question 2

What is a basic quantity

Answer 2

A basic quantity is a quantity that can be measured with an instrument.

Question 3

What is a base unit?

Answer 3

Base units are units of physical quantities. They are not derived from any other units( they are independent of other units)

Question 4

What are the 7 basic physical quantities and their respective base units?

Answer 4

1. Length: metre m
2. Time : second s
3. Mass : kilogram kg
4. Temperature: Kelvin K
5. Electric current: Ampere A
6. Amount of substance: mole mol
7. Light intensity: Candela cd

Question 5

What are derived quantities and units?

Answer 5

•Derived quantities are physical quantities other than basic quantities

Obtained when:

1. Base quantities are multiplied together or divided by one another.
2. Never when base units are added or subtracted.

•Derived units consist of a combination of base units.

Question 6

State the fundamental derived base units.

Answer 6

1. Speed= distance/ time > m/s
2. Force= ma > kg/ m/s^2
3. Frequency= 1/time > s^-1
4. Work done = fd > Nm = kg m^2 s^-2
5. Power= fd/t > kg m^2 s^-3
6. G.p.e= mgh
7. Charge = current x time
8. P.d = work done/ charge
9. Resistance = V/ I
10. Momentum= ft > kg m s^-1

Question 7

How are terms identified in an equation?

Answer 7

They are separated by ‘+’ or ‘-‘ or ‘=‘

Question 8

What is a homogenous equation?

Answer 8

An equation in which each term has the SAME BASE UNITS.

• For an equation to be physically correct, it must be homogenous.

Question 9

Why are prefixes used?

Answer 9

To express very large or small quantities.

Question 10

State the prefixes

Answer 10

1. Tera T: 12
2. Giga G: 9
3. Mega M: 6
4. Kilo k: 3
5. Deci d: -1
6. Centi c: -2
7. Milli m: -3
8. Micro u: -6
9. Nano n: -9
10. Pico p: -12

Question 11

Check approximations pic

Answer 11

.

Question 12

What is the uncertainty of a value?

Answer 12

The uncertainty is an estimate of the difference between a measurement reading and the true value.

Question 13

What are the two types of errors?

Answer 13

Random and systematic errors are two types of measurement errors which lead to uncertainty.

Question 14

What are random errors and it’s effects?

Answer 14

• Random errors cause unpredictable fluctuations in an instrument’s readings as a result of uncontrollable factors, such as environmental conditions.
• varies in MAGNITUDE and DIRECTION

Example: parallax error

Effects:
1. This affects the PRECISION of the measurements taken, causing a wider spread of results about the mean value- causes readings to scatter around the true value.

Question 15

How can random errors be reduced?

Answer 15

1. To reduce random error: repeat measurements several times and calculate an average from them
2. Drawing a line of best fit

Question 16

What are systematic errors and their effects?

Answer 16

• Systematic errors arise from the use of faulty instruments/wrongly calibrated used or from flaws in the experimental method.
• chase readings to deviate in one direction only. (Constant sign)

Effects:
•This type of error is repeated every time the instrument is used or the method is followed, which affects the ACCURACY of all readings obtained.

Question 17

What are the causes of systematic errors and how can they be reduced?

Answer 17

1. Using experiments with zero error.
2. Instrument with wrongly calibrated scale.
3. Reaction time

To reduce systematic errors: instruments should be recalibrated or the technique being used should be corrected or adjusted

A line of best fit can be drawn. If the line does not cut the expected y-intercept, the shift is probably due to a sys error.

Question 18

What is zero error?

Answer 18

• This is a type of systematic error which occurs when an instrument gives a reading when the true reading is zero
• This introduces a fixed error into readings which must be accounted for when the results are recorded

Question 19

What is the precision of an instrument? (Check pic)

Answer 19

• Precision of a measurement: this is how close the measured values are to each other; if a measurement is repeated several times, then they can be described as precise when the values are very similar to, or the same as, each other
• refers to the smallest division of the measuring instrument.
• affected by random error
• The precision of a measurement is reflected in the values recorded – measurements to a greater number of decimal places are said to be more precise than those to a whole number

Question 20

What is the accuracy of an experiment?

Answer 20

• Accuracy: this is how close a measured value is to the true value; the accuracy can be increased by repeating measurements and finding a mean average.
• Affected by systematic errors.

Question 21

Recall practical skills.

Answer 21

Raw data: recorded to the same number of dp as precision of instrument

Calculated data: recorded to the same number of SF as raw data used to calculate it(

Note: all times should be between 20-25s (control oscillations)

Question 22

What are uncertainties?

Answer 22

•There is always a degree of uncertainty when measurements are taken; the uncertainty can be thought of as the difference between the ACTUAL reading taken (caused by the equipment or techniques used) and the TRUE value.

Question 23

How are uncertainties different from errors?

Answer 23

• Errors can be thought of as issues with equipment or methodology that cause a reading to be different from the true value
• The uncertainty is a range of values around a measurement within which the true value is expected to lie, and is an ESTIMATE.

For example, if the true value of the mass of a box is 950 g, but a systematic error with a balance gives an actual reading of 952 g, the uncertainty is ±2 g

Question 24

What are the types of uncertainties?

Answer 24

These uncertainties can be represented in a number of ways:

1. Absolute Uncertainty: where uncertainty is given as a fixed quantity
2. Fractional Uncertainty: where uncertainty is given as a fraction of the measurement
3. Percentage Uncertainty: where uncertainty is given as a percentage of the measurement

Question 25

How to find the uncertainty in different situations? (Check pic)

Answer 25

To find uncertainties in different situations:
1. The uncertainty in a reading: ± half the smallest division

2. The uncertainty in a measurement: at least ±1 smallest division
3. The uncertainty in repeated data: half the range i.e. ± ½ (largest – smallest value)
4. The uncertainty in digital readings: ± the last significant digit unless otherwise quoted

Question 26

What are the rules to follow when calculating uncertainties? (Check pic)

Answer 26

1. Adding / subtracting data – add the absolute uncertainties
2. Multiplying / dividing data – add the percentage uncertainties
3. Raising to a power – multiply the uncertainty by the power

Note:

1. Constants are ignored
2. Uncertainties are always quoted to 1sf only

Question 27

What is the resolution of an instrument?

Answer 27

• The resolution is the smallest change in the physical quantity being measured that results in a change in the reading given by the measuring instrument
• The smaller the change that can be measured by the instrument, the greater the degree of resolution

•For example, a standard mercury thermometer has a resolution of 1°C whereas a typical digital thermometer will have a resolution of 0.1°C
The digital thermometer has a higher resolution than the mercury thermometer