units and measurement

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20 Terms

1
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physical quantites

matter that can be measured

it is expressed with a number with a unit

it is of two types; fundamental and derived

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fundamental quantities

also called base quantities. they are independent of each other

length - metre

mass - kilogram

time - second

electric current - ampere

thermodynamic temperature - kelvin

amount of substance - mole

luminous intensity - candela

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derived quantities

made by multipl. and divis of fundamental quantities

eg: speed = length/time

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radian and steradian

they are two other fundamental units

radian = plane angle = length of arc/ radius = ds/r

steradian = solid angle

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prefixes of units

very large quantities can be expressed w a prefix before the unit signifying a power of 10

kilo = 10Âł

hecto = 10²

deka = 10^1

deci = 10^-1

centi = 10^-2

milli = 10^-3

micro = 10^-6

nano = 10^-9

pico = 10^-12

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dimensions

an expression of a derived quantity by showing the powers of each fundamental quantity

eg: speed = L/T = L^1T^-1 [this is called dimensional formula]

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homogeneity of dimensions in an equation

dimensions of every term in an equation is the same

different dimensions cannot be added & subtracted

no.s are dimensionless

eg: x = ut + 1/2at² => L = L + L

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conversion of units

Q (quantity) = n1u1 = n2u2

=> n2 = [M1/M2]^a [L1/L2]^b [T1/T2]^c

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finding equations using dimensions

Q = kQ1^a Q2^b Q3^c [k = dimensionless constant]

eg: time period of pendulum depends on length, mass, g

t = k l^a m^b g^c

T = k[L]^a[M]^b[LT^-2]^c

T = k[L]^a+c [M]^b [T]^-2c

a = 0 ; c = -1/2 ; b = ½

t = k square root l/g

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limitations of finding equations using dimensions

the expression can only be product type (no + or -)

k is unknown

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significant figures

they are all the certain digits & one uncertain digit (least count)

eg: 10.46cm has 4 SFs and 6 is uncertain

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least count

it is the smallest subdivision of a scale & is uncertain when measuring

eg: mm is smallest subdivision of a cm scale (0.1 cm)

least count of screw gauge = pitch/ no. of divisions

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order of magnitude

it is the expression of big numbers in the form N x 10^n where N is less than 10

eg: 95200000 = 9.52 Ă— 10^5

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rules to determine no. of SFs

1- all non-zero digits are SF [1234 = 4 SFs]

2- all zeros btw two non-zero digits are SF [10.07, 1007 = 4SFs]

3- in whole no.s, trailing zeros are NOT SF [3210 = 3 SFs]

4- if no. is less than 1, all zeros b4 the 1st non-zero digit are NOT SF [0.005704 = 4 SFs]

5- in decimals, trailing zeros are SF [3.500 = 4 SFs]

6- factors are exact & have infinite SF [r = d/2 => 2 = infinite SF]

7- no. of SF does not depend on the unit [164cm, 0.164m, 0.00164km = 3 SFs]

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precision and accuracy

precision = closeness of 2 measurements to each other

accuracy = closeness of a measurement to the actual value

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rounding off SFs

7.82 = 7.8

6.87 = 6.9

16.351 = 16.4

3.250 = 3.2

3.750 = 3.8

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addition and subtraction of SFs

the result should have the same SF as the term with the least decimal places

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multiplication and division of SFs

the result should have the same SFs as the term with the least SFs

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relative error

eg: R.E of 9.24kg

m = 9.24 +- 0.01

RE = (0.01/9.24) x 100 = +- 0.1%

RE = (least count/meaurement) x 100

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arial and linear magnification

arial magnification = area of image/ area of object

linear magnification = sq. root of arial magnification