Q.Colour code of carbon resistance for blue colour is (A) 3
(B) 4
(C) 5
(D) 6

Yellow = 4 in the resistor colour code.

The standard colour code for carbon resistors assigns a digit to each colour (mnemonic: B B ROY of Great Britain had a Very Good Wife):

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In the resistor colour code, the fourth (last) band gives the tolerance; a Gold band means +/-5% tolerance.

Reading the four bands left to right: Yellow (1st significant digit = 4), Violet (2nd significant digit = 7), Brown (multiplier = x10^1), Gold (tolerance band). So the resistor's value is $47\times10 = 470\ \Omega$.

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Decoding $47\ \text{k}\Omega \pm 10\%$ digit-by-digit using the standard resistor colour code gives Yellow–Violet–Orange–Silver.

Working

Colour code reference: Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9; tolerance bands: Gold=$\pm5\%$, Silver=$\pm10\%$.

Given resistance $= (47\pm4.7)\ \text{k}\Omega$. Tolerance $= \dfrac{4.7}{47}\times100 = 10\%$.

• First significant digit "4" → Yellow
• Second significant digit "7" → Violet …

In the colour code the sequence is B-B-ROY-G-B-V-G-W = 0-9; Blue = 6.

The standard resistor colour code (mnemonic "B B ROY of Great Britain has a Very Good Wife") assigns:

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Writing $2.4\times10^6\,\Omega\pm5\%$ as (1st digit)(2nd digit) $\times$ (multiplier), tolerance, and matching each to the standard resistor colour code gives Red-Yellow-Green-Gold.

The colour code

Each colour on a carbon resistor stands for a digit (and, on the third band, a power-of-ten multiplier):

Tolerance bands: Gold $=\pm5\%$, Silver $=\pm10\%$.

Reading off $2.4\times10^6\,\Omega$

Write the resistance using two significant digits times a power of ten:

$2.4\times10^6\,\Omega = 24\times10^5\,\Omega$ …

Green = 5 in the carbon-resistor colour code.

The colour code for carbon resistors assigns a digit to each colour (mnemonic B B ROY of Great Britain): Black 0, Brown 1, Red 2, Orange 3, Yellow 4, Green 5, …

Yellow = 4 in the resistor colour code.

The carbon-resistor colour code assigns a digit to each colour (mnemonic B B ROY of Great Britain...): …

Decoding the 4-band colour code Brown-Black-Brown-Red gives a resistance of $100\,\Omega \pm 2\%$.

For a 4-band carbon resistor, the first two bands give the first two significant digits, the third band gives the power-of-ten multiplier, and the fourth band gives the tolerance. Using the standard colour code (Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9; tolerance: Brown=$\pm1\%$, Red=$\pm2\%$, Gold=$\pm5\%$, Silver=$\pm10\%$):

• 1st band Brown $\to$ digit $1$
• 2nd band Black $\to$ digit $0$ …