To find out how many electrons it takes to equal the mass of a dust particle, you can divide the mass of the dust particle by the mass of an electron.
The mass of the dust particle is \( 7.5 \times 10^{-10} \) kg, and the mass of an electron is \( 9.1 \times 10^{-31} \) kg.
The formula is:
\[ \text{Number of electrons} = \frac{\text{mass of dust particle}}{\text{mass of electron}} = \frac{7.5 \times 10^{-10}}{9.1 \times 10^{-31}} \]
Now, calculate:
\[ \text{Number of electrons} = \frac{7.5}{9.1} \times \frac{10^{-10}}{10^{-31}} = \frac{7.5}{9.1} \times 10^{21} \]
Calculating \( \frac{7.5}{9.1} \):
\[ \frac{7.5}{9.1} \approx 0.823 \]
Now multiply by \( 10^{21} \):
\[ 0.823 \times 10^{21} \approx 8.23 \times 10^{20} \]
Rounding this to two significant figures gives:
\[ \approx 8.2 \times 10^{20} \]
So, it takes approximately \( 8.2 \times 10^{20} \) electrons to equal the mass of a dust particle.
The correct answer is:
8.2×10^20.
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