A red blood cell typically carries an excess charge of about 2.5 Ý 1012 C distri

Question

A red blood cell typically carries an excess charge of about 2.5 Ý 1012 C distributed uniformly over its surface. The red blood cells can be modeled as spheres approximately 7.0 ?m in diameter and with a mass of 9.0 Ý 1014 kg. How many excess electrons does a typical red blood cell have?

Does the mass of the extra electrons appreciably affect the mass of the cell? To find out, calculate the ratio of the mass of the extra electrons to the mass of the cell without the excess charge.

What is the surface charge density on the red blood cell. Express your answer in units of C/m2 ()

Explanation / Answer

excess electrons in a typical Red blood cell = Charge of Red blood cell/ Charge of an electron = (2.5*10^-12)/(1.6*10^-19) = 1.5625*10^7 electrons per Red blood cell

Mass of extra electrons = Number of electrons * MAss of 1 electron = 1.5625*10^ 7 * 9.1*10^-31 = 14.2328125*10^-24 Kg

Mass of Red blood cell = 9*10^-14 Kg

Mass of cell without charge = (9*10^-14) - ( 14.2328125*10^-24) = 9*10^-14 Kg

Ratio of the mass of the extra electrons to the mass of the cell without the excess charge. = (14.2328125*10^-24) / 9*10^-14

Ratio = 1.581 * 10^-10

as the ratio is very less (negligible)

No, mass of the extra electrons does not affect appreciably the mass of the cell

Surface charge density = Charge/surface area

surface area of cell = pi*d^2

d is diameter

surface area of cell = 3.14*(7*10^-6) ^2 = 153.86*10^-12 m^2

Surface charge density = (2.5*10^-12)/(153.86*10^-12) = 0.0162485 C/m^@

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