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Consider a circular disc of radius $$a$$ whose surface density of charge at any point $$(r,\theta$$) is $$\sigma(r,\theta)=\sigma_0r^2sin^2\theta$$ Find the total charge on the disc.

Solution:

Consider a surface element $$rdrd\theta$$ on the disc about the point $$(r,\theta$$). The charge on is $$Q= \sigma(r,\theta)rd\theta dr$$ $$=\sigma_0\int_{0}^{a} r^3dr \int_{0}^{2\pi}sin^2\theta d\theta$$ $$=\sigma_0\frac{a^4}{4}.\pi$$$$=\pi \sigma_0\frac{a^4}{4}$$