Question

# Two 3.00 cm × 3.00 cm plates that form a parallel-plate capacitor are charged to ±...

Two 3.00 cm × 3.00 cm plates that form a parallel-plate capacitor are charged to ± 0.708 nC . What is the electric field strength inside the capacitor if the spacing between the plates is 1.20 mm ? What is potential difference across the capacitor if the spacing between the plates is 1.20 mm? What is the electric field strength inside the capacitor if the spacing between the plates is 2.40 mm ? What is the potential difference across the capacitor if the spacing between the plates is 2.40 mm ?

First you have to calculate the capacitance of a parallel plate capacitor with two plates that each have an area of 9 x 10^-4 square meters and a distance of 1.2 x 10^ -3 meters
ε = 8.854 x 10^-12 farads per meter
C = ε*A/d= (8.854 x 10^-12)(9x 10^-4)/(1.2 x 10^-3)= 6.64 x 10^-12 farads
Since C is defined as Q/V, V = Q/C = (0.708 x 10^-9)/(6.64 x 10^-12) = 106.63 volts
E = V/d = 106.63/(1.2 x 10^-3) = 88858.33 volts per meter

If the plates are separated by 2.4 mm, C = 2.2 x 10^-12 farads, V = 321.82 volts
E = V/d = 321.82/(2.2 x 10^-3) = 146281.8 volts per meter

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