A variable dielectric capacitive displacement sensor consists of two square metal plates of 5 cm in each side, separated by a 1 mm gap. A 1-mm-thick sheet of a dielectric material can also slide between the two plates (same area as the plates). Given that the dielectric constant (i.e. relative permittivity) of air is 1 and that of the dielectric material is 4, for each case, calculate the capacitance of the sensor for the input displacements of x = 0.0, 2.5 and 5.0 cm
Question
A variable dielectric capacitive displacement sensor consists of two square metal plates of 5 cm in each side, separated by a 1 mm gap. A 1-mm-thick sheet of a dielectric material can also slide between the two plates (same area as the plates). Given that the dielectric constant (i.e. relative permittivity) of air is 1 and that of the dielectric material is 4, for each case, calculate the capacitance of the sensor for the input displacements of x = 0.0, 2.5 and 5.0 cm
Solution
The capacitance of a parallel plate capacitor is given by the formula:
C = ε * (A/d)
where:
- C is the capacitance,
- ε is the permittivity of the material between the plates,
- A is the area of one of the plates, and
- d is the separation between the plates.
The permittivity of a material is the product of the permittivity of free space (ε0) and the relative permittivity (εr) of the material. The permittivity of free space is a constant, ε0 = 8.85 x 10^-12 F/m.
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For x = 0.0 cm, the dielectric material is not between the plates, so the permittivity is that of air. The relative permittivity of air is 1, so the permittivity is ε = ε0 * εr = 8.85 x 10^-12 F/m * 1 = 8.85 x 10^-12 F/m. The area of the plates is A = 5 cm * 5 cm = 25 cm^2 = 2.5 x 10^-3 m^2. The separation between the plates is d = 1 mm = 1 x 10^-3 m. So, the capacitance is C = ε * (A/d) = 8.85 x 10^-12 F/m * (2.5 x 10^-3 m^2 / 1 x 10^-3 m) = 22.125 pF.
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For x = 2.5 cm, half of the gap between the plates is filled with the dielectric material. So, the capacitance is the sum of the capacitance with air and the capacitance with the dielectric material. The permittivity of the dielectric material is ε = ε0 * εr = 8.85 x 10^-12 F/m * 4 = 35.4 x 10^-12 F/m. The separation between the plates is now d = 0.5 mm = 0.5 x 10^-3 m. So, the capacitance with the dielectric material is C = ε * (A/d) = 35.4 x 10^-12 F/m * (2.5 x 10^-3 m^2 / 0.5 x 10^-3 m) = 177 pF. The total capacitance is the sum of the capacitance with air and the capacitance with the dielectric material, which is 22.125 pF + 177 pF = 199.125 pF.
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For x = 5.0 cm, the entire gap between the plates is filled with the dielectric material. So, the capacitance is C = ε * (A/d) = 35.4 x 10^-12 F/m * (2.5 x 10^-3 m^2 / 1 x 10^-3 m) = 88.5 pF.
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