PhysCh25章节幻灯片.pptVIP

Dielectric and Gauss’ Law More on capacitance Force E-field Unit : N/C Unit : N Unit : V/m d Voltage Capacitance Energy Unit : V, J/C Unit : F, C/V Unit : J, eV for uniform E-field for capacitor Energy density Unit : J/m3 Discussion : If the upper plate is bigger than the lower plate, do they have the same amount of charge? The whole capacitor is still remained neutral ∴ The two plates have charges of equal magnitude Discussion : The capacitor is kept connected to the battery. If the separation d is doubled, How does the electric field change The charge on the plates? The total energy? Ans : Discussion : The capacitor is disconnected from the battery. If the separation d is doubled, How does the electric field change? The potential? The total energy? Ans : By Gauss’ Law, same q same E E d = V V doubles U doubles Discussion : When we immerse the charged capacitor into pure water (contains no ion), does the E-field increase, decrease or stay the same? What if we heat up the water? Will the dielectric increase or decrease? Temp. KE of H2O decrease the alignment of their molecular dipoles, this decrease the E-field they produce to oppose the e-field in the capacitor ∴ dielectric constant Discussion The freshness of fish can be measured by placing a fish between the plates of a capacitor and measuring the capacitance. How does this work ? ?? The capacitance depends on the dielectric constant of the fish and this in turn depends on the amount of water in the fish’s tissue. Discussion : For a charged capacitor, when we bring a slab of dielectric close to the gap between the plates, What force will you feel? What does this force tell you about the energy stored between plates ? - + + The capacitor exert an attractive force on the dielectric and the potential energy decrease. q is constant, C increases (C =κC0) ∴ U decreases Discussion : How does the following changes? a) E-field E = V / d, b