Unit 2 Day 3: Electric Energy Storage Electric potential energy stored between capacitor plates Work done to add charge to the capacitor plates Energy density of the electric field between capacitor plates Movable parallel plate capacitor: change in electric potential energy vs. work done to move plates

Energy Stored in a Capacitor A charged capacitor stores electric potential energy in the electric field between the plates The potential energy stored in the plates is equivalent to the work done to charge the plates. This work is usually done by a battery As charge is added to the plates, it takes increasingly more work to add additional charge because of electron repulsion

Energy Stored in a Capacitor We can then say that the energy “stored” in the capacitor is: W (J) Q (C)

Movable Parallel Plate Capacitor +Q -Q A The electric potential energy decreases as the plates are pulled apart x + ΔV The separation distance x, is increased to 3x, while the battery remains connected

Movable Parallel Plate Capacitor If the capacitor plate distance is increased with the battery remaining connected: –ΔV is constant –Capacitance decreases –Charge decreases –E decreases –U e stored, decreases –Energy density decreases If the capacitor plate distance is increased with the battery removed: –Charge is constant –Capacitance decreases –ΔV increases –E is unchanged –U e stored, increases –Energy density is unchanged