Conductors in Equilibrium E = 0 inside the conductor. E at the - PowerPoint PPT Presentation

Conductors in Equilibrium • E = 0 inside the conductor. – E at the surface is perpendicular to the surface. – Potential inside conductor is a constant. • Excess charge on the conductor must reside on the surface.

Spherical conductor in Equilibrium • Potential inside the conductor is a constant: kQ  V in R • Potential of conductor is proportional to charge on it.

Capacitor • Two (or more) conductors at different potentials form a capacitor. • A capacitor “stores” charge and energy. • Capacitance: – Charge “moved” from one conductor to the other is proportional to the potential difference   between the conductors: Q C V – Constant of proportionality C is called capacitance . – Capacitance is a geometric and materials property. – Units: Farad = Coulomb/Volt

As the voltage in the circuit is increased, the capacitance 1) increases. 2) decreases. 3) does not change.

Parallel-plate Capacitor  • Electric Field for “infinite” sheet of charge:  E  2 0 Area A d

Parallel-plate Capacitor  0 A • With “nothing” (air) in the gap,  C d • The capacitance changes when a dielectric (insulator) is placed in the gap.

Figure 24.17b

Figure 24.17c

Parallel-plate Capacitor  0 A • With “nothing” (air) in the gap,  C d • The capacitance when a dielectric (insulator)  0 A is placed in the gap is  C d