Presentation on theme: "Chapter 24 Capacitance, dielectrics and electric energy storage"— Presentation transcript:
1 Chapter 24 Capacitance, dielectrics and electric energy storage Basic circuit devicesResistorsCapacitorsInductorsPower supply (Battery, Generator)Use our knowledge of electric fields, potentials, and energy to describe how capacitors work.
3 Active Figure 26.4 (b) When the switch is closed, the battery establishes an electric field in the wire that causes electrons to move from the left plate into the wire and into the right plate from the wire. As a result, a separation of charge exists on the plates, which represents an increase in electric potential energy of the system of the circuit. This energy in the system has been transformed from chemical energy in the battery.Fig 26-4b, p.800
18 Energy Density Energy per unit volume: Consider a Parallel Plate Capacitor:
19 Dielectrics k is the Dielectric Constant - - + + - + + + - - + - A dielectric is a nonconducting material that, when placed between the plates of a capacitor, increases the capacitanceMaterials with Dipoles that can align with an external electric Field. Dielectrics include rubber, plastic, and waxed paper--+ +-+ ++--+-k is the Dielectric ConstantMeasure of the degree of dipole alignment in the material
21 Example values of dielectric constant “Dielectric strength” is themaximum field in thedielectric before breakdown.(a spark or flow of charge)
22 Effect of a dielectric on capacitance Potential difference with a dielectricis less than the potential differenceacross free spaceResults in a higher capacitance.Allows more charge to be stored before breakdown voltage.
23 Effect of the dielectric constant Parallel Plate CapacitorMaterial permittivity measuresdegree to which the materialpermits induced dipolesto align with an external fieldExample modificationsusing permittivity
24 Example – Parallel Plate Capacitor +QWhat is new capacitance?dA-Q
25 DipolesThe combination of two equal charges of opposite sign, +Q and –Q, separated by a distance l-Q+Q
27 Work Rotating a Dipole in an Uniform Electric Field +Q-Q
28 Example P26.9When a potential difference of 150 V is applied to the plates of a parallel-plate capacitor, the plates carry a surface charge density of 30.0 nC/cm2. What is the spacing between the plates?
29 Example P26.21Four capacitors are connected as shown in Figure P26.21.Find the equivalent capacitance between points a and b.Calculate the charge on each capacitor if ΔVab = 15.0 V.
30 Example P26.27Find the equivalent capacitance between points a and b for the group of capacitors connected as shown in Figure P Take C1 = 5.00 μF, C2 = 10.0 μF, and C3 = 2.00 μF.
31 Example P26.35A parallel-plate capacitor is charged and then disconnected from a battery. By what fraction does the stored energy change (increase or decrease) when the plate separation is doubled?. Therefore, the,
32 Example P26.43Determine (a) the capacitance and (b) the maximum potential difference that can be applied to a Teflon-filled parallel-plate capacitor having a plate area of 1.75 cm2 and plate separation of mm.
33 Example P26.59A parallel-plate capacitor is constructed using a dielectric material whose dielectric constant is 3.00 and whose dielectric strength is 2.00 × 108 V/m. The desired capacitance is μF, and the capacitor must withstand a maximum potential difference of V. Find the minimum area of the capacitor plates.