17-6 Electric Dipoles 2 equal point charges Q, of opposite signs, separated by a distance L, are called an electric dipole. Calculate the electric potential at an arbitrary point P…where r is the distance to P… V=(kQ)/r + (k-Q)/(r +  r)=[kQ(  r )]/[r(r +  r)] V=kQlcos  /r 2 r>>l Ql is called the dipole moment V=[kpcos  r 2

Dipole cont’d See example 17-6 p512

17-7 Capacitance A capacitor, sometimes called a condensor, is a device that can store an electric charge and consists of two ll plates separated by a space and connected to two conducting wires. The symbol for a capacitor –ll- Q=CV where C is capacitance and the SI unit for capacitance is the farad (F) Capacitance, C, is constant for a capacitor. C=  o [A/d] where A is area and d is distance between plates and  o =8.85 x C 2 /N. m 2

Capacitance cont’d See 17-7 p514

17-8 Dielectrics In most capacitors, there is an insulating sheet called a dielectric in-between the plates. The value for K  o can be found on the table on page 514 and will be in the possibly useful page information C=K  o [A/d] See Conceptural Example 17-8 p516

17-9 Storage of Electricity A charged capacitor stores electricity…it is usually discharged by depressing one or both plates to send the charge across the space. U= energy= ½ QV= ½ CV 2 = ½ [Q 2 /C] See Example 17-9 page 517 The quantity Ad is the volume between the plates where an electric field exists… u=energy density= energy/volume =1/2  o E 2

17-10 Cathode-ray tube A CRT operates by thermonic emission. The negative electrode or cathode and a positive electrode called an anode exist as two plates in an evacuated tube. When the negative electrode is heated it gives off electrons, originally called cathode rays. (See pg. 519) An oscilloscope is a device for representing an electric signal on a screen, using a CRT.

17-11 The ECG and EKG Each time the heart beats, a change in electrical potential can be detected using electrodes near it. The record of these changes are called an electrocardiogram… or EKG or ECG. Infarcts, which are dead regions of the heart become evident. See QRS group diagram page 520…(contraction of the ventricle and T recovery.