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/r2 r>>l •Ql is called the dipole moment•V=[kpcosr2

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 x10-12C2/N.m2

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 Ko can be found on the table on page 514 and will be in the possibly useful page information C=Ko[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 CV2= ½ [Q2/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 oE2

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.