Transcript 17-6 Electric Dipoles

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 +Dr)=[kQ(Dr)]/[r(r +Dr)]
•V=kQlcosq/r2 r>>l
•Ql is called the dipole moment
•V=[kpcosq]/r2
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=eo[A/d] where A is area and d is distance
between plates and eo=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 Keo can be found on the
table on page 514 and will be in the
possibly useful page information
C=Keo[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 eoE2
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.