Page 7 - PRE-U STPM PHYSICS TERM 2

Physics Term 2 STPM Chapter 13 Capacitors

13.3 Dielectrics
13.3 Dielectrics

Learning Outcomes 2012/P1/Q25, 2018/P2/Q3
Students should be able to:
• defi ne relative permittivity ε (dielectric constant)
r
• describe the effect of a dielectric in a parallel-plate capacitor
13 • use the formula C = ε ε A 13
r 0
d

1. The parallel-plate capacitor discussed in section 13.2 has free space or vacuum in between the plates.

2. The capacitance of a capacitor is greatly increased by having an insulator, known as dielectric, in
between the plates.
3. The dielectric constant or relative permittivity ε of an insulator is defi ned as
r
capacitance of a parallel-plate capacitor with the insulator in between the plates
ε =
r capacitance of the parallel-plate capacitor with free space in between the plates
C
=
C 0
Hence, capacitance of a parallel-plate capacitor with an insulator of dielectric constant ε in between
r
the plates is
ε ε A
C = ε C = r 0
r 0 d
4. Typical values of dielectric constant ε are
r

Air: 1.0006, Paper: 3, Mica: 7, Paraffin wax: 2.5
5. Since the dielectric constant for air ε = 1.0006 = 1.00 (to 3 signifi cant fi gures), parallel-plate capacitors
r
with air between the plates can be taken as a good approximation to capacitors with free space or
vacuum between the plates.
6. The action of an insulator is illustrated as in + – Dielectric
Figure 13.6. + – + – + – + –
(a) The molecules of the insulator are polarised + + – + – + – + – – Original field

by the electric field in between the plates. + + – + – + – + – – Reverse field
(b) This results in the surface of the insulator + + – + – + – + – –
facing the positive plate being negatively + + – + – + – + – –
charged, and the other side being positively + + – + – + – + – –
charged. + –
(c) A reverse electric field is set up. The + + – + – + – + – – Polarized
resultant electric field between the plates molecule

is reduced.
V
(d) Since E = —, when E decreases, the potential
d
difference V between the plates decreases. Battery
Figure 13.6
(e) If the capacitor is still connected to the
battery, charges continue to flow into the
capacitor until the potential difference across the
capacitor equals the e.m.f. of the battery.
(f) Since the capacitor is able to store more charge for the same potential difference, its capacitance
increases.

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