Transcript PHS102 Lecture 25 Sep 2014 DLI Block 2 2nd Floor

DR. A. O. ADEWALE
Course Outline:
Electrostatics, potential and capacitance, dielectrics, production and measurement
of static electricity. Current, Ohm’s law, resistance and resistivity, heating,
Galvanometers, Voltmeters and Ammeters.
D.C. circuits, sources of emf and currents, Kirchhoff’s laws. Electrochemistry. The
Earth’s magnetic fields and induction. Faraday’s and Lenz’s Laws. Force on a
current carrying conductor. Biot-Savart law. Flemmings right and left-hand rules,
motors and generators.
Cells in Series and Parallel
 When two or more cells are in series, with the positive terminal of one
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connected to the negative terminal of another, then the total e.m.f. is
given as,
E = E1 + E2 + E3 +…
and the total internal resistance is
r = r1 + r2 + r3 +…
If one of the cell, say e.m.f. E3, is connected in opposition with the
others, then
E = E1 + E2 - E3 +…
but the total internal resistance remains unaltered.
When similar cells are in parallel, then total e.m.f. = E, the e.m.f. of any
one of them. The internal resistance r is given by 1 1 1 1
    ...
where r1 is the internal resistance of each cell.
r r1 r2 r3
Example
E = 1.5 V + 1.5 V = 3.0 V
E = 20 V - 12 V = 8 V
E = 12 V
Current in a typical circuit
E
I
Rr
Terminal p.d
V  IR 
ER
Rr
Example
 Consider the circuit diagram in Figure 13.15. Calculate
the terminal p.d. across AB.
Class Work
The figure shows a battery of 12 V and negligible
internal resistance connected to three resistors.
Calculate the p.d. V across the 6 Ω resistor and the
current passing through the 8 Ω resistor.
Example
 Consider the circuit diagram in Figure 13.15. Calculate
the terminal p.d. across AB.
Class Work
The figure shows a battery of 12 V and negligible
internal resistance connected to three resistors.
Calculate the p.d. V across the 6 Ω resistor and the
current passing through the 8 Ω resistor.
Practice Questions
 1. What is the magnitude of a point charge whose
electric field 50 cm away has magnitude 2.0N/C?
 2. In a rectangular coordinate system, two positive
point charges of magnitude 10-8C each are fixed at the
points (0.1, 0) and (-0.1, 0). Find the magnitude of the
electric field at point (0.2, 0).
 3. What is the magnitude of an electric field in which
the force on an electron is equal in magnitude to the
weight of an electron?
Practice Questions
 4. A small object carrying a charge of 5 x 10-9 C
experiences a downward force of 20 x 10-9 N when
placed at a certain point in an electric field. What is
the electric field at the point?
 5. Find the electric field at a point 0.2 m from a charge
of 20 C, what force will the field exert on a charge of
10 C, placed at that point?
 6. What is the electric field E experienced by a charge
of magnitude 5 nC at a point where the electric force is
2 x 10-4 N in the x direction? If an electron is placed in
this field, what force will be exerted on it?
Practice Questions
 7. An oil drop of mass 2 x 10-14 kg carries a charge of 8 x
10-18 C. The drop is stationary between two parallel
plates 20 mm apart with a p.d. of V between them.
Calculate V.
 8. Two charges q1 = 10 µC and q2 = -12 µC are within a
spherical surface of radius 10 cm. What is the total flux
through the surface?
 9. A charged oil drop of mass 1.0 x 10-14 kg remains
stationary when situated between two parallel metal
plates 25 mm apart and a p.d. of 1000 V is applied to
the plates. Find the charge on the drop.
Practice Questions
 10. Two plates of a capacitor are 2.50 m apart while the
potential difference between the plates is 72.0 volts.
What is the electric field intensity on each of the
plate?
 11. Find the potential difference required to give a
helium nucleus (Q = 3.2 x 10-19C) 48 x 103eV of kinetic
energy.
 12. Two point charges Q1 = 10C and Q2 = -2C are
arranged along the x-axis at x = 0 and x = 4m
respectively. Find the positions along the x-axis where
V = 0.
Practice Questions
 13. A 1F and a 2F capacitor are connected in series
across a 1000 volt supply line. Find the charge on each
capacitor and the voltage across each.
 14. A parallel plate air capacitor is made of 0.2 m
square tin plates and 1 cm apart. It is connected to a
50 V battery. What is the charge on each plate?
 15. The plates of a parallel-plate capacitor are 2 mm
apart and 5 m2 in area. The plates are in vacuum. A
potential difference of 2000 volts is applied across the
capacitor. Calculate the magnitude of the electric field
between the plates.
Practice Questions
 16. A capacitor of capacitance C is fully charged by a 200 V
battery. It is then discharged through a small coil of
resistance wire embedded in a thermally insulated block of
specific heat capacity 2.5 x 102 Jkg-1K-1 and of mass 0.1 kg. If
the temperature of the block rises by 0.4 K, what is the
value of C?
 17. A parallel plate air capacitor is made using two plates 0.2
m2 spaced 1 cm apart. It is connected to a 50 V battery.
What is the charge on each plate?
 18. The dimensions of plates of a parallel plate capacitor are
8 cm by 8 cm, and they are separated by a distance of 2
mm. Calculate the capacitance if air is between the plates.
Calculate also the capacitance if a glass of dielectric
constant 5 fills the space between the plates.
Practice Questions
 19. A 6 F capacitor and a 3 f capacitor are connected in
series across an 18 volts d.c source. Calculate the charge on
each capacitor.
 20. What area of the plates of a parallel-plate capacitor
gives 1 F, if the separation of the plates is 1mm and the
plates are in vacuum?
8. Two charges q1 = 10 µC and q2 = -
12 µC are within a spherical surface
of radius 10 cm. What is the total
flux through the surface?
9. A charged oil drop of mass 1.0 x
10-14 kg remains stationary when
situated between two parallel
metal plates 25 mm apart and a p.d.
of 1000 V is applied to the plates.
Find the charge on the drop.
15. The plates of a parallel-plate
capacitor are 2 mm apart and 5
m2 in area. The plates are in
vacuum. A potential difference
of 2000 volts is applied across
the capacitor. Calculate the
magnitude of the electric field
between the plates.