買二第張六第6日七十月二年酉己腐 WAH KIU YAT PO
1969FUIZ
物理科
(二十二) ·陸永熾
PHYSICS (22)
Solution
41.(4) Charzing an Electr
Induction
one Positively br
(2)
A
74)
Bring a negatively charged rod third rubber rubbed with fur) near the disc of the elect-
roscope. Positive charges will be attracted to the disc while negative ones are repelled to the other end of the rod. The gold leaves, consequently, diverge.
2. Touch the disc with a finger. This permits
some of the negative charges (fres electrons), which are repelled by the negatively charged body, to go to the ground through the finger, The gold leaves collapse now.
3. On the withdrawal of the finger, the bound
charge (positive charge) remains in the disc.
4. The rod is then withdrawn. The electroscope
has, now received a positive charge deficienc of electrons), and the leaves diverge.
(s) Electrophorous
This instrument works by electrostatic induction and can be used to produce any number of positive charges. It consists of a circular slab of hard rubber together with a brass dise on an insulating handle. V
The hard rubber is given a negative charge by flicking it with a piece of fur, and then the brass disc is placed on top of it. This results in positive and negative charges becoming induced respectively on the under and upper sides of the brass disc. The brass disc 19 then: earthed momentarily by touching it with the finger so that the negative induced charge is repelled to earth. On removing the dise from the slab it is found to be positively charged and a spark may be drawn from it. The brass dise
may be charged very many times before the slab. needs recharging,
The positive charge on the disc possesses no available energy while it rests on the slab. They are neutralised by the negative charge on the slab, The disc will only release electric energy when t mechanical energy is done against the force of attracting between the charges on the disc and the slab. This work is converted into electrical energy by producing a spark.
(b) Distribution of charge over the surface of
Bpherical com
The charge is distributed equally over the surface of a spherical conductor, as the curvature is the same on the surface of it.
Distribution of charge over the surface of a pear-shaped conductor
Charge is mostly
concentrated at
place where the
surface is aharply curved. This is particularly noticeable at
the pointed end of the pear-shaped conductor, where the curvature is greatest.
報日怯
proof plane and a gold-leaf electroscope ara used to investigate the distribution of charge over the surface of a conductor. The proof plane is pressed into contact with the surface at various places in turn and then transferring the charge to the electroscope. The divergence of the ear will give a rough massure of the amount of charge or surface density of charge. there,
How an électri
wiridmill work
Van de Graaf
eneratar
四明星 日三月四年九六九一座公年八十五圃民资中育教儒费
(1) Two res
stances in seriest
The windmill consists of four wires arranged as the spokes of a wheel on an insulated pivot and having their ends bent at right angles. When connected to a high voltage terminal, of a Van de Graaf generator, the surface density of charge. on the points becomes exceedingly high, that charges are overcrowded at the pointed ends, As a result of repulsion on like charges, an electric wind streams away from each of the four points. The resulting reaction on the wires causes the mill to rotate in the opposite direction.
(d) A bird sitting on a high-voltage wire is
unaffected since it provides no complete circui to elsewhere. No current passes through the bird. However, if a person touches the live mins terminal & rather high current passes through his body to the earth. A fatal accident may result.
(1) The modern electron theory teaches that all:
the elements are composed of atoms. The different structure of atoms gives their distinctive properties. An atom
consists of protons, which is
post
charged, and neutrons of neutral charge as the nucleus; and the negatively charged electrons revolving round the nucleus. Normally, the number of revolving electrons is the same as the mumber of protons in the nucleus Hence, their charges neutralise one another, and the atom is said to be uncharged. The protons and neutrons are closely bound together and cannot be disrupted except by tremendous forces. Electrons, on the other hand, move with relative eage from atom to atom. Setting up a charge on a body is always explained in terms of the loss or gain of electrons on the atom. An excess of electrons in an atom means that the atom is negatively charged. A deficiency of electrons means that it is positively charged. That is why matter is claimed to be electrical in nature.
(11) When a negatively charged body is brought
near, but not touching, a neutral body, some of the electrons in the ball are. repelled to the far side. The side nearer the negative rod now has a deficiency of electrons, and is therefore positive, The attraction between the charged body and the noarer side is greater than the repulsion of the two like charges, which are farther apart; hence the two bodies attract. When contact is made, the unlike inducing and induced charges neutralise one another The two bodies now have like charges and like charges repel. This explains why a neutral- body is first attracted and then repelled by a charged body.
(111) Flames or a glowing body emits electrons,
They can ionize the air as well as discharge a positively charged body as it brought near to it.
iv) The atoms of some materials have a large
number of free electrons. These materials are good conductors. If a potential difference is applied across such a conductor, free electrons will drift from atom to atom. The drift of free electrons in a conductor is called an electric current. Some materials conducts little or does not conduct electrical charges at all. They have no free electrons on then. Such materials have their atoms hold on to their electrons very strongly. They are known as insulators (vd) When a body is positively charged, it is in
deficiency of electrons, that is, tha number of positive protons out-numbers the electrons..
The force or tendency of driving an electric current through a conductor is called an electromotive force (e.m.f.) Electro-motive force results from the fact that electrons. repel each other. Electrons, therefore, tend to be repelled out of bodies that have a large concentration of electrons and to enter bodies that either lack electrons or have very few excess electrons. If two bodies with equal concentrations of electrons are connected by a conducting wire, no electrons will flow from one to the other. There is no e.m.f.
(b) The current passing through a wire at constant
temperature is proportional to the potential difference between its ends. This is known as Ohm's Law,
The resistance of a conductor is the ratio of the potential difference across it to the current flowing through it.
If R ohms is the combined resistance
V volts, the total p.d. across them, and
Amperes, the current passing,
VIR (Ohm's Law).
sum of individual p.d.a. across
and
TR
IH = I{R,
RR 2
(same current passing thro
R2 and R2l
(11) Two resistances in parallel:
The Potential differences Acroas Rare equal,
Therefore,
but
and
sus of individual current
When three coils of 1 on each are arı series:
The resultant resistance
3 ohms,
R3
When joined in parallel:
The resultant resistance can be calculated as
chri
12V-
9 ohms
When 6 cells are.in series, the total e.nf. 18 6 x 2 - 12 volts.
The total interm1 resistance - 0.1 x 6
The current in the circuit is then,
E
R
12
1.25 ampy
voltmeter is put across the battery and it reads
12- Ir
121.25 x 0.6
• 11.25 volts
The current in the circuit 18 1.25 amp, and th voltmeter reading is 11.25 volts.
Total emf E- 12 volts Total internal resistance r
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