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1770英文中學會考試題預習專欄

物理科

堅道英文書院主編

(廿九)

PHYSICS (29)

ELECTRIC CURRENT.

Current is defined as the flow of charge, and inten- sity of current (I) is defined as the time rate of flow of charge, 1.0..

Direction of current:

(1) Conventional current. Since the concept of positive charge za basic to electricl terminology, the convention direction of current is from positive to negative in a simple conductor ( a simple conductor ig one which does not include any source of potential rise, such as a battery.

(2) Electron current. Electrons are negatively

charged particles, which therefore flow in the opposite direction from positive charges. Therefore, electron flow is the current from negative to positive."

Le 18 assumed that flow of positive charge in oge direction is equivalent to the flow of negative. charge (electron) in the opposite direction,

BATTERY the energy source of an electric circuit, is to develop and maintain the difference in poten- tial necessary to move charges in a circuit consist- ing of an external conducting path which offers re-f sistance to the flow and the battery itself,, Electrically, the battery develops electromotive for- ce (enf) by chemical action or by some other method of converting energy to electrical energy from sone other form, and the circuit deeplays external re- sistance (R) as well as internal resistance (s) or the battery,

OHM'S LAW

Under constant physical conditions, the potential d difference between two enda of a device or conduct or is proportional to the current flowing through it RESISTANCE

The constant of the ratio or the potential differen- ce between the ends of a device to the current flow- ing through the device is known as the resistance

and has a unit of OHMa..

of the deviões MA DÜN

One ohm is the resistance of an electrical element when a potential difference of one volt between the ende of the element causès, & current of one amperE to flow through it.

where Vis the potentia, airrerenos in volta.

"Volt is defined as the potential difference between two points in a conductor when a Joule of work is required to transfer one coulomb from one point to the other."

is the current in amperes.

SPECIFIC RESISTANCE

The resistance of a conductor of length Lom and cross section area. A so.cm.

R

In series

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tv, t vet v

Effects of Electric current,

In parallel

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There are three ma jor effects which have contributed most of the information accumulated concerning the slectric currents.

(1) the heating effect

(2) the chemical effect, and

(3) the magnetic effect..

Heating effect of current

The rate of heating varies directly as the square of the current intensity as the current is passing the device.

or 1 = 12R

(B) 18 the energy of heat in calories and that (JH): axpresses heat in mechanical energy units. joules.

Therefore.

(watt or doule/sec)

Chemical Effect of Cürrent

Certain chemical solutions are capable of passing e electric current, when a battery is connected across

These two electrodes which dip into the solution..

They contain positive. the small particles and negative ions, which that carry the charge. The electrode connected to the positive terminal of the battery is called the anode, and is the electrode at which current ie sait to enter the electrolyte. The electrode connected to the negative terminal is the cathode, and in the one at which the current leave the electrolyte. The negative ions which migrate to the anode are the anions; the positive ione which migrate to the cath- ode are the cations,

solutions are electrolyte to

Metallic substances in solution, and hydrogen, having positive valence, form positive ions; nonmetals and acid radicale, having negativa valence, form negá- tive ions. Thus metallic

ions are attracted to- wards the cathode of an electrolyte cell, where their charges are neutral- ized and they collect. This is the basis of electroplating.

Faraday's Law of Electrolysis

2.

The mass of a sucevance liberated at an electrode. is proportional to the quantity of electricity (ie. to the number of coulombs) that has passed through the electrolyte.

The masses of different substances liberated by the same quantity of electricity are proportional to their chemical equivalents.

The chemical equivalent of an element is its atomic weight divided by its valence. When the chemical e equivalent of a substance is expressed in grams, it is called the gran-equivalent of any substance, Electrochemical equivalent of a substance is the n number of grams of that substance liberated by 1 coulomb. Hence,

Electrochemical equivalent of a substance

gran-equivalent of substance 96,500 coulomba

The mass, in grams, of a subtance literated by an electric current electrochemical equivalent of substance x number of coulombs used.

determine the intensity of current in, the potential differences across, and the amount of resistance of various portions of a circuit. Thus such problems

involve applications of Oha's Law, Joule's Law, and

the concepts associated with resistance of electro- motive force.

Sten to carry out:

(2

Draw a circuit diagram of the situation. Label every component of the circuit, includ- ing the known and unknown quantities, specify- ing the quantity or quantities to be determined Check the units and unify them. Consider the possible combinations or electro- motive forces and possible combinations or resistances in the hope that the circuit may be simplified so that Ohm's Law can be utilized

Example 29-1.

Four reainters are connected as anown In ulagram. Two cells and are connected in opposition.

E

It is required to determine the current flowing in the main, the voltage drop across AB and the current passing the 3-ohm resistor.

Solution

E,= 4 valte =2 votte

Combination of cells: It is clear that the two cells are in series, whence the total emf (1) the sum (algebraic sum) of the two

E

42 = 2 võita.

(Note that ♪ la negative with respect

Moreover, this equivalent battery has an internal resistance of

Combination of resistance:

ohms)

It is also evident that the three resistances B B3, and B. represent a parallel combination whose combined resistance is given by:

1 - 1 1 - 1

0.5

1.08

* 1/1.08 - 0.93 onms

Thus the entire circuit can be replaced by a simpa equivalent circuit with a total external resistance ɔf (6 +0,93) ohms.

Applying Ohm's Law to this simple circuit..

R-6.934

Answer:

6.93 +1.5.

0:24 amperea

the current in

the main circuit ië 0.24

amperes.

Consider the potential difference of each parts in the cirücft and applying Ohm's Law again te thần. The voltage drop across A and Bia

Answer:

INM (0.24)(0.93) · 0.22 volta

The voltage drop across AB is 0.22 vo

Answer: The current 1, passing the resistor

0.22/3 0.73 amperes.

Where

is a constant called specific re818- tance or resistivity and depends on

the material of which he conductaudut

is made and expressed in oha-on.

Resistors in series

The total resistance offered by several resistances connected in series. equal to the sum of the s9- perete resistances,

16

Thus; + R2

Current in every resistor is the same.

13-14

Voltage across the entire combination is the sum. the voltages across independant resistors.

where z.

atomic weight: valence x 96.540

Combination of Celle

Cells in series.

*} each call

Cells in parallel

Resistors in parallel

The reciprocal of the resultant resistance 18 toe sus of the reciprocals of independent resistors.

Total emf - ne External resistance Internal resistance

ne

Example 29-21

Compare the cost of operating 3 lamps in series and in parallel on a 220 volt circuit, if each lamp has a resistance of 100 ohms what is the power consumption in each case, and how many calories of heat are generated in each case during one hour?

Solution;

Lamps in

QQ Q

ov.

Since power)

Lamps in parallel

ppp

IV watts

✰ - ✰, - . - *, - 200

1 2

H = 100/3 ohms

∙100

otal emf External resistance » internal resistance=r/n

-(3) (100)

ne

nR + r

300 ohms

20% 300

2.2

2.2(200)

220 100

Current in the main is the sum of ourrente în eaón branches.

+

Voltares across each resistor are equal.

= V3

The above two equations explain in what conditions is a series arrangement of cella or a parallel ar- rangement of cells chosen.

Problem Proqeaures

Problems in electric current phenomena are grequent- ly directed towards analyzing electric circuits, to

Watca

Pawer in parallel) Power (in series).

The heat generated is given by Ji

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x-3 x 220 watt

220