ELECTRIC FIELD AND ELECTRIC FIELD INTENSITY

ELECTRIC FIELD AND ELECTRIC FIELD INTENSITY

လွ်ပ္စစ္စက္ကြင္း ႏွင့္  လွ်ပ္စစ္စက္ကြင္းျပင္းအား
When a positive charge  q  is brought close to another positive charge  Q , which is placed at a point, the charge  q  is found to be acted upon by a repulsive force. The repulsive force becomes greater as  q  gets nearer to  Q. That is,  Q  has a field surrounding it where electric forces due to it may act.  In other words,  Q  produces or set up an electric field around it.

Positive Charge q တစ္ခုကို အမွတ္တစ္ေနရာေတြထားထားေသာ အျခားေသာ Positive charge  Q  တစ္ခုနားကို ယူေဆာင္တည္ရွိေစေသာအခါ ၎ charge   q   ကို တြန္းအား (repulsive force) တစ္ခုသက္ေရာက္ေနပါသည္။ Charge   q   သည္  Charge  Q  နားသို႔ ပိုပိုနီးလာေလ တြန္းကန္အားသည္လည္း ပိုပိုၾကီးလာေလျဖစ္သည္။ ဆိုလိုသည္မွာ Charge  Q   ေၾကာင့္ electric force တို႔ျဖစ္ေပၚေနျပီး ၎ Charge  Q ပတ္၀န္းက်င္တြင္ electric field သည္ Q ကို ၀ိုင္းရံထားသည္။ တစ္နည္းဆိုေသာ္ Charge  Q  သည္ ၎ပတ္၀န္းက်င္တြင္ လွ်ပ္စစ္စက္ကြင္းကိုတည္ေဆာက္ျဖစ္ေပၚေနေစပါသည္။

Fig. 8.3  Electric filed around charged bodies

In Fig. 8.3 the force is exerted on  q  directly by the electric field produced by  Q.  Even though  q  is removed, the electric field of  Q  still exists. In addition, if  q  is placed at any other point in the vicinity of  Q,  it will be found that a repulsive force due to electric field of  Q  still acts on  q.  Since  Q  also experiences a repulsive force.   q  is said to produce an electric field in its vicinity. An electric field, therefore, can be defined as a region where electrical forces act.

အထက္ပါပံုတြင္ Charge Q မွ ထုတ္လႊတ္လိုက္ေသာ electric field သည္ charge  q  ေပၚသို႔ Force ကိုတိုက္ရိုက္သက္ေရာက္ေစပါသည္။  charge q  ကို ဖယ္ရွားလိုက္ေသာ္လည္း Charge Q ၏ လွ်ပ္စစ္စက္ကြင္းသည္ ဆက္လွ်က္တည္ရွိေနပါဦးမည္။  ထိုျပင္  charge q ကို charge q ၏အနီးနားပတ္၀န္းက်င္၏မည္သည့္ေနရာတြင္ ထားထားေစကာမူ charge Q ၏ လွ်ပ္စစ္စက္ကြင္းေၾကာင့္ျဖစ္ေပၚေနေသာ repulsive force (တြန္းအား) သည္ charge q ေပၚတြင္ဆက္လွ်က္သက္ေရာက္ေနပါဦးမည္။ အဘယ္ေၾကာင့္ဆိုေသာ္ Charge Q သည္ repulsive force ကို ကိုယ္တိုင္လုပ္ေဆာင္ေနေသာေၾကာင့္ျဖစ္ပါသည္။ Charge q သည္လည္း ပတ္၀န္းက်င္တြင္ လွ်ပ္စစ္စက္ကြင္းကိုထုတ္လႊတ္ေနသည္ဟုေျပာ၍ရႏိုင္ပါသည္။ ထို႕ေၾကာင္း electric field ဆိုမွာ electric forces မ်ားသက္ေရာက္ေနေသာ ေနရာအပုိင္းအျဖစ္ အဓိပၸါယ္သတ္မွတ္ႏိုင္ပါသည္။

Not just the positive charges but the negative charges as well are surrounded by electric fields.

Positive charges မ်ားသာမက Negative charges မ်ားကို electric field မ်ား၀န္းရံထားၾကသည္။

We may say, then, that any electric charge gives rise to an electric field in its vicinity.

မည္သည္ electric charge မဆို ၎တို႔ပတ္၀န္းက်င္တြင္ electric field ကို ထုတ္လႊတ္ျဖစ္ေပၚေနေစသည္ဟု ကြ်န္ေတာ္တိုေျပာႏိုင္ပါသည္။

In order to test whether an electric field exists at a certain point, a test charge must be placed at that point. If in electric force is exerted on the test charge, then we can say that an electric filed exists at the point under consideration. Generally, a unit positive charge is considered as a test charge.

သိက်ေသာအမွတ္တစ္ေနရာတြင္ electric field တည္ရွိသည္မတည္ရွိသည္ကို စမ္းသတ္ရန္အလိုငွာ စမ္းသပ္လွ်ပ္စစ္တစ္ခုကို ထိုအမွတ္ေနရာတြင္ထားရွိေစရပါမည္။ ၎ test charge ေပၚသို႔ electric force ကိုသက္ေရာက္ေစလွ်င္ ထိုအေျခအေန၌ ၎အမွတ္တြင္ electric field သက္ေရာက္ေနသည့္ဟူ ကြ်န္ေတာ္တို႕ေတြေျပာႏိုင္ပါသည္။ အမာ်းအားျဖင့္ 1 Unit Positive Charge ကိုသာ test charge တစ္ခုအျဖစ္မွတ္ယူၾကသည္။

The Electric field Intensity

 We have seen that when an electric charge is placed in an electric filed a force is exerted on it. If the charge is moved from a point to another point in the field, the magnitude and direction of the force acting upon it will change. This means that the the magnitude and direction of the force acting upon the charge will change in accordance with the change in position of the charge. It is necessary to know the electric field intensity in order to specify an electric filed. The electric filed intensity is defined as follows.

Electric field တစ္ခုအတြင္း electric charge တစ္ခုကိုထားထားေသာအခ်ိန္တြင္ force တစ္ခုသက္ေရာက္ေစေၾကာင္းကို ကြ်န္ေတာ္တို႔ေတြသိျမင္ခဲ့ၾကၿပီးျဖစ္သည္။ ထိုစက္ကြင္းအတြင္း charge ကို point တစ္ေနရာမွ အျခား point တစ္ေနရာသို႔ ေရြ႕ေျပာင္းလိုက္လွ်င္ ၎ charge ေပၚသို႔သက္ေရာက္ေနေသာ force magnitude (ပမာဏ) ႏွင့္ direction (ဦးတည္ဘက္) တို႔သည္လည္း ေျပာင္းလဲးသြားလိမ္မည္။ ဆိုလိုသည္မွာ -  charge ေပၚသို႔သက္ေရာက္ေနေသာ force ၏ magnitude and direction တို႔သည္ ၎ charge ၏ position (တည္ေနရာ) ေျပာင္းလဲျခင္းႏွင့္အညီလိုက္၍ေျပာင္းလဲေနပါလိမ္မည္။ Electric field ကို တိတိက်က်သိေစရန္အလို႔ငွာ electric field intensity (လွ်ပ္စစ္စက္ကြင္းျပင္းအား) ကို သိရန္လိုအပ္ပါသည္။ electric field intensity ကို ေအာက္ပါအတိုင္း အဓိပၸါယ္မွတ္ယူသည္။

The electric field intensity at a point in an electric field is the electric force acting upon a unit positive charge placed at that point. The electric field intensity is a vector quantity. The electric field intensity is represented by  E.

electric field တစ္ခုအတြင္း အမွတ္တစ္ေနရာ၌ရွိေသာ electric field intensity ဆိုသည္မွာ ထိုအမွတ္၌ထားထားေသာ အေပါင္းတစ္ယူနစ္ charge အေပၚသို႔သက္ေရာက္ေနေသာ electric force ျဖစ္သည္။ electric field intensity သည္ vector quantity အမ်ဳိးအစားတစ္ခုျဖစ္သည္။ electric field intensity ကို အဂၤလိပ္အကၡရာ  E  ျဖင့္ကိုယ္စားျပဳသည္။

In Fig. 8.3 the force on  q  exerted by  Q  is  F.

အထက္ပါပံု Fig.8.3 တြင္ charge Q အားျဖင့္ charge q ေပၚသို႔သက္ေရာက္ေနေသာ force သည္  F ျဖစ္သည္။

The force exerted by  Q  on a unit positive charge is F/q.
အေပါင္းတစ္ယူနစ္ charge ေပၚသို႔ charge Q ေၾကာင့္သက္ေရာက္ေစေသာ force သည္ F/q ျဖစ္သည္။

Since the force acting upon a unit positive charge is the electric field intensity, the electric filed intensity  E  can be expressed as

တစ္ယူနစ္ positive charge ေပၚတြင္သက္ေရာက္ေနေသာအားသည္ electric field intensity ျဖစ္ေသာေၾကာင့္ ၎ electric field intensity E ကို ေအာက္ပါ equation ျဖင့္ ေဖာ္ျပတြက္ခ်က္ႏိုင္ပါသည္။

From equation we see that the direction of  E  is the same as that of  F.

အထက္ပါ equation မွ electric field intensity (E) ၏ ဦးတည္ဘက္သည္ Force ၏ ဦးတည္အတိုင္းျဖစ္သည္ကို ကြ်န္ေတာ္တိုေတြ႕ျမင့္ၾကရသည္။

In the SI system the unit of electric force  F  is newton (N)  and the unit of electric charge  q  is coulomb  (C). Thus, the unit of electric field intensity  E  is newton per coulomb (N C⁻¹).

SI system တြက္ခ်က္ရာတြင္ Force ၏ unit သည္ newton (N) ျဖစ္၍ electric charge q  ၏ unit သည္ coulomb (C) ျဖင့္ တြက္ခ်က္ရသည္။ ထို႕ေၾကာင္ electric field intensity E ၏ ယူနစ္သည္ newton per coulomb (N C⁻¹)  ျဖစ္သည္။

Equation can be rewritten as
တြက္ခ်က္မူ equation ကို ေအာက္ပါအတိုင္း ေရးသားႏိုင္သည္။

F = qE

If the values of  q  and  E  are known the force  F  acting upon  q  can be calculated from the about equation.
 Charge q ႏွင္ electric field intensity တို႔၏ တန္ဖိုးကိုသိရွိထားလွ်င္ charge q ေပၚတြင္ သက္ေရာကေနေသာ force F တန္ဖိုးကို အထက္ပါ equation မွတြက္ခ်က္ယူႏိုင္သည္။

Calculation of the Electric Field Intensity from Coulomb's Law

The electric filed intensity at a point, a certain distance from the charge, can be calculated by using Coulomb's law.

 ၎ charge မွ သိက်ေသာအကြာအေ၀းရွိ အမွတ္တစ္ေနရာ၌ရွိေသာ electric field intensity ကို Coulomb's law ကို အသံုးျပဳၿပီး တြက္ယူႏုိင္သည္။

Fig. 8.4 Force on a charge in an electric field

We shall consider the electric field surrounding the charge  Q, shown in Fig. 8.4 and then find the electric field intensity at a point  A  in the field, at a distance  r  from   Q .  Suppose that a small positive charge  q  is placed at   A.

By Coulomb's law the force  F  on  q  due to  Q  is

The force on a unit positive charge due to  Q  is by definition the electric field intensity; thus

Charge Q ေၾကာင့္ တစ္ယူနစ္ positive charge (+1 C) ေပၚတြင္သက္ေရာက္ေသာအားသည္ electric field intensity ျဖစ္သည္။

Equation gives that the magnitude of the electric field intensity at the point   A.

အထက္ပါ equation သည္ အမွတ္ A ေနရာတြင္တည္ရွိေနေသာ electric field intensity ၏ ပမာဏ (magnitude) ကိုတြက္ခ်က္ေပးသည္။

In the SI system the unit of the charge  Q  is coulomb (C)  and the unit of distance  r  is metre  (m).  When these units and the SI unit  of   1/4𝛑𝛆₀  are used in equation, the unit of  E  is newton per coulomb  (N C⁻¹)

SI system တြင္ charge Q ၏ unit သည္ coulomb (C) ျဖစ္ၿပီး၊ အကြာအေ၀း  distance r ၏ unit သည္ metre  (m) ျဖစ္သည္။ အထက္ပါ equation တြင္ charge Q ႏွင့္ distance r တို႔၏ unit မ်ားႏ်ွင့္ 1/4𝛑𝛆₀ တို႔၏ unit မ်ားကို အသံုးျပဳလိုက္ေသာအခါ electric field intensity  E  ၏ unit သည္ newton per coulomb  (N C⁻¹) အျဖစ္ရရွိသည္။

The direction of the electric filed intensity at the point  A  is along the line joining  q  and  Q  and away from  Q.

အမွတ္ A ၌ ရွိေသာ electric field intensity ၏ ဦးတည္ဘက္သည္ charge  q  ႏွင့္ charge  Q  တို႔ ဆက္သြားထားေသာ မွ်ဥ္းလႈိင္းအတိုင္းသာျဖစ္ျပီး charge Q ၏ အေ၀းသို႔သာ ဦးတည္းသည္။ (မွတ္ခ်က္ q₁ ကို Q ဟုမွတ္ယူသည္။)

If a negative charge  Q  is put in place of the positive charge  Q,  the magnitude of the electric field intensity at the point  A  will not change,  But its directions will be along the line joining  q  and Q and toward  Q.

positive charge Q ေနရာတြင္ negative charge Q  ကိုအစားထုိးလွ်င္ အမွတ္ A ၌ ရွိေသာ electric field intensity E ၏ magnitude ပမာဏသည္ မေျပာင္းပဲ၊ ၎ electric field intensity E ၏ ဦးတည္ဘက္သည္ charge q ႏွင့္ Charge Q ကိုဆက္သြယ္ေသာ line တစ္ေလွ်ာက္သာျဖစ္ျပီး charge Q ဆီသို႔ဦးတည္သြားမည္ျဖစ္သည္။

If the resultant electric field intensity at a point due to two or more charges to be found, the vector sum of electric field intensities at that point must be taken.  This means that if the electric field intensities at a point due to the charges are   E₁,  E₂,  E₃  ........... , then the resultant electric filed intensity  E at that points is

အမွတ္တစ္ေနရာ၌ရွိေသာ  electric field intensity သည္ charge ႏွစ္ခု သို႔မဟုတ္ မ်ားစြာေသာ charge မ်ားေၾကာင့္ျဖစ္ေပၚေနလွ်င္ ထိုအမွတ္၌ရွိေသာ electric field intensity ၏ vector ေပါင္းလဒ္ကို တြက္ယူရမည္ျဖစ္သည္။ ဆိုလိုသည္မွာ အမွတ္တစ္ေနရာ၌ရွိေသာ electric field intensity သည္ E₁,  E₂,  E₃  ........... , တို႔ျဖစ္ေနၾကလွ်င့္ ထိုအမွတ္၌ရွိေသာ အသာတင္ electric field intensity တန္ဖိုးသည္ ေအာက္ပါ vector sum အတိုင္းျဖစ္သည္။

E  =  E₁  +  E₂  +  E₃  +  ..................