Week 9 – SS1 Second Term Physics Notes

 WEEK NINE
TOPIC:
Electric Field – Line of Forces, Properties of Line of Force, Description and Properties of Force Field
CONTENT

• Electric Field
  
• Line of Force and properties
  

      
   

• Electric Field

Electric field is defined as any region where a charge experiences a force of electrical origin. There are two types of charges namely : positive and negative charges.

 Lines of Force
An electric field has been defined as a region where an electric force is experienced. Electric fields can be mapped out by electrostatic lines of force. An electrostatic line of force may be defined as a line whose tangent is in the direction of the force on small positive charge at that point. Arrows on the lines of force show the direction of the force on a positive charge. The force on a negative charge is in the opposite direction.

 Since the direction of a field varies from point to point, lines of force are usually curves.

 
 
Fig 4.1: Lines of Electrostatic force

 (i) Isolated positive charge        (ii) Isolated negative charge    
(iii) Unlike charges – positive and negative charges      (iv) like charges – positive and positive charges

 Properties of Lines of Force
1.    Lines of force never intersect.
2.    Lines of force are usually curves, as the direction of a field varies from point to point.
3.    In a uniform field, the lines of force are straight, parallel and uniformly spaced.
4.    No lines of force originate or terminate in the space surrounding a charge.
5.    Every lines of force in an electrostatic field is a continuous line terminated by a positive charge at one end and a negative charge at the other end.

 Coloumb’s Law states that the force experience by two charges Q1Q2 separated by a distance r is directly proportional to their product of their charges and inversely proportional to the square of their distance apart. It is also known as inverse square law.
Q1 Q2

 F α Q1Q2
Fα 1/r2
F α Q1Q2

  r²
F = K Q1Q2 __________ (i)
r²
K = 9 x 10⁹ Nm² C^-2 ⁄ f/m
K = 1
4ΩЄ₀
(Permitivity of Free Space vacuum)

 EVALUATION

1. State four properties of line of force.
2. State coulomb’s law.

• Electric Field Intensity (E)
  

  It is simply the force per unit charge. It is a vector quantity and measured in
  E = F/Q _________ (iii)

• Electric Field Potential (V)
  

It is the work done in moving a unit positive charge from it point of infinity to a point in the field. It can also be defined as work done per unit charge, it is a scalar quantity and measured in joules per coloumb or volt.
V = W
Q

• Flux Density D.
  

  It is denoted by D. It is simply charge by unit area. It is measured in Cm-2/ Clm²
      D = Q
  A

   EVALUATION
  

1. Define A. electric field intensity B. electric field potential
2. Differentiate between electric potential and electric potential energy
   

    Reading Assignment
   New School Physics pg 72 & 73

    WEEKEND ASSIGNMENT
   

3. The SI unit of flux density is A. Cm² B. Cm C. Cm^-2 D. C
4. Electric potential is a scalar quantity A. true B. false C. cannot say D. none of the above
5. Electric field intensity is a scalar quantity A. true B. false C. cannot say D. none of the above
6. The SI unit of electric potential energy is ………. A. volt B. Joules per coulomb

   C. Joules D. Cm²

7. Line of force are maginary A. true B. false C. cannot say D. none of the above

    THEORY

8. The force acting on an electron carrying a charge of 1.6X10^-19 C in an electric field of intensity 5X10⁸ Vm^-1.
9. 1f 20MJ of work is done in moving a 5µC of charge between two points in an electric field, the potential difference between the two points is?