Before you learn Electric field, it is advisable you learn Electrostatics . Click on it
Electric Field can be defined as a region where electric force is experienced. In an Electric field, there will either be force of attraction or repulsion. Let's take a look at Coulombs law.
Coulombs law of electrostatics
This law states that the electric force of attraction or repulsion between two points charges (q1 and q2) is directly proportional to the product of the magnitude of thier charges and inversely proportional to the square of thier distance apart.
Mathematically,
The electric force(F) obeys the inverse square law.
Inverse square law is any physical law stating that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity.
Let's take some questions on electric force.
- Calculate the magnitude of the force of repulsion between two equal charges of 3× 10⁻⁶C separated by a distance of 2m.
Solution
q = 3× 10⁻⁶C, r= 2m
- Calculate the force of attraction between two point charges of +3×10⁻⁶C and -2×10⁻⁶C separated by a distance of 1m.
Solution
q1 = +3×10⁻⁶C , q2= -2×10⁻⁶C , r =1m
Electric Field Intensity
This describes the strength of the Electric field. It is a vector quantity and its unit is NC⁻¹ or Vm⁻¹. Electric Field Intensity is the ratio of Electric force to the charge.
i.e E= F/q
Another formula for calculating electric Field Intensity is
The formula above shows that Electric field intensity also obeys inverse square law.
Let's take some calculations on this
- Calculate the force acting on an electron of charge 1.5 x 10⁻¹⁹C placed in an electric field of intensity 10⁵Vm⁻¹.
Solution
q= 1.5 x 10⁻¹⁹C, E= 10⁵Vm⁻¹.
- A charge 50μC has an electric field strength of 360 NC⁻¹ at a certain point. The electric field strength due to another charge 120μC. kept at the same distance apart and in the same medium is
Solution
q₁=50μC= 50× 10⁻⁶C , q₂ = 120μC= 120×10⁻⁶C , E₁ = 360 NC⁻¹, E₂= ?
Electric field line
This is an imaginary line drawn in such a way that the direction of the line at any point is the same as the direction of the field at that point. What are the properties of electric field lines
- They do not cross each other
- They start only on positive charges and end only on negative charges.
- They show the direction of the electric field
- The closer the lines are together, the stronger the electric field.
For a positive charged body, the lines radiate outward
For a negative charged body, the lines radiate inward.
The basic law of electrostatics is that unlike charges attract while like charges repel.
The neutral point can be defined as the point where the resultant electric field intensity is zero.
Electric Potential
This is the product of electric field intensity and the distance. i.e
Electric Potential is also the workdone in moving a unit charge from infinity to the point of consideration. Electric potential is sometimes called potential difference.
V= W/q
W= Vq
Let's take some questions on this
- A student is at a height 4m above the ground during a thunderstorm. Given that the potential difference between the thunderstorm and the ground is 10⁷V, the electric field created by the storm is
Solution
- If two charged plates are maintained at a potential difference of 3 kV, the work done in taking a charge of 600μC across the field is
Solution
V= 3kv= 3000v, q= 600μC= 600×10⁻⁶C
Electric discharge and lightning conductor
During lightning, electric charges can be discharged into the atmosphere. These charges can destroy electronics in building. Hence, there is need to provide lightening conductors to prevent lightening damages. Lightening Conductors should have high conductivity and high tensile strength and they must convey charges from the atmosphere to the earthcrust.
According to lee-pail experiment, charges are distributed uniformly outside a uniform body but they are concentrated at the pointed end of a pair-shaped body.
Exercises
- If a charge of 2μC is situated in a field of intensity 400N/C, the force on the charge is ?
- The electric field intensity in a place where a charge of 10⁻¹⁰C experiences a force of 0.4N is ?
- Aluminium is sometimes used as the leaf of an electroscope because it (a) is a good insulator (b) is a light material (c) is a good conductor (d) can be converted into thin sheets
- When a negatively charged rod is brought near the cap of a negatively charged gold-leaf electroscope, the leaves will (a) collapse (b) become positively charged (c) negatively charged (d) diverge more