Electric Field Lines

Introduction

We know that in the gravitational field an object creates gravitation around it and if any other object is placed inside that region, it will experience a gravitational force. The gravitational field is created due to the masses present but the electric field is created due to charges and the time-varying magnetic fields.

Electric Field

We define the electric field as the region or space around a charge within which if we place another charge, then it will experience an electric force on it which can be attractive or repulsive depending on the polarity of the source and the test charge.

If we bring a test charge at any point in the region of electric field of the source charge Q, then the test charge will experience a force –

F⃗ =qE⃗ ⇒E⃗ =F⃗ /qF→=qE→⇒E→=F→/q

Here, the text charge is so small that it will not disturb the configuration of other charges if present.

As we know that the electric field due to the point charge Q is given by

E=kQq/(qr2)⇒E=kQ/r2E=kQq/(qr2)⇒E=kQ/r2

So, we can infer that the electric field doesn’t depend on the value of test charge. Since the electric field depends only on the distance r, then at equal distances from the charge Q, the magnitude of electric field will be same.

Since electric field is a vector quantity, so if there are system of charges present, then the net electric field at any point will be the vector sum of individual electric fields due to the point charges. This is also called the superposition principle.

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Electric Field Lines

To represent or visualize an electric field, the concept of electric field lines was introduced. Electric field lines, also called as lines of electric force were first introduced by Michael Faraday.

Electric field line is defined as hypothetical line or curve drawn in such a way that a tangent to the curve at any point is in the direction of the net electric field at that point. They have no physical appearance. As electric field is a vector quantity and the magnitude of electric field due to a point charge varies as the inverse of the square of distance between the charge and the point where we have to find the electric field, so if we represent electric field lines for a point charge using a vector, then as the distance increases from the point charge, the length of the vector will also decrease or we can say that the length of the vector is proportional to the strength of the electric field.

Electric field lines due to positive and negative charges:

Since gravitation is only an attractive force, the gravitational field lines look similar to the field line due to the negative point charge, while electric field lines can be in either direction, i.e., radially outward or inward.

Due to a positive charge, the electric field line always points radially away from the charge and points radially towards the charge due to the negative charge.

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Properties of Electric Field Lines

• Electric field lines always start from a positive charge and terminate at a negative charge, however for a single point charge, electric field lines terminate at infinity.

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• They are perpendicular to the surface containing the charge or the surface of a conductor.

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• The density of electric field lines gives the magnitude or strength of the electric field, i.e., near the charge, electric field lines are much closer to each other, so the strength of the electric field is more but as we move away from the charge, the electric field lines moves apart, i.e, electric field strength is less.
• In a charge-free region, electric field lines are continuous and smooth.
• Electric field lines never form a closed loop.
• If we draw a tangent on the electric field line, then that tangent will be towards the net electric field direction at that point.
• The electric field lines never intersect each other at any point. If they intersect at any point, then we will have two net electric fields at a single point which is not possible.

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• The number of lines emerging from or ending on a charge is proportional to the magnitude of charge.

Example

If a charge Q1Q1 has a magnitude of +16C and another Q2Q2 has a magnitude of -4C, then the number of lines originating from Q1Q1 will be more than that from Q2Q2.

• Electric field lines never pass through a conductor means that the electric field inside a conductor is always zero.
• In a given region if the electric field is zero, then electric field lines do not exist.
• If the electric field lines are straight, at an equidistant from each other or if they are parallel, then the electric field is uniform. And for non-uniform electric field, the field lines will not be parallel.

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Electric Field Lines Due to Some Charge Configurations

• Electric field lines for a combination of two positive or negative charges: For a system of two positive charges, the field lines show the repulsion between them.
• Electric field lines for a combination of one positive and one negative charge: