[Physics Class Notes] on Line of Force Pdf for Exam

A line of Force can be defined as a way or path, be it straight or curved, and the tangent at any given point to it gives the direction of the electric field intensity at that point. The conception of electric lines of forces was founded by Michael Faraday in the year 1837. A force is experienced when a unit charge or point charge is placed in the electric field of another charged particle. The direction of this force can be represented by the imaginary lines, which are called electric lines of force. Electric lines of force are also known as electric field lines.

In Physics, lines of force are the path followed by an electric charge. They are free to move in an electric field or a mass free to move in a gravitational field. Lines of force are lines in any source field, where the tangent drawn at any point gives the field direction at that point and the density of that gives the magnitude of the field. 

The concept of lines of force was introduced in the 1830s by the English scientist Michael Friday. Here we will discuss magnetic lines of force as well as electric lines of force and their properties.

In physics, lines of force are the imaginary line that represents a field of force, such as an electric or magnetic field.  If we draw a tangent at any point on the lines of force gives the direction of the field vector at that point.

Magnetic Lines of Force

Since no isolated unit pole has ever been discovered in the case of a magnetic field, the field lines are called lines of force only. An electric charge that travels along a magnetic field line undergoes no magnetic force.

Properties of Lines of Force 

Below we have discussed the properties of magnetic lines of force:

• Magnetic lines of forces always emerge from the north pole and it merges at the south pole.

• As the distance between the poles increases, the density of magnetic lines also decreases.

• The direction of field lines inside the magnet starts from the South Pole and ends at the North Pole.

• Two magnetic lines of force can never intersect each other at any point.

• The strength of the magnetic lines of force is similar throughout the magnetic field and it is proportional to how close the lines of force are.

Force Line

Force lines are a technique that we use in solid mechanics for the visualization of internal forces in a deformed body. A forced line graphically represents the internal force that is acting within a body across imaginary internal surfaces. These lines of forces show the internal forces and their directions.

Important Characteristics of Electric Lines of Force

• They contract longitudinally.

• Electric lines of force expand laterally.

• They cannot intersect with each other.

• Two electric lines of force travelling in the opposite direction attract each other.

• Two electric lines of force travelling in the same direction repel each other.

• The lines of force are imaginary but the field it represents is real.

• There are no lines of force inside the conductor.

Electric Lines of Force 

Electric Lines of Force Definition – Electric field lines are the way of visualizing electric fields. Electrical lines of force are described as a way or path. It can be straight or curved lines so that the tangent gives the direction of the intensity of the electric field at that point.

An electric line of force is an imaginary line that is a continuous line or curve that we draw in an electric field such that tangent to it at any point gives the direction of the electric lines of force at that point. The direction of a line of force is in the direction where a small free positive charge will move along the line. It always travels from positive charge to negative charge.

The electric lines of force represent the field of a positive electric charge in space.  It consists of a family of straight lines that radiate uniformly in all directions from where the charge originates. A second positive charge is placed in the field that will travel radially away from the first charge.

Properties of Electric Field Lines

• The field lines never intersect each other.

• Electric lines of force contract longitudinally.

• They expand laterally.

• The magnitude of charge and the number of field lines, both are proportional to each other.

• The starting point of the field lines is at the positive charge and the ending point is at the negative charge.

• A single charge must be used for the field lines that either start or end at infinity.

Properties of Electric Lines of Force

• Each electric line of force is conceptually imagined; it does not have any physical existence.

• The total number of lines of force that emerges from a charged body is equal to the charge of the body measured in Coulomb.

• Each electric line of force originates from the positive charge and is terminated to the negative charge.

• A tangent drawn at any point on an electric line of force indicates the direction of the electric field lines at that point in the field.

• Each electric line of force emerges normally from the surface of the charged body.

• Electric lines of force contract only longitudinally.

• Electric lines of force can expand laterally.

• There are no two electric lines of force crossing each other.

• In the same direction, the electric lines of force repel each other.

• The electric lines of force in opposite directions attract each other.

• No lines of force can exist inside the conductor.

Opposite Charges Attract

If two opposite charges placed close to each other get attracted. This is because the force present between them is attractive. If we consider two opposite charges, positive and negative. Then, for positive charge, the electric lines of force move away from the centre of positive charge and for a negative charge, the electric lines of force move towards the centre of the negative charge.

Like Charges Repel

Two positive charges move in the electric lines of force when both the forces are positioned close to each other, both the charges will try to move in the direction of electric lines of force. It is observed that the positive charge at the left side will try to move towards the positive charge at the right side, whereas the electric lines of force of the right side’s positive charge oppose this movement. 

In a similar way, the positive charge at the right side will also experience an opposing force from the left side’s positive charge. Therefore, both the charges, positive charge and negative charge will have a repulsive force from each other.

Difference between Magnetic and Electric Lines of Force

The differences between magnetic and electric lines of force are given below:

Magnetic Lines of Force

1. It is always in a close loop.

2. Magnetic lines of force can pass through iron more easily as compared to air.

3. Outside the body of the magnet, the direction of magnetic field lines is from the north pole to the south pole.

4. Magnetic fields formed from the motion of the charge, or the current.

Electric Lines of Force 

1. They never form a close loop.

2. Electron moves from positive to negative 

3. They have a static charge.

4. Electric fields are the result of the strength of the charge.

Line of Force Examples

1. If p and p’ make angles 0 and 0 with the axis, then the equation to a line of force is cos 0 – cos B’= constant. 

2. A line of force is considered as proceeding from the north pole towards the south pole of the magnet, its direction being that in which an isolated north pole would be urged along with FIG.

3. The resultant magnetic force at every point is in the direction of the normal (n) to the surface. Hence, cut the equipotential surfaces at right angles.

However, the magnetic influence itself is entirely invisible, its existence and strength can be judged by such lines of force which indicate the direction in which the magnetic influence is acting. The intensity of the field at any point can be achieved by the density of the electric lines of force at that point. 

When two opposite charges are placed close to each other, the electric lines of force present between them will become shorter in length. When two similar charges are placed closer to each other, the length of electric lines of force present between those charges will enlarge.