Attraction and Repulsion

Attraction and Repulsion

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Do you know what attraction and repulsion mean? They're words we use to explain how things behave when they're close to each other. For example, when it's hot outside, going to the beach can be an attractive idea. But when food starts to go bad, it can have a repulsive smell. In physics, these words mean something different. They're used to describe non-contact forces. Did you know that water can be attracted to a plastic comb? It's true! Just try running a plastic comb through your hair a few times and then hold it close to a thin stream of running water.

A comb can attract water after using passing it through your hair
A comb can attract water after using passing it through your hair

What is the meaning of attraction and repulsion in physics?

In physics, there's a special type of force called non-contact forces. These forces can affect how an object moves without actually touching it. Some examples of non-contact forces include gravity, electrostatic, and magnetic forces. Non-contact forces can be either attractive or repulsive. An attractive force is when objects exert a pull on each other, making them want to move closer together. A repulsive force is when objects push each other away. Gravity is always attractive, which is why we feel pulled towards the center of the Earth no matter where we are. But electrostatic and magnetic forces can be either attractive or repulsive, depending on the charge of the objects or the poles of the magnets involved.

Attraction and repulsion of charges

An electrostatic force is an interaction that occurs between charged particles.

There are two types of charges: positive and negative. If two objects have the same type of charge the electrostatic force between them will be repulsive. This means that if a positively charged object gets close to another, they both will push each other away. The same occurs for two negatively charged objects.

Objects with the same charges feel a repulsive force
Objects with the same charges feel a repulsive force

On the other hand, opposite charges attract each other. So when one positively charged object is near a negatively charged object, they pull towards each other, tending to move closer.

Objects with opposite charges feel an attractive force
Objects with opposite charges feel an attractive force

Law of attraction and repulsion of electrostatic forces

Charles-Augustin de Coulomb developed a formula to quantify the strength of electrostatic forces between two charged objects. This formula is now known as Coulomb's law and it states that the force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. This formula is expressed as:

F = k * (q1 * q2) / r^2

where F is the force between the two objects, q1 and q2 are the charges of the two objects, r is the distance between them, and k is the Coulomb's law constant. The value of k depends on the medium that the charged objects are in. For example, if the objects are in a vacuum, the value of k is approximately 8.987 551 7923(14) x 10^9 kg⋅m3⋅s−4⋅A−2 .

Attraction and repulsion between magnets

A magnet is any object or material that produces a magnetic field.

Regardless of its shape, every magnet has two poles called north (N) and south (S) poles. Similarly to the case of electric charges, unlike magnetic poles attract.

Opposite poles of two magnets feel an attractive force
Opposite poles of two magnets feel an attractive force

Furthermore, same magnetic poles repel each other.

Same poles of two magnets feel a repulsive force
Same poles of two magnets feel a repulsive force

The Earth itself behaves like a giant magnet, but its magnetic north pole is actually a geographic south pole, and its magnetic south pole is a geographic north pole. This means that a freely moving magnet, such as the needle in a compass, will align its magnetic north pole with the Earth's geographic north pole, and its magnetic south pole with the Earth's geographic south pole. This is why the poles of magnets are named the way they are.

Magnets work by the magnetic forces that occur in atoms, forming magnetic zones in the material that act like tiny magnets themselves. Usually, these zones are randomly aligned, but there are certain materials called ferromagnetic materials that have the property of aligning their magnetic zones when exposed to an external magnetic field. These materials maintain this alignment even after the magnetic field is removed, and become magnets themselves. An object that generates a magnetic field due to its internal structure is called a permanent magnet. Ferromagnetic materials are strongly attracted to magnets, and some examples include iron, cobalt, and nickel.

Examples of attraction and repulsion

Now that we understand how electrostatic and magnetic forces work, we will explore some daily examples where we can observe attractive and repulsive forces in action.

Electrostatic forces

Static electricity is a common example of how objects can become charged through the transfer of electrons. For instance, when a balloon is rubbed against hair, lots of electrons are transferred from the hair to the balloon. This results in the hair becoming positively charged due to a lack of electrons, and the balloon becoming negatively charged due to the excess of electrons it gained. This charge imbalance can cause the balloon to stick to certain surfaces, such as the wall, due to the attraction between opposite charges. This phenomenon can also be observed when rubbing a plastic comb against a wool sweater, causing the comb to become negatively charged and attract small pieces of paper or hair.

Rubbing a balloon on hair is an example of static charge build up created  by? - Quora
A balloon can be negatively charged by rubbing it against your hair

Now, the balloon is attracted to the positively charged hair. The electric force can be seen at a distance when the balloon is moved closer to the hair - the hair is pulled towards the balloon's surface. Both the hair and the balloon are attracted to each other.

The balloon attracts the positively charged hair

Similarly, if two balloons are rubbed against some hair, both of them get negatively charged. If they are put near each other, they push each other away.

Two negatively charged balloons repelling each other
Two negatively charged balloons repelling each other

As you might have deduced, this is the same reason why the comb attracts the stream of water after you pass it through your hair, it obtains an electrostatic charge!

Magnetics forces

As we explained before, you can see attraction and repulsion due to magnetic forces when playing with two magnets. However, if you just have one magnet, you can still see how this will attract some metals. In particular, iron, cobalt, and nickel are strongly attracted to magnets since they are ferromagnetic.

A magnet attracts small pieces of ferromagnetic metal

A magnetic field can be generated by a current flowing through a conductor. This will make the conductor behave like a magnet. By looping a wire over a metal core and connecting it to a battery we create an electromagnet. The end connected to the positive terminal of the battery will act like the south pole of the magnet, and the end connected to the negative terminal will behave like the north pole.

A current flowing through a wire can turn it into an electromagnet

Attraction and Repulsion - Key takeaways Attraction and repulsion are characteristics of non-contact forces such as electrostatic and magnetic forces. Non-contact forces are interactions that can affect a body's motion without the need to touch it. Examples of non-contact forces include electric and magnetic forces. Objects having opposite charges experience an attractive electrostatic force, pulling them towards each other. Objects having the same charge experience a repulsive electrostatic force, pushing them away from each other. Like poles of two magnets experience an attractive magnetic force, pulling them towards each other. Opposite poles of two magnets experience a repulsive magnetic force,  pushing them away from each other. Coulomb's law states that the force exerted between two charged objects, whether a force of attraction or repulsion, can be determined by the charge of both objects, the distance between them, and the medium in which the charges are.

Attraction and Repulsion

What does attraction and repulsion mean?

Attraction and repulsion are characteristic of non-contact forces experienced by two objects when they are moved towards or away from each other. For example, electric and magnetic forces are non-contact forces that can be either attractive or repulsive.  

What is attraction and repulsion in terms of magnetism?

Attractive magnetic forces pull two objects towards each other. For example, the different poles of two magnets are attracted to each other. On the other hand, like poles experiment a repulsive force that pushes them away from each other. 

What is an example of attraction and repulsion in terms of charge?

An example of attraction and repulsion in terms of charge involves static electricity. Rubbing a balloon on your hair will cause the balloon to steal electrons from your hair, charging the balloon negatively and your hair positively. If the balloon gets near the hair, it will attract it. Two ballons charged in the same way obtain the same type of charge, and thus, they repel each other.

How are attraction and repulsion related?

Attraction and repulsion are opposite characteristics of non-contact forces. An attractive force pulls objects together, whereas a repulsive force pushes objects away.

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