Attraction and Repulsion

Meaning of attraction and repulsion in physics

As we mentioned before in physics, there is a special type of force called non-contact forces.

Non-contact forces can be attractive or repulsive.

Attraction and repulsion of charges

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, StudySmarter Originals

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, StudySmarter Originals

Law of attraction and repulsion of electrostatic forces

In 1785, Charles-Augustin de Coulomb, measured the magnitude of electrostatic forces between charged objects using a torsion balance that he invented. He used a needle with a brass disc on one end and a counterweight on the other. The needle was suspended by a fibre so it could turn. He then charged a metallic sphere and the brass disc with the same type of charge and measured how far apart they pushed each other away by using a scale attached to the outer part of the device.

After repeating this process for different amounts of charges, he was able to come up with a formula to quantify the strength of electrostatic forces. That formula is now known as Coulomb's law and it depends on the charge of the objects and the distance between them.

$F=\frac{\text{k}{Q}_1{Q}_2}{d^2}$

In this formula, the force$F$is measured in newtons$\text{(N)}$,$Q_1$is the charge of the first object in coulombs$\left(\mathrm{C}\right)$, similarly,$Q_2$is the value of charge for second object, and$d$is the distance between them, measured in metres$\left(\mathrm{m}\right)$. The constant$\text{k}$is called coulomb's law constant and it depends on the medium that the charged objects are in. For example, if the objects are in a vacuum the value of$\text{k}$is approximately$9\times {10}^9\frac{{\text{Nm}}^2}{{\text{C}}^2}$.

Attraction and repulsion between magnets

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.

Examples of attraction and repulsion: Electrostatic forces

A great and very common example involves static electricity. A balloon can be negatively charged by rubbing it against your hair. When rubbing, lots of electrons 'jump' from the hair to the balloon. This results in the hair being positively charged due to the lack of electrons, and since the balloon gained those electrons, it is now negatively charged.

A balloon can be negatively charged by rubbing it against your hair, StudySmarter Originals

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, StudySmarter Originals

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, StudySmarter Originals

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!

Examples of attraction and repulsion: 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.