Why charges attract

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Asmis , Simon Grabowsky , Jonas Warneke. Angewandte Chemie , 27 , Angewandte Chemie International Edition , 56 27 , The electric force is a non-contact force. Any charged object can exert this force upon other objects - both charged and uncharged objects.

One goal of this unit of The Physics Classroom is to understand the nature of the electric force. In this part of Lesson 1, two simple and fundamental statements will be made and explained about the nature of the electric force. These two fundamental principles of charge interactions will be used throughout the unit to explain the vast array of static electricity phenomena. As mentioned in the previous section of Lesson 1 , there are two types of electrically charged objects - those that contain more protons than electrons and are said to be positively charged and those that contain less protons than electrons and are said to be negatively charged.

These two types of electrical charges - positive and negative - are said to be opposite types of charge. And consistent with our fundamental principle of charge interaction, a positively charged object will attract a negatively charged object. Oppositely charged objects will exert an attractive influence upon each other.

In contrast to the attractive force between two objects with opposite charges, two objects that are of like charge will repel each other. That is, a positively charged object will exert a repulsive force upon a second positively charged object. This repulsive force will push the two objects apart. Similarly, a negatively charged object will exert a repulsive force upon a second negatively charged object. Objects with like charge repel each other. This electric force exerted between two oppositely charged objects or two like charged objects is a force in the same sense that friction, tension, gravity and air resistance are forces.

And being a force, the same laws and principles that describe any force describe the electrical force. According to Newton's third law, a force is simply a mutual interaction between two objects that results in an equal and opposite push or pull upon those objects. Let's apply Newton's third law to describe the interaction between Object A and Object B, both having positive charge. Object A exerts a rightward push upon Object B. Object B exerts a leftward push upon Object A.

See diagram at right. These two pushing forces have equal magnitudes and are exerted in opposite directions of each other. Each object does its own pushing upon the other. Because of the away from nature of the mutual interaction, the force is said to be repulsive. Now let's apply the same action-reaction principle to two oppositely charged objects - Object C positive and Object D negative.

Object C exerts a leftward pull upon object D. Object D exerts a rightward pull upon Object C. Again, each object does its own pulling of the other. Just as before, these two forces have equal magnitudes and are exerted in opposite directions of each other. However in this instance, the direction of the force on Object D is towards Object C and the direction of the force on Object C is towards object D.

Because of the towards each other nature of the mutual interaction, the force is described as being attractive. Quantum fields to be exact. A field is a mathematical object that takes a value at every point in space and at every moment of time. Quantum fields are fields that carry energy and momentum and obey the rules of quantum mechanics. One consequence of quantum mechanics is that a quantum field carries energy in discrete "lumps".

We call these lumps particles. Incidentally this explains why all particles of the same type e. The fields take values in different kinds of mathematical spaces that are classified by special relativity. The simplest is a scalar field. A scalar field is a simple number at every point in space and time. Another possibility is a vector field: these assign to every point in space and time a vector an arrow with a magnitude and direction.

There are more exotic possibilities too. The jargon term to classify them all is spin , which comes in units of one half. This means that these particles often behave like you expect classical particles to behave. We call these matter particles, and all the basic building blocks of the world electrons, quarks etc.

On the other hand, integer spin particles obey Bose-Einstein statistics again a consequence of relativity. This means that these particles "like to be together," and many of them can get together and build up large wavelike motions more analogous to classical fields than particles.

These are the force fields; the corresponding particles are the force carriers. This fact and the previous one are called the spin-statistics theorem. Because of special relativity, the interaction between a particle and a force carrier has to take a specific form depending on the spin of the force carrier this has to do with the way space and time are unified into a single thing called spacetime. For every unit of spin the force carrier has you have to bring in a minus sign this minus sign comes from a thing called the " metric ", which in relativity tells you how to compute distances in spacetime; in particular it tells you how space and time are different and how they are similar.

Now for gravity the "charge" is usually called mass, and all masses are positive. So you see gravity is universally attractive! So ultimately this sign comes from the fact that photons carry one unit of spin and the fact that the interactions between photons and matter particles have to obey the rules of special relativity.

Notice the remarkable interplay of relativity and quantum mechanics at work. When put together these two principles are much more constraining than either of them individually! Indeed it's quite remarkable that they get along together at all. A poetic way to say it is the world is a delicate dance between these two partners. Now why do atoms and molecules generally attract? This is actually a more complicated question!

The force between atoms is the residual electrical force left over after the electrons and protons have nearly cancelled each other out. Here's how to think of it: the electrons in one atom are attracted to the nuclei of both atoms and at the same time repelled by the other electrons.

Both are expressed through field contours; field forces act in the direction of the contours, and the distance between the contours indicates their magnitude. The closer the contours are together, the larger the force. In an electric field the direction of the field is given by the electric charge of the charged sources. If the charge is negative, the field is directed toward the charge.