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Section 8 Static electricity: Part 8.2 What is an electric field? Maps of repulsion and attraction of static charges - rules on size of charge and distance apart

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8.2A What is an electric field?

Reminder of what the rules are on bringing electrically charged objects together?

An electric field is created around any particle or object that has an overall positive or negative electric charge.

Any pocket of static charge on an object, when brought near another object that is also carrying a static electrical charge then one of two things can happen.

When an electrically charged object is placed in an electric field e.g. from another charged object, it will experience a force due to the interaction of two electric fields.

The magnitude and direction of this force is determined by three factors, which are all to do with the strength of the electric field.

Also note that both charged objects experience a force e.g. both the objects Q and q move apart or come closer together.

 

1. Direction of the force

If the static charges are alike, two positive or two negative charges, then a force of repulsion occurs between the objects, pushing them apart.

This is called electrostatic repulsion, a non-contact force.

If the static charges are different, a positive and a negative charge, then a force of attraction occurs between the objects, attracting them together.

This is called electrostatic attraction, a non-contact force.

This non-contact force acts due to the presence of an electric field around each object which do NOT have to be in contact with each other.

You can demonstrate rule 1. by suspending two different plastic rods of different static charges and bring them towards each other e.g. two of the same charge brought near each other OR two plastic rods of opposite charge.

 

Rule 2. The size of the charge

For oppositely charged objects, the greater the two electrical charges e.g. Q+ and q-, the greater the force of attraction between the charged objects. The strength of the electric field is increased by increase in the magnitude of the charges.

If the two charges are of the same sign (+ + or - -) then the greater the magnitude of the charges, the greater the force of repulsion pushing them away from each other.

 

Rule 3. The distance between two charged objects

For oppositely charged objects (+ve and -ve), the smaller the distance between the two static charges, Q+ and q-, the greater the force of attraction between the charged objects.  The strength of the electric field is increased by decreasing the distance between the charges.

If the two charges are of the same sign (+ + or - -) then the closer they are, the greater the force of repulsion pushing them away from each other.

Footnote:

(at higher level i.e. A level, the force of attraction is proportional to charge Q+ x charge q- / d2)

Electric charge is measured in coulomb units, denoted by C

See Calculation of the charge passing through a point in a circuit Q = It


8.2B More on electric fields - 'maps' of the electric field lines

Can we map out the field lines of a static electric field (as with magnets!)?

An electrical field is created around any electrically charged object.

The electric field of an object is the region that will experience a force if a second charged object is moved into it.

The nearer you are to a charged object AND the bigger its charge, the stronger the electric field effect is.

the electric field lines of force around a sphere static electricity on a spherical surface

The orientation of field lines around a charged object

The diagram above shows the direction of the field lines in an electric field emanating ('spreading out') from two isolated spherical objects, one positively charged Q, and one negatively charged object q.

The lines of the electric force field go from positive to negative and at right angles to the surface of the charged object.

The closer together the field lines are, the stronger the electric field - you can tell from the diagram the field is strongest the closer to the centre of charge you get.

The diagram above forms the basis for showing the electrical field around an object for an isolated positively charge object and an isolated negatively charged object.

By convention the field lines run from positive to negative as indicated by the added arrowheads.

 

direction of electric field lines attraction repulsion positive and negative

Electrostatic force field diagrams when charges interact

The above diagrams show the maps of field lines for electrically charged objects when you get:

Note the electrostatic charge convention that the field direction is from positive to negative.

(i) ATTRACTION: when two oppositely charged objects approach each other (+ve and -ve)

Here the field lines join up between the two charged particles.

If the charged objects are free to move towards each other, the field lines will strengthen, become closer together, as the attractive force increases.

(ii) REPULSION: when two objects of the same charge approach each other (two +ve or two -ve).

Here the field lines do not join up, but are pushed away from each other.

If the charged objects are free to move apart, the field lines will weaken as they become further apart, and the attractive force decreases.

The closer the field lines are together the stronger the electrical field effect.

This happens as you get closer to any charged object OR if the charge is increased, which also increases the electric field strength.

You can see this as the field lines get closer to the source of the static charge.

See also the three rules in the previous section.

 

Note: The greater the potential difference between the charges, and the closer the charge fields are, the more likely you are to see the spontaneous discharge of the electrical energy and a visible spark.

 

Parallel charged plates

field lines for parallel charged plates brought near each other static electricity The electric field between parallel plates

The electric field between two oppositely charged parallel plates is quite uniform with all the field lines parallel to each other and all exactly at right angles to the surfaces of the plates.

Therefore the strength of the electric field is the same at any point between the plates and will only differ at the ends of the plates where the field lines become curved.

INDEX physics notes on static electricity - electrostatics & electric fields


Keywords, phrases and learning objectives on static electricity

Be able to describe and explain what is an electric field of static electricity is.

Be able to describe 'maps' of the field around an static electric charge e.g. between two spheres or two plates.

Know the rules of repulsion and attraction of static charges e.g. what happens when two charged objects interact.

Know the rules on force of repulsion or attraction in terms of the size of the static charge and the distance between the centres of the two statically charged objects.


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