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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 on attractive force:

At a higher level i.e. UK A level, the force of attraction is proportional to

charge Q+ x charge q- / d²

Q+ and q- are the two static electric charges and d the distance between the centres of the two charges.

The same equation applies for the repulsive force if both charges were +ve or -ve.

 

Electric charge is measured in coulomb units, denoted by C

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

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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 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.

 

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

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

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Key points about static electricity - electric fields and charge size and distance factors

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

A comprehensive and exam-board-aligned summary of electric fields, tailored for IGCSE/GCSE Physics students across WJEC, CCEA, CIE, AQA, Edexcel, and OCR:

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What Is an Electric Field?

An electric field is a region around a charged object where another charge experiences a non-contact electrostatic force.

• Created by charged objects (positive or negative).
• The field exerts a force on other charges placed within it.
• Direction of the field is defined as the direction a positive test charge would move.

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Electric Field Lines: Visual Maps

Electric field lines are used to represent the field:

Charge Type	Field Line Direction	Field Line Pattern
Positive	Outwards	Radial (like sun rays)
Negative	Inwards	Radial (towards centre)
Opposite Charges	From + to –	Curved lines connecting them
Like Charges	Away from each other	Repelling, never crossing

• Lines never cross.
• Closer lines = stronger field.
• Uniform field: Between two parallel plates → straight, equally spaced lines from + to –.

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Attraction and Repulsion

• Like charges repel, opposite charges attract.
• These forces are non-contact and act through the electric field.
• The force experienced depends on:
  • Size of the charges: Larger charge = stronger field.
  • Distance apart: Closer = stronger force.

Rule of Thumb:

• Force proportional to 1 / distance²
• Force proportional to charge size

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Real-World Examples

• Van de Graaff generator: Shows repulsion (hair standing up).
• Lightning: Discharge due to strong electric fields between cloud and ground.
• Balloon and wall: Balloon sticks due to induced attraction.

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Exam Board Coverage

All major boards include the following concepts
Definition of electric field
Field line diagrams
Attraction and repulsion
Effect of charge size and distance apart
Real-world examples (e.g. lightning)

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Student Tips for Exam Success

• Draw field lines with arrows: away from +, toward –.
• Use correct terminology: “non-contact force,” “field strength,” “radial field.”
• Explain interactions: e.g. “The negative charge experiences an attractive force toward the positive charge.”
• Link to real-world examples: sparks, photocopiers, paint spraying.
• Don’t say “positive charge moves” unless it’s a test charge in a field.
• Practice sketching: isolated charges, opposite charges, parallel plates.

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Keywords, phrases and learning objectives on static electricity - electric field diagrams and rules on charge size and distance apart

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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Based on the syllabus-specifications for students taking the IGCSE/GCSE level physics examinations summary revision notes and key points on electric field maps of repulsion & attraction for students taking the AQA igcse/gcse physics notes on electric field maps of repulsion & attraction, Edexcel gcse physics notes on electric field maps of repulsion & attraction,  OCR 21st century GCSE physics notes on electric field maps of repulsion & attraction, OCR gateway GCSE physics notes on electric field maps of repulsion & attraction, WJEC gcse physics notes on electric field maps of repulsion & attraction, CCEA gcse physics notes on electric field maps of repulsion & attraction for students taking CIE Cambridge igcse physics, or any other GCSE or IGCSE level physics exams notes on electric field maps of repulsion & attraction, useful for US grade 9-10 physics courses, Explaining the importance of electric field rules on how attractive force depends on quantity & distance apart of charges in GCSE level physics, What you need to know about electric field rules on how attractive force depends on quantity & distance apart of charges for GCSE level physics, Explaining what is the use of electric field rules on how attractive force depends on quantity & distance apart of charges knowledge in GCSE level physics, Examples of electric field rules on how attractive force depends on quantity & distance apart of charges explained when studying GCSE level physics, What is the significance of electric field rules on how attractive force depends on quantity & distance apart of charges in GCSE level physics, Describing and explaining the theory of electric field rules on how attractive force depends on quantity & distance apart of charges when studying GCSE level physics, revision notes for electric field rules on how attractive force depends on quantity & distance apart of charges in exams, online exam help for electric field rules on how attractive force depends on quantity & distance apart of charges, revision notes for electric field rules on how attractive force depends on quantity & distance apart of charges, what do I need to learn about electric field rules on how attractive force depends on quantity & distance apart of charges for by GCSE physics exam? revision summary for electric field rules on how attractive force depends on quantity & distance apart of charges, help in teaching electric field rules on how attractive force depends on quantity & distance apart of charges, learning notes for electric field rules on how attractive force depends on quantity & distance apart of charges, help to understand the electric field rules on how attractive force depends on quantity & distance apart of charges topic in preparation GCSE physics exam question, how to prepare for questions involving electric field rules on how attractive force depends on quantity & distance apart of charges in a GCSE physics examination?

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INDEX physics notes on static electricity - electrostatics & electric fields