Lesson 4 of 5
Electric fields and force direction
I can describe an electric field as a region where a charge experiences a force.Syllabus
Syllabus K323, 13(c). Describe an electric field as a region where a charge experiences a force.
I can draw the field of an isolated point charge and use the force on a positive test charge to define its direction.Syllabus
Syllabus K323, 13(d). Draw the field of an isolated point charge and use the force on a positive test charge to define its direction.
I can draw the field patterns for two point charges of like or unlike sign.Syllabus
Syllabus K323, 13(e). Draw the field patterns for two point charges of like or unlike sign.
Make a guess. You are not marked.
A small positive charge is released near a negatively charged sphere. Which way does it move?
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Towards the sphere, along the field lines, which point inwards.
Field lines show the direction of the force on a positive charge. Near a negative sphere they point inwards, so the positive charge is attracted towards it.
An electric field is a region in which an electric charge experiences a force. A field diagram describes the effect of the source charges at different positions.
Unlike charges attract and like charges repel. A field-line arrow uses one fixed convention: it points in the direction of force on a positive test charge.
Draw the required point-charge patterns
A point-charge model treats the source's size as negligible compared with the distances being considered. For the test-charge interpretation, imagine a charge small enough not to change the source arrangement significantly.
A field arrow gives the force direction on a positive test charge
These are schematic field lines, not visible threads or particle tracks. Lines that reach a frame continue beyond the drawing. Their number is not a supplied measure of field strength.
One isolated positive charge
One isolated negative charge
Equal unlike charges
Two equal positive charges
Two equal negative charges
- One isolated positive charge
- Draw radial lines pointing outwards. A positive test charge is repelled from the source.
- One isolated negative charge
- Draw radial lines pointing inwards. A positive test charge is attracted towards the source.
- Two equal unlike charges
- Lines run from the positive charge towards the negative charge, with curved lines above and below the line joining them. Arrowheads must agree with the positive-test-charge convention.
- Two equal positive charges
- Lines leave both charges and curve away from the middle region. They do not connect positive to positive.
- Two equal negative charges
- The pattern has the corresponding shape to two equal positive charges, with arrows reversed to point into the negative charges.
Field lines do not cross: crossing would give two different field directions at one position. Lines may leave the edge of a diagram to represent a field extending beyond the drawing; they should not simply end at an unlabelled point in empty space. At a position where the resultant field is zero, there is no nonzero field-direction arrow to draw.
The lines are a representation, not visible threads around the charges. Their drawn number is not a count of particles. Do not infer an exact numerical field strength by counting an illustrator's lines.
A negative charge has the opposite force direction
Keep the field direction fixed when changing the test charge
Green arrows show the supplied field; orange arrows show electric force. Blue arrows or paths show motion. A force direction need not be the current direction of motion.
Apply the same rule to a particle separator
The field changes sideways motion. It does not instantly replace the particles' upward motion. Gravity and air resistance are ignored in this comparison.
Read the convention
The field arrow points right at a marked point
- A positive test charge has electric force right.
- A negative test charge has electric force left.
Changing the test-charge sign reverses its force. It does not reverse the existing field established by the source charges.
Force changes motion; it does not specify the velocity an object already has. A charge moving left can experience force right, so it may initially slow before reversing. If electric force is the only unbalanced force, its direction gives the acceleration direction, consistent with resultant force and acceleration.
Apply a supplied field to a new arrangement
In the separator diagram, positive and negative particles enter moving upwards. The field points left. Ignore gravity and air resistance for this comparison.
The positive particles experience force left and begin to bend towards a left collector. Negative particles experience force right and begin to bend towards a right collector. Both initially continue moving upwards while their horizontal velocities change. The force does not instantly turn either velocity through a right angle.
A field line is not automatically a moving particle's path. It shows force direction for a positive test charge at each point. The charge's sign, existing velocity and other forces also matter when predicting its motion.
Check your understanding
An existing electric field points to the right at a marked position. A small negative test charge is placed there without significantly changing the field. Which statement is correct?
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Its electric force points left, while the existing field still points right.
Changing the test-charge sign reverses its electric force, not the field established by the source charges. This force can change the charge's motion; it does not specify its current velocity.
Practise an exam question
Write your answer first, then tick the mark points it makes.
Exam-style question
Define an electric field. Sketch the electric field lines around an isolated, positively charged sphere, and between two parallel metal plates, one positive and one negative. [3]
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