Electric field
The force per unit of positive test charge at a point, E = F/q. For a point charge E = k q / r squared.
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Analogy
A weather map that shows how strongly a charge would be pushed at each spot.
Example
E = k q / r^2E = 8.99e9 x 2e-9 / 0.25E = 71.9 N/C
In practice
A 2 nC charge makes a field of about 72 N/C at 0.5 m.
Practice
Flashcards
- What is the SI unit of electric field?Newtons per coulomb (N/C), the same as volts per metre.
Interview questions
- Use Gauss's law to find the field outside a charged conducting sphere of charge Q and radius R.
- Choose a spherical surface
- Field is the same size everywhere on it
- E x 4 pi r^2 = Q / epsilon0 gives E = kQ/r^2
Questions people ask
What is a test charge?
A test charge is a tiny positive charge used to probe a field. It is small enough that it does not disturb the charges creating the field. The field at a point is the force on the test charge divided by its charge, so it does not depend on the test charge.
What does the field of a dipole look like?
A dipole is a positive and a negative charge close together. Field lines leave the positive charge and curve around to enter the negative one. The field falls off faster than for a single charge, as 1/r cubed far away, because the opposite charges nearly cancel.