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Electric fields

Any electrically charged object will have an electric field around it. The strength of the force another charge experiences inside the electrical field is a measure of the strength of the electric field.

Charges can be point, plate, or spherical. These all have 'force lines', indicating the direction of force a positively charged particle experiences.

The direction of the force is the direction of the electric field lines. The arrow on a field line indicates which direction a positive charge brought into the field will move.

Spherical charges

Spherical charge
Spherical charges have electrical fields like the spokes of a bicycle wheel

If a metallic sphere is negatively charged, the lines of force point towards the sphere, indicating that a positive charge released near the sphere would be attracted to the sphere.

With a positively charged sphere, the field lines point away, indicating that a positive charge would be repelled.

Negative charges move in the opposite direction to the field lines, since they are attracted by positive charges.

The force F is inversely proportional to the square of the distance r separating the charges Q and q:

$$F = qE = k{Qq}/{r^2}$$

where k is Coulomb's constant, $9.0 × 10^{9} N m^2 C^{-2}$, and E is the strength of the electric field at that point.

Point charges

Point charges
Point charges: lines of force indicate which direction a positive charge would move

A point charge is like a very small sphere. It makes it easier to understand fields if we imagine the force starting from a single point, rather than spread across a large surface.

Just like a sphere, a single point charge will have electric field lines coming out from it like the spokes of a bicycle wheel.

Now, what happens if two point charges are brought close together? What do the electric field lines look like then?

If the two charges are opposite (positive and negative), then the electric field lines pass from the positive charge to the negative charge. The lines are curved, because the force depends on how close the field is to the charges. Near the positive charge, there is more push force from the positive charge than pull force from the negative.

Plate charges

Parallel plates
Plates with opposite charges have an electric field between them that never changes intensity

A flat metallic plate can be charged on one side, to create an electric field that decreases in strength the further away from it a charge is.

However, if there are two plates of opposite charge close to each other, an electric field is generated between them that does not change in strength anywhere between the plates.

Two parallel plates are called a capacitor. The capacitor is an electronic device that helps control current and voltage in a circuit.

The Maths

The strength of an electric field is:

$$E = F/q$$

where F is the force (newtons N) and q is the charge (coulombs C). The unit of field strength is therefore N/C.

A coulomb is a number of charges: 6.241 × $10^{18}$ electrons make a charge of -1.0 C, and 1.6 × $10^{18}$ protons make a charge of +1.0 C.

Content © Andrew Bone. All rights reserved. Created : April 10, 2014

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