Learning Resource Topics Overview 1 Charge & Coulomb’s Law 2 Electric Current 3 Voltage 4 Resistance 5 Ohm’s Law 6 Power & Energy 7 Elements of Electric Circuits 8 Kirchhoff’s Law 9 Series Circuits 10 Parallel Circuits 11 Analysis of Combination Circuits 12 Capacitance 13 Inductance

# Charges and Electric Force

Although we regularly transfer charges between different parts of an electric circuit, we do nothing to change the total amount of charge. Clearly, we neither create nor destroy electrons or protons when operating electric circuits.

Moving charges represents an electric current. In a neutral state (zero charge), electrons will neither leave nor enter the neutrally charged body should it come in contact with other neutral bodies. If, however, any number of electrons is removed from the atoms of a body of matter, there will remain more protons than electrons and the whole body of matter will become electrically positive.

Should the positively charged body come in contact with another non-charged body, or having a negative charge, an electric current will flow between them. Electrons will leave the more negative body and enter the positive body. This electron flow will continue until both bodies have equal charges. When two bodies of matter have charges and are near one another, an electric force (F) is exerted between them.

The existence of such force, where current does not flow, is referred to as static. The force of attraction or repulsion (see Figure 1.1) exerted between two charged bodies is directly proportional to the product of their charges (Q) and inversely proportional to the square of the distance (d) between them.

This relationship between attracting or repelling charged bodies was first discovered by a French scientist named CHARLES A. COULOMB and accordingly is known as Coulomb’s Law, which can be expressed mathematically by the following equation

 (1.1)

where

F is a vector [Focus on Math] quantity, which represents the electrical force acting on charge Q2 due to charge Q1 measured in newtons (N)

â [Focus on Math] is a dimensionless unit vector with a unity magnitude pointing from charge Q1 to charge Q2, and

0 is a universal constant called the electrical permittivity of free space
[ 0 = 8.854 × 10-12 farad per meter (F/m)].

The two charges are assumed to be in free space (vacuum) and isolated from all other charges. Note: the arrows indicate the direction of the force vector F.

Figure 1.1 The force of attraction or repulsion

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