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## 9.4 Chapter summary (ESBPQ)

Presentation: 23ZB

• Objects can be positively, negatively charged or neutral.

• Coulomb's law describes the electrostatic force between two point charges and can be stated as: the magnitude of the electrostatic force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them.

$$F=\frac{k{Q}_{1}{Q}_{2}}{{r}^{2}}$$
• An electric field is a region of space in which an electric charge will experience a force. The direction of the field at a point in space is the direction in which a positive test charge would moved if placed at that point.

• We can represent the electric field using field lines. By convention electric field lines point away from positive charges (like charges repel) and towards negative charges (unlike charges attract).

• The magnitude of the electric field, E, at a point can be quantified as the force per unit charge We can write this as: $$E= \frac{F}{q}$$ where F is the Coulomb force exerted by a charge on a test charge q. The units of the electric field are newtons per coulomb: $$\text{N·C^{-1}}$$ .

• The electric field due to a point charge $$Q$$ is defined as the force per unit charge:

$$E= \frac{F}{q} = \frac{kQ}{{r}^{2}}$$
• The electrostatic force is attractive for unlike charges and repulsive for like charges.

 Physical Quantities Quantity Unit name Unit symbol Charge ($$q$$) Coulomb $$\text{C}$$ Distance ($$r$$) meters $$\text{m}$$ Electric field ($$E$$) Newtons per Coulomb $$\text{N·C^{-1}}$$ Force ($$F$$) Newtons $$\text{N}$$

Table 9.1: Units used in electrostatics