Electrostatics

• Two types of electric charge:– Negative (excess electrons)– Positive (lacking electrons)

• e = elementary unit of charge

= magnitude of charge on an electron

= 1.60 x 10-19 Coulomb

Types of materials

• Conductors: electric charges move freely (ex. Metals)

• Insulators: Electric charges do not move freely (ex. Plastic, rubber, glass, etc)

• Semiconductors: Mix of conductor and insulator properties (silicon)

• Superconductors: electric charges move without resistance.

Particle Mass Electric Charge

Electron me=9.110x10-31 kg q=-e

q=-1.60x10-19 C

Proton mp=1.673x10-27 kg q=+e

q=+1.60x10-19 C

Neutron mn=1.675x10-27 kg q=0

q=0 C

Coulombs Law

The electric force between two point charges is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them, and directed by the line joining the two charges.

€

Fe =kq1q2

r2

Electric force

Charge 1

Charge 2

Radius

8.99 x 109 N m2 C-2

Determine the force on q1, as shown above

Electric field

• A region in space surrounding a charged object in which a second charged object experiences an electrical force.

• Similar to gravitational field around a planet…but the mass is smaller and the force is stronger

Diagram symbols

+ charged sphere

+ point charge

- point charge

Rules of electric field lines

1. Lines never cross2. Show the direction of force on a small

positive test charge3. Out of positive, toward negative4. Direction of electric field is tangent to field

lines5. Density of field lines is proportional to field

strength (density=intensity)6. Perpendicular to surface

Draw the field lines, assuming +1 and -1

Calculating

eFEq

= eF Eq=

Electric Field

Force

Charge

Electric Field

€

E =k

Qq

r2

q=k

Q

r2

Charge creating the field

Charge affected by the field

Electric fieldradius

€

E =kQ

r2eFEq

=eF Eq=

€

Fe =kq1q2

r2k = 8.99 x 109 N m2 C-2

Summary