1. Physics
Section 16-3
The Electric Field

2. History
Michael Faraday (1791–1867) was the son of a blacksmith, and started his career as a bookbinder’s apprentice. He used the opportunity to read many books on science. He became interested in chemistry and physics. Faraday proposed our current concept of the electric field.

3. Faraday never had the proper education given to an English gentleman
Faraday never had the proper education given to an English gentleman. As such, his mathematical skills were basic. However, he was a brilliant experimentalist. He discovered, or laid the ground work for, electric and magnetic fields, electromagnetic induction, diamagnetism, the laws of electrolysis, electrochemistry (he popularized the terms ion, anode, cathode, and electrode), electric motors and generators, and organic chemistry (he discovered benzene for example).
He has not one but two units named for him — the farad (F), the unit of capacitance, and the faraday, the charge of one mole of electrons.
(1 faraday = C)

4. Field lines
Faraday imagined invisible lines of electrical force emanating from charged particles or metal plates.
The convention is to imagine a positive test charge within the field. It will move along a path toward a negative charge, or away from a positive charge. + F + F q0 q0 q q

5. Analogy to gravity + + + + + + + + + + + + + + Fg Fe Fe Fg m q0 q0 m
Gravity only attracts, while charge can either attract or repel. However, there are a number of analogs between gravity and the electric field and force.
gravity
electric field + + + + + + + + + + + + + +
gravitational field
electric field Fg Fe P g = P + E = m q0 q0 m
in m/s2
in N/C Fe Fg
or N/kg – – – – – – – – – – – – – –

6. Electric Field StrengthFe
lim E =
units: N/C q0 q0
Remember, there is an electric field strength at a point created by a charge, even if no test charge exits!
From Coulomb’s Law:
Fe = r2
q q0 ke P q r2
q q0 ke q Fe E E = = = ke q0 q0 r2

7. Fe E = q E = ke r2 + q0 (5.0 × 10–4 N) = E = 500 N/C (1.0 × 10–6 C)
1. A positive charge possesses an electric field around it. A positive test charge q0 = +1.0 × 10–6 C experiences a force of 5.0 × 10–4 N on it. What is the magnitude and direction of the electric field strength at the location of the test charge? + E = q0 Fe
(5.0 × 10–4 N) = E =
500 N/C
(1.0 × 10–6 C)
away from the positive charge
2. Calculate the magnitude and direction of the electric field strength at a point P which is 30 cm to the right of a point charge q = –3.0 × 10–6 C. E = q r2 ke
(3.0 × 10–6 C) = E
(9 × 109 Nm2/C2)
(0.30 m)2
E =
3 × 105 N/C
toward charge q

8. Field lines for multiple charges

9. Visualizing the field
Oppositely charged electrodes placed in a liquid containing polymer chain molecules that align with the field.

10. Charges on conductors
The excess charges in the left sphere repel each other.
When they touch the conductors share charge evenly.
Potential (V) is a measure of charge density. A small sphere with the same charge as a large one will have a higher potential. When they touch, the potential equalizes, so charges move from high to low potential until electrostatic equilibrium is reached.
Charge is evenly distributed on the surface of a sphere. [a]
Charge only exists on the outer surface of a hollow conductor. [b]
Charge collects at points. [c]