Applications of Electric Fields Lesson 4

Applications of
Electric Fields

Lesson 4

Focus Question

What is a capacitor and how does it work?

New Vocabulary

electric potential difference
volt
equipotential
capacitor
capacitance

Review Vocabulary

work: the transfer of energy that occurs when a
force is applied through a distance; equal to the
product of the system’s displacement and the
force applied to the system in the direction of
displacement

Energy and Electric Potential

• The electric potential difference (ΔV), which often

is called potential difference, is the work (Won q’)
needed to move a positive test charge from one
point to another, divided by the magnitude of the
test charge.

Electric Potential Difference

Multiple Choice

What is the electric potential difference?

• The transfer of energy that occurs when a force is applied through a distance

• The work needed to move a positive test charge from one point to another, divided by the magnitude of the test charge

• The change in electric potential energy per unit charge

• The product of the system's displacement and the force applied to the system in the direction of displacement

Energy and Electric Potential

Energy and Electric Potential

You also can think of electric potential difference as the
change in electric potential energy (ΔPE) per unit
charge.

Energy and Electric Potential

• ΔV is measured in joules per coulomb (J/C). One (J/C)

is called a volt (V).

• Whenever the electric potential difference between

two or more positions is zero, those positions are
said to be at equipotential.

Multiple Choice

What does it mean when two positions have an electric potential difference of zero?

• They have the same electric potential energy

• They have the same electric field strength

• They are at equipotential

• They have the same charge

Multiple Choice

Which unit is used to measure electric potential difference?

• Joules per coulomb

• Newtons per meter

• Farads

• Volts

Electric Potential in a Uniform Field

• You can produce a uniform electric field by placing

two large, flat conducting plates parallel to each
other.

• One plate is charged positively, and the other plate is

charged negatively.

• The magnitude and the direction of the electric field

are the same at all points between the plates, except
at the edges of the plates, and the electric field
points from the positive plate to the negative plate.

Multiple Choice

How can a uniform electric field be produced?

• By placing two large, flat conducting plates parallel to each other

• By connecting a power supply to a capacitor

• By charging a conductor with excess electrons

• By using a magnetic field

Electric Potential in a Uniform Field

• The electric potential is higher near the positively

charged plate and lower near the negatively
charged plate.

• You can represent the electric potential difference

(ΔV) between two points a distance (d) apart in a
uniform field (E) with the following equation.

Electric Potential Difference
in a Uniform Field

Millikan’s Oil-Drop Experiment

• Robert Millikan performed an experiment to test

whether charge exists in discrete amounts.

• Millikan was able to measure the magnitude of

the elementary charge with his experiment.

• His results showed why the net charge on an

object must be some integer multiple of the
elementary charge.

Millikan’s Oil-Drop Experiment

Millikan used two parallel plates to produce a
uniform electric field in his apparatus.

Millikan’s Oil-Drop Experiment

• Millikan sprayed fine oil drops from an atomizer.

The drops were charged by friction.

• Earth’s gravitational force pulled the drops

downward. A few drops entered the hole in the
top plate.

• Millikan adjusted the electric field between the

plates until the downward force from Earth’s
gravitational field and the upward force from the
electric field were equal in magnitude.

Multiple Choice

What did Robert Millikan's oil-drop experiment demonstrate?

• The existence of discrete amounts of charge

• The relationship between electric potential and charge

• The behavior of electric fields near conductors

• The storage of electrical energy in capacitors

Electric Fields Near Conductors

• Because electrons have like

charges, they repel each other. In a
conductor they are free to move,
so they spread far apart in a way
that minimizes their potential
energy.

• The charges come to rest on the

surface of the conductor. It does
not matter if the conducting
sphere is solid or hollow.

Electric Fields Near Conductors

• The electric field is zero

everywhere inside a closed,
charged metal container.

• The electric field at the surface

depends on the shape of the
conductor; free charges are
closer together at the sharp
points of a conductor.

Capacitors

• Energy can be stored in an electric field. A device

for storing electrical energy is called a capacitor.

• If you connect a 1-V power supply across a

capacitor, the potential difference between the
two plates would be 1 V.

• This would result in a net positive charge (+q) on

one plate and a net negative charge of equal
magnitude (-q) on the other plate.

Multiple Choice

What is the purpose of a capacitor?

• To produce a uniform electric field

• To store electrical energy

• To measure electric potential difference

• To create an equipotential surface

Capacitors

• The graph of q v. ΔV

is a straight line.

• The slope of the line

in a net charge
versus potential
difference graph is a
constant and is called
the capacitance
(C) of the capacitor.

Multiple Choice

What is the relationship between net charge and potential difference in a capacitor?

• They are directly proportional

• They are inversely proportional

• They have no relationship

• It depends on the shape of the capacitor

Capacitors

• Capacitance is measured in farads (F),

where 1 F = 1 C/V.

Capacitance

Multiple Choice

What is the unit of capacitance?

• Joules per coulomb

• Newtons per meter

• Farads

• Volts

Multiple Choice

What is the typical range of capacitance for capacitors used in modern electronics?

• 1 F to 10 F

• 10 pF to 500 μF

• 1 μF to 10 μF

• 1 nF to 100 nF