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WELCOME TO PERIOD 14
Homework Exercise #13 is due today.
Watch video 3 Edison’s Miracle of Light for
class discussion next Tuesday or Wednesday.
PHYSICS 1103 – PERIOD 14
•What is an electric circuit?
•How do capacitors store energy?
•Why are capacitors useful?
•Remember to put away your phone.
No calls or texting during class.
Tin can voltmeter
Place negative charge on the inner Leyden jar.
9000 V
0V
Measuring voltage across a battery
1) Turn the dial to the voltage symbol
V
2) Attach the two wire leads to the two outlets
on the lower right of the meter.
V
V W
COM
3) To measure voltage across a battery, touch
the ends of the leads to each end of the
battery.
Measuring voltage across a bulb
1) Turn the dial to the voltage symbol
V
V W
COM
V
2) Attach the two wire leads to the two outlets on
the lower right of the meter.
3) To measure voltage across a bulb, connect
the ends of the leads to each side of the bulb.
Voltage boosts and drops
Voltage Boosts
The potential energy per unit of charge, or voltage,
increases when charge flows through a battery.
When batteries are connected in series, charges get a
voltage boost from each battery.
Voltage Drops
Voltage drops occur as current flows through load
devices (resistors) in the circuit.
The voltage boost from the battery is divided among
the load devices in the circuit.
The sum of the voltage boosts and drops in a closed
circuit are equal.
In a circuit, VOLTAGE BOOSTS = VOLTAGE DROPS
Electric charge does work
The attraction or repulsion between charged objects
results in an electrical force on the objects.
Electrical forces store electrical potential energy
when work is done to push charges of the same sign
together.
Electrical potential energy is converted into electrical
energy when the charges are allowed to move apart.
This electrical energy can be used to do work, such
as lighting a bulb.
Electric circuits
A closed circuit has a complete conducting pathway.
A closed circuit allows current
to flow through the circuit and
light the bulb.
Current cannot flow through
an open circuit. The bulb does
not light.
Filament
Conducting
pathways
Insulating ring
Example of an electric circuit
Negative charge flows from the inner Leyden jar, through the
bulb filament, and onto the positively charged outer jar.
This flow of charge is
an electric current.
Electric current
Current =
amount of charge moved
time
I 
Q
t
I = current (in amperes)
Q = charge (in coulombs)
t = time elapsed (in seconds)
Measuring current through a bulb
1)Turn the dial to the current symbol ( A ).
2) Place one wire lead in the bottom right
outlet (marked “COM”). Place the other lead
in the top left outlet (marked “10 A”).
A
3)To measure the current through the bulb,
clip the ends of the leads into the circuit.
10 A
COM
Storing charge on capacitors
• A capacitor stores positive and negative charge on metal plates.
• An insulating layer keeps the positive and negative charges
separated.
• The charge-holding capacity of a capacitor is called its
capacitance.
• Capacitance is measured in units of farads.
• The greater the capacitance, the more charge Q can be stored at
voltage V
Q

C V
Q = charge (in coulombs)
C = capacitance (in farads)
V = voltage (in volts)
A capacitor example: Leyden jars
Positive charge is stored on the outer metal Leyden jar and
negative charge is stored on the inner jar. The insulating plastic
cup keeps the positive and negative charge separated.
Separating Leyden jars
When Leyden jars
are separated, the
voltage across the
jars increases.
Predict what will happen
to the capacitance when
the jars are separated.
Hint: Q = C V
Capacitors vs. batteries (Activity 6)
b) Charge per capacitor: Q = C V = 1 farad x 1.5 V
= 1.5 coul
c) Battery charge per hour:
0.8 coul x 3,600 sec = 648 coul
sec
hour
hour
d) How many charged capacitors equal the energy
provided by one battery in one hour?
648 coul x 1 capacitor = 432 capacitors !!!
hour
1.5 coul
hour
Capacitor discharge
• Capacitors discharge their stored charge exponentially.
• The amount of charge in the capacitor decreases by ½ during
each time period.
Charge Q (in microcoulombs)
1800
1600
1400
1200
1000
800
600
400
200
0
0
3
6
9
Time (in seconds)
12
15
Electrical potential energy of a capacitor
Electrical potential energy is the product of the amount of
charge and the voltage of that charge.
Epot
= Q V
But the electrical potential energy of a capacitor is:
Ecap
= ½Q V
Why?
• The equation uses the average voltage of the charges.
• Initially, the voltage of the charges is very low. As the
capacitor fills, the voltage increases.
• The factor of ½ in the equation comes from taking the
average of the initial and final voltage of the charges.
Capacitor discharge
a) Does the capacitor discharge more quickly through the
rod or through the bulb?
b) In which case is more energy released?
c) In which case is more power produced?
Power = Energy
time
BEFORE THE NEXT CLASS…
Read textbook chapter 15
Complete Homework Exercise 14
Bring a blank Activity Sheet 15 to class.
Watch video 3 Edison’s Miracle of Light for
discussion during period 15