Transcript Slide 1

Lesson 4
 Charge
does not flow on its own. An electric
charge has a certain amount of electrical
potential energy because of the electric field
set up by the power supply.
 Work
is done by the power supply to increase
the electric potential energy of each
coulomb of charge from a low to a high
value.
 As
the charge flows through the load, its
energy decreases.
A
load converts electrical energy into another
form of energy. You can compare this to the
water flowing past a water wheel. The wheel
converts some of the energy of the water into
motion. The water has more energy before
the wheel than after the wheel.
 The
electrical potential energy for each
coulomb of charge in a circuit is called the
electric potential difference (V)
 Voltage
 The
formula can be rearranged
 Energy
 Charge
 Where
E is the energy required to increase
the electric potential of a charge, Q.
Potential difference is often called voltage.
 One
volt (V) is the electric potential
difference between two points if one joule of
work (J) is required to move one coulomb (C)
of charge between the points.
 What
is the potential difference across an air
conditioner if 72 C of charge transfers 8.5 x
103 J of energy to the fan and compressor?
Q
= 72 C
 E = 8.5 x 103 J
V = ?
V
= 1.2 x 102 V
 Therefore, the potential difference or
voltage in the air conditioner is 1.2 x 102 V
A
static electric shock delivered to a student
from a “friend” transfers 1.5 x 101 J of
electric energy through a potential
difference of 500 V. What is the quantity
charge transferred in the spark?
E
= 1.5 x 101 J
 V = 500 V
Q = ?
Q
= 0.03 C
 Therefore, the charge transfer between
friends is 0.03 C.
 Recall
that and
 So E = VQ and Q = It
 Therefore, E = VIt
 One
1.5 V (AA) battery runs a portable MP3
Player that draws 5.7 x 10-3 A of current for
about 6 hours before it runs out. How much
energy does the battery transfer?
V
= 1.5 V
 I = 5.7 x 10-3 A

E=?
E
= 185 J
 Therefore, the battery transfers 185 J of
energy
A
coffee maker draws about 5.0 A of current
for 270 s using 1.6 x 105 J of energy. What is
the potential difference across the coffee
maker?
 I = 5.0 A
 t = 270 s
 E = 1.6 x 105 J
 V =?
V
= 119 V
 Therefore, the potential difference across
the coffee maker is 119 V
 Potential
difference between any two points
can be measured using a voltmeter. A
voltmeter must be connected in parallel with
a load in the circuit in order to compare the
potential before and after the load.
 Electrical
energy always originates from
some other form of energy.
 Some common sources include:
 Voltaic cells – Chemical potential energy
released during a reaction as electrons are
driven between two different metals
 Piezo-electricity
– Crystals that produce a
small electric potential when a mechanical
force is placed on them.
 Thermoelectricity
– Two different types of
metal joined together and subjected to
temperature differentials.
 Photo
electricity – Light energy absorbed by
electrons of certain metals causes charge
flow.
 Electromagnetic
induction in generators –
Kinetic energy forces conductors to rotate in
a magnetic field.
Explain the difference between current and voltage.
You go to a store and buy a 12 V car battery. All the
batteries are 12 V, but they differ in cranking amps.
What should you look for in a winter battery?
A 12 V car battery delivers 1. 3 x 104 J of energy to
the starter
1.
2.
3.
A.
B.
C.
How much charge does it deliver?
How many electrons does the battery transfer
Given that you keep the key turned for 2.5 s (time to turn
the car over in order to start the car), how many amps are
delivered to the starter motor?
Lightning transfers charge between a charged cloud
and the ground. If the voltage difference between
two is 1.3 x 108 V and 3.2 x 109 J of energy is
transferred, find
4.
A.
B.
C.
The total charge moved between the two potential energy
surfaces.
The number of electrons making up the charge
The current delivered if the lightning takes 25 microseconds
to hit the ground.