Questions to think about
• What makes the bulb to glow
• What are the batteries for
• What does an electric circuit do
battery
+
-
The SI unit of electromotive force is the volt: 1V = 1J/1C
Duracell
1.5 V
The electromotive force of a source of electricity is
defined as the amount of work done by the source on
charged particles per unit charge passing the source.
(The source performs work against the electric field.)
Example 1:
The amount of work done by a 1.5 V battery on an anion with charge 1.6 ⋅ 10-19 C
is (1.6 ⋅ 10-19 C ) ⋅ 1.5 V = 2.4 ⋅ 10-19 J
Example 2:
The amount of work done by a 1.5 V battery on a cation with charge -1.6 ⋅ 10-19 C
is (-1.6 ⋅ 10-19 C ) ⋅ (-1.5 V) = 2.4 ⋅ 10-19 J
If no energy is lost in the battery, the electromotive force of the battery
is equal to the potential difference produced by the battery.
What happens when you connect a
wire across the leads?
• This creates an electric field whose field
lines are parallel to the wire and whose
strength is equal to voltage/(length of
wire). As a result, the electrons accelerate
opposite the field, from the negative end of
the battery to the positive.
• But, this is not a good thing to do!
flashlight
reflector
-
+
-
+
Duracell
1.5 V
bulb
conductive
strip
switch
Duracell
1.5 V
contact
spring
batteries
A combination of electrical elements, connected with each other by a
conductive paths (wires) is called a circuit.
If there is no closed path for the charged particle, the circuit is open.
If there is a closed path for the charged particle, the circuit is closed.
Moving charged particles transport energy from one element to
another.
electric (conduction) current
The flow of charge is described by a physical quantity called
(conduction) current.
I
Current across a surface is defined
as the rate at which charge crosses
the surface:
dq
I=
dt
I
By convention, the direction of
the current is consistent with the
flow of the positive charge.
Where dq is the total charge of all
particles, which crossed the surface
in a small amount of time dt
The SI unit of current is the ampere: 1A = 1C ⋅ 1s.
When 1 A flows in the wire, there are about 6,250,000,000,000,000,000
electrons passing through a cress sectional surface each second.
electric power
1.5 V
+
When charge dq passes an element,
the work done by the element is
equal to the product of the potential
difference across the element and
the charge
dW = ( Vf - Vi ) ⋅ dq
Duracell
1.5 V
The rate at which energy is
transferred is called the power
-
0V
1.50 V
dW
P=
dt
The watt is the SI unit of power: 1W = 1J/1s
The electric power delivered to an electrical element is equal to the product of the
potential drop across the element and the current through the element.
P=
dW
dq
= (Vf − Vi ) ⋅ = V ⋅ I
dt
dt