A physical model of current
Defining and describing current
Lesson 1:
ELECTRICAL CURRENT
Announcements
HW 1 Current due Tuesday
Photoelectric Lab simulation due today
5pm by email to mperkins@ortn.edu
No pity, no excuses, no sob stories…
NO MERCY!!!! 
Exam: Electrostatics – Monday 3/4/2013
Lesson 1: Introduction to Current
AP Physics B Objectives
III.C.1. Current, Resistance, & Power
a) Students should understand the definition of electric current, so
they can relate the magnitude and direction of the current to the
rate of flow of positive and negative charge.
Student Objectives
Students will be able to
1) Define electric current.
2) Calculate the electric current in a circuit.
3) Apply conservation of charge to junctions in wires.
A physical model of electrical
current
Current is the flow of electrical charge from
one point to another…specifically positive
charge!
Charges flow in response to an electric field.
What are charge carriers?
In a normal electrical circuit with metal wire, it
is the electrons that are the “charge carriers”.
So if the electrons move one way, which way
does the current move?
How is current created?
Current is created when an electric field
exists in a conductor.
The loosely held electrons in the conductor
respond to the electric force, and move
opposite the direction of the field.
To sustain the current, the electric field must
be sustained. Batteries and capacitors do a
good job of providing an electric field.
What do we mean by
conservation of current?
Charge cannot be
created or destroyed.
The flow of charge
cannot be created or
destroyed, either.
Current cannot be
created or destroyed.
Mathematical definition of
current
Current is defined as the flow of positive
charge.
I = Q/t



I: current in Amperes or Amps (A)
Q: charge in Coulombs (C)
t: time in seconds
Sample Problem 1.1:
How many electrons per hour flow past a point in a circuit if
it bears 11.4 mA of direct current? If the electrons are
moving north, in which direction is the current?
Mathematical definition of
current conservation
A point where wire branches meet is
called a junction.
The sum of currents entering a junction
equals the sum of currents leaving the
junction.
Iin = Iout
or
Iin – Iout= 0
Sample Problem 1.2:
Find the missing currents
a)
8A
15 A
?
b)
2A
4A
?
Sample Problem 1.3:
Find the missing current.