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Principles of Physics
So far, we’ve talked about non-moving charge
We know that charged objects :
 exert an electrostatic force on each other
 move when the force is unbalanced
 gain and lose energy as they move toward and
away from other charged objects
Moving toward a like charge → increase in PE
 Moving away from like charge→ decrease in PE

We also know that a voltage exists between charged
objects


Charged objects are not useful unless moving
They move if there is a voltage
Circuit
A system in which charged objects move in response to a
voltage source
Must include:
voltage source (battery)
closed path for charges to follow (wires)
method of controlling rate of flow (resistor)
Current (I): flow of electrons (negative charge)
In circuits charge is moving in solid materials
(copper wires)
 Only electrons can move in solids

(Positive charge may flow in liquids or gases)
Current/electron flow is calculated by
I=q
t
I = Current
q = charge (C)
t = time (s)
Current units: 1 C/s = 1 Ampere = 1 Amp = 1 A
+
Voltage supply (power supply)
Resistor
Wire
Electrons flow from the negative side of the
voltage supply (the short side)
-
+
Conventional current flows
from the positive side of the
voltage supply (the long side)
Junction: where multiple wires (paths) meet
All current flowing into a junction = all current
flowing out of a junction.
Current flowing in:
1.5 A + 2.5 A = 4 A
1.5 A
1.0 A
2.5 A
3.0 A
Current flowing out:
3.0 A
IN = OUT
*Always add x to the side with less current
4 A = 3.0 A + x



When one electron leaves from the negative
side of the voltage supply another one enters
the positive side
The rate that electrons leave is current
How fast they leave depends on resistance in
the circuit

A material with high resistance typically causes
the electrons to experience more collisions as
they move through
RESISTOR
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Resistance, R, is related to:
•Type of material
•Some materials have more resistance than others
•Length
• Electrons will experience more collisions as they
travel through the longer resistor
•Cross sectional area
• Increasing cross sectional area decreases
resistance because more electrons can get
through at one time
• Temperature
• In metallic conductors, as temperature
increases, resistance increases