Download Physics 215: Physics for Engineers Part I

Physics 212 and 222
Current and Resistance
TOC
Current
Batteries
Basic Circuit Elements
1
Physics 212 and 222
Current and Resistance
•Current is a measure of the movement of charge
TOC
2
Physics 212 and 222
Current and Resistance
TOC
Current is defined as the amount of charge that passes through
a surface in a given amount of time.
Q
I
t
The units of current are amps
Q
The direction of the current is opposite to the flow of electrons.
It may be thought of as the flow of positive charge.
3
Current always flows from higher potential to lower potential.
Physics 212 and 222
Current and Resistance
TOC
A battery uses chemical reactions to cause a potential
difference between the terminals
+
-
The maximum value of the potential difference of a battery is
called its voltage
In a circuit diagram we draw it like this
V
+
4
-
Physics 212 and 222
Current and Resistance
This is how an alkaline cell (AA battery) works.
TOC
Zn + 2 OH- —> ZnO + H2O + 2 e2 MnO2 + H2O + 2 e- —>Mn2O3 + 2 OH-
Cathode (MnO2)
Mn2O3
MnO2
MnO2
5
Anode (Zn)
ZnO
OH
HOH
O
2 - Zn
Zn + 2MnO2 —> ZnO + Mn2O3 + 1.5 V
Physics 212 and 222
Current and Resistance
Current is defined as the amount of charge that passes
through a surface in a given amount of time
TOC
Q
I
t
The units of current are amps (A)
The direction of current is opposite to the flow of electrons
The current may be though of as the flow of positive charge
Current always flows from higher potential to lower potential
In other words, current flows from the positive terminal of the
battery through the circuit to the negative terminal
V
In a circuit diagram we draw it like this
6
+
I
-
Physics 212 and 222
Current and Resistance
TOC
The following circuit elements and their symbolic
representation are the ones most commonly found in circuits.
Name
Battery
Symbol
-
+
V
Resistor
Capacitor
Inductor
V
R
Ω
F
C
H
L
Switch
S
7
Units
Physics 212 and 222
Current and Resistance
TOC
Current
Batteries
Basic Circuit Elements
8