Transcript Name Symbol Units

Circuits
Current, Voltage and Resistance
TOP
Current
Batteries
Basic Circuit Elements
Resistors
1
Circuits
Current, Voltage and Resistance
Current is a measure of the movement of charge
TOP
2
Circuits
Current, Voltage and Resistance
TOP
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.
Circuits
Current, Voltage and Resistance
TOP
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
4
Circuits
Current, Voltage and Resistance
This is how an alkaline cell (AA battery) works.
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Zn + 2 OH- —> ZnO + H2O + 2 e2 MnO2 + H2O + 2 e- —>Mn2O3 + 2 OH-
Cathode (MnO2)
Mn2O3
MnO2
MnO2
Anode (Zn)
ZnO
OH
HOH
O
2 - Zn
Zn + 2MnO2 —> ZnO + Mn2O3 + 1.5 V
5
Circuits
Current, Voltage and Resistance
TOP
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 (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
-
Circuits
Current, Voltage and Resistance
TOP
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
Circuits
Current, Voltage and Resistance
TOP
8
Circuits
Current, Voltage and Resistance
Resistors slow down the electrons
TOP
I
9
Circuits
Current, Voltage and Resistance
Resistors slow down the electrons
TOP
+
-
-
10
+
Circuits
Current, Voltage and Resistance
TOP
In most instances, the voltage across a piece of material is
proportional to the current that flows through it
The constant of proportionality is called resistance, R, and has
units of Ohms (Ω)
This is known as Ohm’s law and is usually written as
V  IR
In a circuit diagram we draw resistance like this
R
11
Circuits
Current, Voltage and Resistance
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As area increases, more electrons
pass through it. Thus, the current is
greater and the resistance is smaller
As length increases, the electrons are
blocked for a longer time, slowing them
down even more. Thus, the current is
smaller and the resistance is greater
For many materials, the resistance is proportional to the length of
the resistor and inversely proportional to the cross-sectional area
L
R
A
The constant of proportionality is called resistivity, ρ, and is
different for different materials
12
Table 28.2 in the text gives resistivity for some materials
Circuits
Current, Voltage and Resistance
Power is defined as the change in energy per unit time
TOP
In the case of electric potential energy, we can write
U qV
P

 IV
t
t
The power may be written in different forms using Ohm’s law
P  IV
PI R
2
13
V2
P
R