circuits current, voltage and resistance 1 top current batteries basic circuit elements resistors
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CircuitsCurrent, Voltage and Resistance
1
TOP
Current
Batteries
Basic Circuit Elements
Resistors
CircuitsCurrent, Voltage and Resistance
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TOP
Current is a measure of the movement of charge
CircuitsCurrent, Voltage and Resistance
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TOPCurrent is defined as the amount of charge that passes through a surface in a given amount of time.
t
QI
The units of current are amps
The direction of the current is opposite to the flow of electrons. It may be thought of as the flow of positive charge.
Current always flows from higher potential to lower potential.
Q
CircuitsCurrent, Voltage and Resistance
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TOPA 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
CircuitsCurrent, Voltage and Resistance
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Zn + 2 OH- —> ZnO + H2O + 2 e-
2 MnO2 + H2O + 2 e- —>Mn2O3 + 2 OH-
Zn + 2MnO2 —> ZnO + Mn2O3 + 1.5 V
Cathode (MnO2) Anode (Zn)
H2OMnO2
MnO2 ZnOH-
OH-
ZnOMn2O3
This is how an alkaline cell (AA battery) works.
CircuitsCurrent, Voltage and Resistance
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Current is defined as the amount of charge that passes through a surface in a given amount of time
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
In a circuit diagram we draw it like this
The units of current are amps (A)
-+
V
I
+ -
t
QI
CircuitsCurrent, Voltage and Resistance
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The following circuit elements and their symbolic representation are the ones most commonly found in circuits.
BatteryV
- + V
Name Symbol Units
Resistor
Capacitor
Inductor
Switch
RΩ
CF
LH
S
CircuitsCurrent, Voltage and Resistance
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CircuitsCurrent, Voltage and Resistance
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Resistors slow down the electrons
I
CircuitsCurrent, Voltage and Resistance
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Resistors slow down the electrons
+-
+-
CircuitsCurrent, Voltage and Resistance
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In most instances, the voltage across a piece of material is proportional to the current that flows through it
This is known as Ohm’s law and is usually written as
The constant of proportionality is called resistance, R, and has units of Ohms (Ω)
IRV
In a circuit diagram we draw resistance like this
R
CircuitsCurrent, Voltage and Resistance
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For many materials, the resistance is proportional to the length of the resistor and inversely proportional to the cross-sectional area
Table 28.2 in the text gives resistivity for some materials
The constant of proportionality is called resistivity, ρ, and is different for different materials
A
LR
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
As area increases, more electrons pass through it. Thus, the current is greater and the resistance is smaller
CircuitsCurrent, Voltage and Resistance
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Power is defined as the change in energy per unit time
In the case of electric potential energy, we can write
The power may be written in different forms using Ohm’s law
IVt
qV
t
UP
IVP
RIP 2
R
VP
2