1. Voltage, Current, Charge and Energy
Introduction
Voltage, Current, Charge and Energy
(Sections 1.3 – 1.5)

2. Voltage, Current Charge and Energy
Consider potential energy of a mass held at a height m h g
E = m g h
Notice that height is a double-ended measurement
It is the distance between two points
We say it is an across variable
11/16/2018
Voltage, Current Charge and Energy

3. Galileo & the Falling Bodies Experiment

4. Same two masses
Different
place
speed
time
energy

5. Mass and place are separable
E = m x g h
Together represent place
gh represents the mechanical potential of a location independent of the mass
for example, the top of Prescott St.
We may write
gh = E / m
Or even
gh = dE / dm

8. The fundamental equation of gravity
Gravity is the property that defines mass
F =
G m1 m2 r2
All our other equations derive from this one
The fundamental equation of electricity
Electricity is the property that defines charge
F =
k q1 q2 r2
The equations are virtually identical!

9. The mechanical equivalent is the flow of mass
Voltage and Current
Voltage is the electrical equivalent of mechanical potential
It represents an electrical hill
v = dw dq
We get potential energy by placing some quantity of charge on top of the hill
v = voltage in volts
w = energy in joules
q = charge in coulombs
i = current in amperes
t = time in seconds
In circuits we also pay attention to the current - the amount of charge that flows per unit time
i = dq dt
The mechanical equivalent is the flow of mass

10. Voltage is an across measurement
It’s our electrical hill h
Height is measured as the difference between its top and its bottom
Current is a through measurement
It flows through a wire
Like water flows through a pipe

11. An ideal electrical componentv + - i 1 2
It represents positive charge flowing from terminal 1 to terminal 2
If i is positive
It represents positive charge flowing from terminal 2 to terminal 1
If i is negative
It is the voltage drop from terminal 1 to terminal 2
It is the voltage rise from terminal 2 to terminal 1
If v is positive or
It is the voltage rise from terminal 1 to terminal 2
It is the voltage drop from terminal 2 to terminal 1
If v is negative or