3. Non-Ohmic Devices
Some of the most interesting devices do not follow Ohm’s Law
Diodes are devices that let current through one way much more easily than the other way
Superconductors are cold materials that have no resistance at all
They can carry current forever with no electric field

4. Power and Resistors
The charges flowing through a resistor are having their potential energy changed Q
Where is the energy going?
The charge carriers are bumping against atoms
They heat the resistor up V

5. Uses for Resistors You can make heating devices using resistors
Toasters, incandescent light bulbs, fuses
You can measure temperature by measuring changes in resistance
Resistance-temperature devices
Resistors are used whenever you want a linear relationship between potential and current
They are cheap
They are useful
They appear in virtually every electronic circuit

6. Direct Current (DC) Circuits
Circuit Basics
These circuit elements and many others can be combined to produce a limitless variety of useful devices
wire
open switch
closed switch
2-way switch
Two devices are in series if they are connected at one end, and nothing else is connected there
Two devices are in parallel if they are connected at both ends + –
ideal battery
1.5 V
47 F
capacitor
4.7 k
resistor

7. Resistors in Parallel and in Series
When resistors are in series, the same current must go through both of them
The total voltage difference is
The two resistors act like one with resistance
When resistors are in parallel, the same potential is across both of them
The total current through them is
The two resistors act like one with resistance

8. Parallel and Series - Formulas
Capacitor
Resistor
Inductor*
Series
Parallel
Fundamental Formula
* To be defined in a later chapter

9. The Voltage Divider
Many circuits can be thought of as a voltage divider
Intentionally or unintentionally
What’s the voltage drop across each of the resistors? R1 + – E R2
The larger resistor gets most of the voltage + –
120 V
If Mr. Curious has a resistance of 10 k and the light bulb has a resistance of 240 , how bright is Mr. Curious?
Not very bright

10. Ideal vs. Non-Ideal Batteries
Up until now, we’ve treated a battery as if it produced a fixed voltage, no matter what we demand of it
Real batteries also have resistance
It limits the current and therefore the power that can be delivered
If the internal resistance r is small compared to other resistances in the problem, we can ignore it E + –
ideal battery + – E r
realistic battery
A 30 V battery with 10  of internal resistance is connected to a 50  resistor. What is the actual voltage across the 50  resistor? + –
30 V
10 
50 

11. Kirchoff’s Laws
12 V I1
Kirchoff’s Laws help us figure out where and how much current is flowing in a circuit
The first step is to assign a direction and a current to every part of a circuit
Items in series must have the same current in them
Then you apply the two laws, which can be thought of as conservation of charge and conservation of voltage, which you apply to vertices and loops respectively. + –
3  I2 + –
5 
6 V
Kirchoff’s First Law:
The total current into any vertex equals the current out of that vertex
Kirchoff’s Second Law:
The total voltage change around a loop is always zero
4  I3
These yield a series of equations, which you then solve