1. Physics 3 – Nov 20, 2018 P3 Challenge –
What is the drift speed of electrons in a 1.50 mm diameter copper wire that carries a 20.0 mA current? Copper has a density of free electrons of 8.5 x 1028 /m3.
Get out 5.1 #10-14 for HMK check

2. Objectives/Agenda/Assignment
5.2 Resistance
Assignment:
p225 #15-22
Agenda:
Homework Review
Ohm’s Law
Resistance
Power

3. V = IR In data booklet as R = V/I
Ohm’s Law
The current through a conductor is proportional to the potential difference across it. I V A plot of I vs V is a line with a positive slope.
V = IR In data booklet as R = V/I
R is the resistance of the conductor. Unit: Ohm, 

4. Ohm’s Law A plot of V vs I is a line with a slope = R
Many conductors obey Ohm’s law but not all do. We will only be dealing with those that obey Ohm’s law.
Graphs of V vs I are not always linear.
Examples: ideal Ohm’s law, real lamp, diode, thermistor

5. Resistance
Source of resistance are collisions of moving electrons with the atoms of the conductor.
The kinetic energy of the electrons moving (current) gets transferred to the wiggling of the conductor atoms (kinetic energy of atoms = temperature).
Resistance creates a heating in the conductor.

6. Resistance Factors Factors effecting resistance: R =  L/A
Nature of material  = resistivity of the material (tables exist)
Length (proportional)
Cross-sectional area (inversely proportional)

7. Sample Problems
The resistivity of copper is 1.68 x 10-8 m. What is the length of a copper wire with a diameter of mm with a resistance of 2.00 ?
What is the voltage across a 5.00  resistor when a A current passes through it?
What is the current through a 25.0  resistor when a 12 V voltage is placed across it?

8. Electric power P= IV All three forms are included in the data booklet.
Recall the work done when moving a charge through a potential difference: W = QV
Power = W/t = QV/t
I = Q/t
P= IV All three forms are included in the data booklet.
P= IV Sub V= IR P = I2R Recall Unit: Watt, W = J/s
P=IV Sub I = V/R P= V2/R

9. Electric Circuit Symbols
Wires are used to connect these elements and are
An alternate common symbol for a resistor is a zizag line:

10. Drawing Circuits
Circuit diagrams are drawn using only horizontal and vertical lines to create boxes.
Two basic ways multiple items can be connected:
Series: all along one path
Parallel: each along its own path
Notice: The longer line on the cell represents a higher potential.

11. Resistors in Series
When multiple resistors are connected in series, they can be combined together and replaced by an equivalent total resistor.
The resistance of total resistor is the simple sum of the resistors connected in series.
RT = R1 + R2 + R3 + R4
The current going through all resistors connected in series will be the same.

12. Resistors in Parallel
Resistors connected in parallel, can also be combined together and replaced by an equivalent total resistor.
The reciprocal of the resistance of total resistor is the sum of the reciprocal of the resistors connected in parallel.
𝟏 𝑹 𝑻 = 𝟏 𝑹 𝟏 + 𝟏 𝑹 𝟐 + 𝟏 𝑹 𝟑 + 𝟏 𝑹 𝟒
The voltage across all resistors connected in parallel will be the same.

13. Measuring Circuits
An ammeter is an instrument used to measure current.
Because current is constant in series, ammeters are placed in a circuit in series next to an element/wire you want to find the current for.
A voltmeter is an instrument used to measure potential/voltage.
Because potential is constant in parallel, voltmeters are placed in a circuit in parallel across an element you want to find the voltage for.

14. Solving Circuits  Or RT
Solving a circuit means to identify the current, potential, and resistance for each element shown.
It’s often convenient to organize given information and missing information to calculate within an Ohm’s law table:
Use Ohm’s Law, RT calculations to complete the table.
Element
Potential , V =
Current , I x
Resistance, R R1 R2
 Or RT

15. Sample problem Strategy:
1) Determine number of different currents. Label on diagram.
2) Create an Ohm’s law table
3) Solve what you can using Loop rule, Current rule, Ohm’s law or methods to find total resistances.

16. Exit slip and homework
Exit Slip – A 6  Resistor and a 4  Resistor are connected to a 12V power source in series. Sketch and solve the resulting circuit.
What’s due? (homework for a homework check next class)
Circuits Worksheet p225 #15-22
What’s next? (What to read to prepare for the next class)
Read 5.2 p