1. Foundations of Physical Science
Unit 3: Electricity and Magnetism

2. Chapter 8: Electrical Relationships
8.1 Ohm's Law
8.2 Work, Energy, and Power

3. Learning Goals Measure how current changes when voltage is increased.
Measure how current changes when resistance is increased.
Describe how voltage, current, and resistance are related.
Use Ohm’s law to solve circuit problems.
Explain why resistors are used in a circuit.
Define power as the rate at which energy flows.
Describe relationships between work, energy, and power.
Calculate power use in a circuit.
Rank the amount of power used by various household appliances.
Estimate the cost per month of using a common household appliance.
Use dimensional analysis to find out what we buy from electric utility companies.
Explain how to choose a safe extension cord.

4. Vocabulary horsepower kilowatt kilowatt-hour Ohm’s law potentiometer
resistor
watt

5. 8.1 Ohm’s Law How are voltage, current, and resistance related?
German physicist Georg Ohm experimented with circuits to find an exact mathematical relationship between voltage, current and resistance.
Ohm's Law can be used to predict any one of the three variable if given the other two.

7. Ohm’s Law: I = V/R Current and voltage are proportional to each other
Current and resistance are inversely proportional to each other

8. Example
How much current flows through a lamp with a resistance of 60  when the voltage across the lamp is 12 V?
V/R = I
12 V/60  = 0.2 A

9. Example
What is the resistance of an electric frying pan that draws a current of 12 A when connected to a 120 V circuit?
Rearrange Ohm’s Law:
R = V/I
120 V/12 A = 10 

11. Graphing and Ohm’s Law
A current vs. voltage graph shows us if resistance changes
A simple resistor obeys Ohm’s law-its current vs. voltage graph is a straight line
Resistance is the same at all values of voltage and current
For a diode, the graph is not a straight line
A diode only allows charge to flow in one direction!
The current is zero when voltage is negative
Diodes are made from semiconductors

12. Temperature and Ohm’s Law
Resistance can change when it is cooler or warmer
Atoms gain energy when they are heated up
With extra energy, the atoms move around more
Extra collisions mean that hot metal has more resistance than cold metal

13. Resistors Components that are used to control current in many circuits
Made from materials that keep the same resistance over a wide range of temperatures and currents
Colored strips tell the resistance of each resistor

14. Resistors Fixed: always has the same value Variable: the amount varies
3 way light switch
Each setting connects the circuit to a different resistor; 3 levels of current control the brightness of the bulb
Variable: the amount varies
Dimmer switch
As you turn the dimmer from low to high, it changes resistance, which also changes current

15. Electrical Controls
Potentiometer: a variable resistor that can change from low to high.
They are wired so that as you turn the knob, it changes the distance the current has to flow.

16. Work, Energy, and Power
How much does it cost to use the electrical appliances in your home?

17. Electric Power Power = current x voltage
When current is in amperes and voltage is in volts, then power is expressed in watts
Watts = amperes x volts

18. Paying for Electricity
You are not charged by the volt, amp or ohm
You are charged by the watt (or kilowatt)
Power: the rate at which energy is flowing
Watt (W): a unit of power
We use watts as a standard unit because energy can be converted into many different forms.

19. Where does electrical power go?
Transformed into different forms
Light
Motion
Electrical
Thermal (heat)
The watt is still used to describe the flow of energy in these cases

20. Kilowatt-hour Utility company charges by kilowatt-hour
You can estimate the cost of running an appliance for a period of time

21. Example

22. Electricity, Power, and Heat
How do you get more power when you need it?
Increase voltage: disadvantage? Standard outlets are 120 volts and this is hard to change
Increase current: disadvantage? More energy transformed into heat; wire could melt and start a fire!

23. Electricity, Power, and Heat
What do we do?
Decrease resistance, more current can flow with less voltage change along the wire
Less power is lost as heat
Wires come in different sizes (thick wires has lower resistance and can carry more current than a thin wire)
Length of a wire also affects resistance (longer-more resistance)

24. Electricity, Power, and Heat