Name: __________________________________________ Date: ______/______/______614 Intro Physics

Notes: Electric Current, Circuits, Ohm’s Law and Power

Static Electricity vs. Electric Current: Static Electricity vs. Electric Current:

1. Previously, we learned about static electricity. Static electricity deals with charges that

are _______________.

2. Now we are going to electric current, which deals with ____________________ charges.

3. Which kind of charge moves: positive charges (protons) or negative charges (electrons)?

Batteries

4. Batteries convert ______________________ energy into _____________________ energy.

5. You can think of a battery as a device that pushes electrons out of the negative end. The electrons return to the positive end.

6. Basically, batteries give electrons energy, causing them to move (which is electric current).

Two Kinds of Electric Current: AC/DC7. ________________________ Current (DC): an electric current in which electrons flow

in only one direction

a. a good source of direct current is a __________________________

8. ________________________ Current (AC): an electric current in which electrons constantly change direction

a. Wall outlets provide alternating current

9. Many devices have converters built in which turn alternating current into direct current.

Light Bulbs10.Label the parts of the light bulb with words from the word bank

Word Bank (some words are used more than once:Glass Insulator FilamentMetal Side Metal BottomWire connecting to the bottom Wire connecting to the side

Electric Circuits: An electric circuit is the path that an electric current follows.

11.Electric circuits have three parts:

a. A source of electric energy (The source of electric energy “pushes” electrons through the circuit.

o examples: wall outlet or battery.

b. A load or device that uses electric energy

o examples: light bulb, appliance, etc.

c. wires

NOTE: **There must be an unbroken path for electricity to flow.**

Series Circuit: a circuit in which electric current follows ______________ path.The following diagram shows a series circuit with two bulbs. Notice that there is only one path for the electric current to follow.

12. Imagine you have a battery, several identical light bulbs, and several wires.

a. First, you set up a series circuit like the one above. What do you think would happen if one of the bulbs burned out? Why would this happen?

b. Next, you again build a series circuit with two light bulbs (like the one above). Then, you add a third light bulb so that you now have three light bulbs in series. How does the brightness of each bulb compare to when there were only 2 bulbs in series? Why does this happen?

Parallel Circuits: a circuit in which electric current can follow ________________ paths.The following diagram shows a parallel circuit with three bulbs. Notice that there are three different paths the current can flow through.

13. Imagine you have a battery, several identical light bulbs, and several wires.

a. First, you set up a parallel circuit like the one above. What do you think would happen if one of the bulbs burned out? Why would this happen?

b. Next, you again build a circuit with three light bulbs in parallel (like the one above). Then, you add a fourth light bulb so that you now have four light bulbs in parallel. How does the brightness of each bulb compare to when there were only 3 bulbs in parallel? Why does this happen?

Three Important Properties of Electrical Circuit: Voltage, Current, Resistance

14.___________________________: energy available to move charges through a circuit

a. Voltage is measured in units called volts.

b. Batteries come in a range of voltages

c. The voltage of most American wall outlets is ____________ volts.

15.___________________________: flow of electric charge through a circuit

a. The unit of electric current is the ____________________ (or ____________).

b. The amount of electric current depends on the number of charges flowing through a wire.

16.___________________________: opposition to the flow of electric current. (conductors tend to have very low resistances and allow current to flow through them easily, insulators tend to have very high resistances and do not allow current to flow through them easily)

a. The unit for measuring resistance is the ohm (symbolized by ).

b. Resistance can be useful:o Light bulbs light up because of the resistance of the wire inside the

bulb.o The wire inside the bulb resists the electric current.o The resistance heats up the wire so that it glows.

c. There are four factors which affect the resistance of a wire:o Length: the longer a wire is, the more resistance it haso Diameter: the thicker a wire is, the less resistance it haso Material: Wires made of poor conductors have more resistance than

wires made of good conductorso Temperature: As a wire gets hotter, its resistance increases .

Short Circuit: unintended direct contact between two points in an electrical circuit

17. A short circuit provides a ______________ resistance path, resulting in a __________________ current passing through it.

18.The high current may cause the circuit to __________________________ dangerously, which could cause a fire.

Ohm’s Law: Term Symbol Units Definitioncurrent I amps flow of electric charges through a conductor voltage V volts energy available to move charges through a

circuitresistance R ohms opposition to the flow of electric current

The current, voltage, and resistance vary from circuit to circuit, but there is a simple relationship among them: Ohm’s Law

Ohm’s Law states that the current (I) in a wire is equal to the voltage (V) divided by the

resistance (R). (equation form:

€

I = VR

)

Ohm’s Law is typically written in the following form (this is how it is written on the MCAS formula sheet):

€

V =I R

Using Ohm’s Law: Sample ProblemsPractice Problem 1 A light bulb with a resistance of 5 ohms is

connected to a battery. The current is 4 amps. What is the voltage of the battery?

SolutionStep 1 Write your knowns

Step 2 Write the formula

Step 3 Substitute given numbers and solve for the unknown

Step 4 Write the final answer with units!

Practice Problem 2: Suppose a 12 volt battery is connected to a circuit with a resistance of 4 ohms. What is the current? (Show Your Work!)

Practice Problem 3: A toaster is plugged into a wall outlet, such that V = 110 volts and I = 10 amps. What is the toaster’s resistance? (Show Your Work!)

Electric Power & EnergyBatteries supplies the energy to the circuit, and the energy is used by the load.

19.Review: Power is measured in units of ____________________.

20.Basically, the power rating tells you something about the rate at electrical energy is being converted to some other form (thermal, light, mechanical, etc.)

o A 100 Watt light bulb is converting electrical energy into heat and light energy at a greater rate than a 40 Watt light bulb.

The Power EquationThere are three variables that are important for the power equation: P, I, & V.

21.Use your MCAS Formula Sheet to determine what each of the symbols stand for and fill the terms into the table (notice that it is a capital “P” and a capital “V”:

Symbol Term Units

P Watts (W)

I Amps (A)

V volts (V)

22.Find the formula which uses these three variables (P, I, V) and write it in the space below:

Power Equation:

Problem 4 A motor connected to 120 V draws a current of 10 A. What power is being consumed?

Solution Solve this problem in the space below. Show all work and follow good problem solving strategy.

Problem 5 A blow-dryer uses 1800 Watts is connected to a 120V outlet. What is the current going through the blow-dryer?

Solution Solve this problem in the space below. Show all work and follow good problem solving strategy.

Buying Electrical Energy23.As described previously, power can be defined as energy converted per unit of time:

€

Power = Energytime

24.Rearranged, this gives us

€

Energy = Power × time

25.So, if you multiply the power rating by the time, you can determine the amount of energy used

a. For these types of problemms, you typically measure

o Power in kilowatts (kW) (1 kW = 1000 W)

o Time in hours (h)

b. Using these units, you get energy in units of kilowatt-hours (kWh)26.Example : Marcy leaves her porch light on constantly (all day/everyday). When her

friend Larry tells her that she is wasting a lot of energy (and money) doing this, she says that it isn’t very much. The bulb Marcy is using is a 60-Watt bulb.

a. How many kilowatts is a 60-Watt bulb?

o Divide the Watts by 1000 to get the number of kilowatts

o

€

60 atts1000 atts / kilowatt

= 0.06 kW

b. If Marcy leaves the light on 24 hours a day for 30 days, how many hours was the bulb on for?

o

€

24 hoursday

× 30 days = 720 hours

c. How much energy was used by the bulb?

o Multiply the power (in kW) by the number of hours used to calculate the energy (use:

€

Energy = Power × time )

o

€

0.06 k × 720 h = 43.2 kh

d. If energy costs $0.20/kWh, how much does it cost for Marcy to keep the bulb lit for 30 days?

o Multiply the number of kilowatt-hours (kWh) by the cost per kilowatt-hour to determine the cost

o

€

43.2 kh ×$0.20kh

=$8 .64

e. Who do you think is correct: Marcy or Larry? Explain your choice.