670 Intro Physics Notes: Electric Current and Circuits

Transcription

1 Name: Electric Current Date: / / 670 Intro Physics Notes: Electric Current and Circuits 1. Previously, we learned about static electricity. Static electricity deals with charges that are at rest. 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)? negative charges- electrons moving Batteries chemical electrical 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). Conductors and Insulators 7. A material through which electric charge can flow easily is called a. 8. Examples of conductors are: copper silver o Most are good conductors of electricity. a. b. gold metals c. 9. A material through which electric charge does not flow easily is called an. 10. Examples of insulators are: paper rubber a. b. glass c. conductor insulator

2 Two Kinds of Electric Current: AC/DC 11. With static electricity, the electric charges build up on objects rather than flow. a. These charges can jump between objects. b. To run, appliances in your home need a steady flow of charges, which static electricity does not provide. Direct 12. Current (DC): an electric current in which electrons flow in only one direction a. a good source of direct current is a Alternating 13. Current (AC): an electric current in which electrons constantly change direction a. Wall outlets provide alternating current batter 14. Many devices have converters built in which turn alternating current into direct current. Light Bulbs 15. Label the parts of the light bulb with words from the word bank Word Bank (some words are used more than once: Glass Insulator Filament Metal Side Wire connecting to the bottom Metal Bottom Wire connecting to the side glass filament wire connecting to the side glass metal side insulator wire connecting to the bottom insulator metal bottom

3 Electric Circuits 16. An electric circuit is the path that an electric current follows. 17. Electric circuits have three parts: a. A source of electric energy battery o examples: wall outlet or b. A load or device that uses electric energy o examples: light bulb, appliance, etc. c. wires 18. There must be an unbroken path for electricity to flow. 19. The source of electric energy pushes electrons through the circuit. Measuring Electricity: Voltage and Current Voltage 20. : energy available to move charges through a circuit V a. The symbol for voltage is. b. Voltage is measured in units called volts. c. Batteries come in a range of voltages 120 d. The voltage of most American wall outlets is volts. 21. Electric current: flow of electric charge through a circuit a. The symbol for current is. I Ampere Amp b. The unit of electric current is the (or ). c. The amount of electric current depends on the number of charges flowing through a wire.

4 Measuring Electricity: Resistance 22. The symbol for resistance is. Resistance 23. : opposition to the flow of electric current Conductors 24. tend to have very low resistances and allow current to flow through them easily Insulators 25. tend to have very high resistances and do not allow current to flow through them easily 26. The unit for measuring resistance is the ohm (symbolized by Ω). 27. Resistance can be useful: R a. Light bulbs light up because of the resistance of the wire inside the bulb. b. The wire inside the bulb resists the electric current. c. The resistance heats up the wire so that it glows. Short Circuit 28. Short Circuit Definition: unintended direct contact between two points in an electrical circuit low 29. A short circuit provides a resistance path, resulting in a current passing through it. high (large) overheat 30. The high current may cause the circuit to dangerously, which could cause a fire.

5 Ohm s Law: Term Symbol Units Definition current I amps flow of electric charges through a conductor voltage V volts energy available to move charges through a circuit resistance 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 = V R ) 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 Problems Practice Problem 1 Solution Step 1 Write your knowns 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? Step 2 Step 3 Write the formula 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!) V = 12 volts R = 4 Ω I =? V = I R or I = V R I = (12 volts)/(4 Ω) I = 3 A

6 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 & Energy Batteries supplies the energy to the circuit, and the energy is used by the load. Watts 31. Review: Power is measured in units of. 32. 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 Equation There are three variables that are important for the power equation: P, I, & V. 33. 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 I V Watts (W) Amps (A) volts (V) 34. Find the formula which uses these three variables (P, I, V) and write it in the space below: Power Equation:

7 Problem 4 Solution A motor connected to 120 V draws a current of 10 A. What power is being consumed? Solve this problem in the space below. Show all work and follow good problem solving strategy. Problem 5 Solution A blow-dryer uses 1800 Watts is connected to a 120V outlet. What is the current going through the blow-dryer? Solve this problem in the space below. Show all work and follow good problem solving strategy.

8 Buying Electrical Energy 35. As described previously, power can be defined as energy converted per unit of time: Power = Energy time 36. Rearranged, this gives us Energy = Power " time 37. 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) 38. 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 Watts 1000 Watts /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? 24 hours " 30 days = 720 hours o day 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 kw " 720 h = 43.2 kwh 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 kilowatthour to determine the cost o 43.2 kwh" $0.20 kwh = $8.64 e. Who do you think is correct: Marcy or Larry? Explain your choice.