Notes: Ohm s Law and Electric Power

Transcription

1 Name: Date: / / 644 Intro Physics Notes: Ohm s Law and Electric Power Ohm s Law: Important Terms Term Symbol Units Definition 1. current I amps flow of electric charges through a conductor 2. voltage V volts energy available to move charges through a circuit 3. resistance R ohms opposition to the flow of electric current Ohm s Law: The Relationship 4. The current, voltage, and resistance vary from circuit to circuit, but there is a simple relationship among them: Ohm s Law 5. 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 ) 6. Ohm s Law is typically written in the following form (this is how it is written on the MCAS formula sheet): V = IR Using Ohm s Law: Sample Problems Sample Problem 1 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? V =? I = 4 amps R = 5 ohms Step 2 Step 3 Write the formula Substitute given numbers V = IR V = 4 amps " 5 ohms V = 20 volts

2 Sample Problem 2 Step 1 Write your knowns HINT: underline numbers and things you are looking for in the problem Suppose a 12 volt battery is connected to a circuit with a resistance of 4 ohms. What is the current? V = 12 volts I =? R = 4 ohms Step 2 Write the formula V = IR Step 3 Substitute given numbers 12 = (I) (4) 12 = 4 I 12 4 = 4I 4 3 = I I = 3 amps Sample 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? Step 1 Write your knowns HINT: underline numbers and things you are looking for in the problem V = 110 I = 10 amps R =? Step 2 Write the formula V = IR Step 3 Substitute given numbers 110 = 10 (R) 110 = 10 R = 10R = R R =11 ohms

3 Using Ohm s Law: Problems for you to try Problem 1 A 12 volt battery is hooked up to a light bulb with a 2 ohm resistance. What is the current? Step 1 Write your knowns V = HINT: underline numbers and things you are looking for in the problem I = R = Step 2 Write the formula Step 3 Substitute given numbers Problem 2 A 12 volt battery is hooked up to 2 light bulbs in series,

4 Step 1 Write your knowns which have a total resistance of 4 ohms. What is the current through the bulbs? HINT: underline numbers and things you are looking for in the problem Step 2 Write the formula Step 3 Substitute given numbers

5 Problem 3 A light bulb with a resistance of 10 ohms is connected to a battery and the current is 2 amps. What voltage is being used? Solve this problem in the space below. Show all work and follow good problem solving strategy. Problem 4: Look at the circuit diagram to the right and find out the current moving through the battery. V= 12v 4Ω Solve this problem in the space below. Show all work and follow good problem solving strategy.

6 Electric Power & Energy 7. Batteries convert energy into energy. 8. A light bulb converts energy into energy. 9. Power is measured in units of. 10. When you buy light bulbs, you must choose how many Watts you want. a. A light bulb with a higher wattage would be bright than a light bulb with a lower wattage. b. A brighter light bulb is releasing more light (and thermal) energy. c. Where is the energy coming from to light the bulb? 11. Items besides light bulbs are rated in terms of power. Motors are another example. 12. 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. 13. Suppose you want to leave your front light on for 10 hours each night. Which would cost more to keep lit: a 100 Watt light bulb or a 60 Watt light bulb (or would it be the same)? Explain your answer.

7 The Power Equation 14. There are three variables that are important for the power equation: P, I, & V. 15. 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) 16. Find the formula which uses these three variables (P, I, V) and write it in the space below: Power Equation: 17. Use the problem solving skills you have been practicing to solve the following problem. Problem 4 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.

8 Buying Electrical Energy 18. As described previously, power can be defined as energy converted per unit of time: 19. Rearranged, this gives us Power = Energy time Energy = Power " time 20. So, if you multiply the power rating by the time, you can determine the amount of energy used a. For these types of problems, 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) 21. 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 kilowatt-hour 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.