# Electric current/kWh

Dx
I have two questions that i need help with, plz?

1) A 9V battery costs $1.49 and will run a portable CD player for 6 hours. Suppose the battery supplies a current of 25mA to the player. What is the cost of energy in dollars per kWh? I started out using ohms law but am lost for there. How do i set up this for a formula i can solve for dollars per kWh? 2) The monthly(30days) electric bill included the cost of running a AC unit for 2hr/day at 5000W, and a series connection of ten 60W lightbulbs for 5hr/day. How much did these items contribute to the cost of the monthly electric bill if electricity costs 8.0 per kWh? 1st i added the ten 60W = 600W. Next, i took (2hr/d)*(30d) = 60h. Also took (5hr/d)*(30d)=150h, i added 5KW + 600W = 5.6KW for total wattage. I plugged in a formula like so (5.6k)*(2.1k)* ??? How do i find the daily cost of electricity, my answer is 2.86x10^-1, i got that from (8/kWh) / 4weeks = ans. Then ans / 7 days = 2.86 x 10^-1 But i am not right, what am i doing wrong, can you help me solve this plz? Last edited by a moderator: ## Answers and Replies Related Introductory Physics Homework Help News on Phys.org Tom Mattson Staff Emeritus Science Advisor Gold Member Originally posted by Dx I have two questions that i need help with, plz? 1) A 9V battery costs$1.49 and will run a portable CD player for 6 hours. Suppose the battery supplies a current of 25mA to the player. What is the cost of energy in dollars per kWh?

I started out using ohms law but am lost for there. How do i set up this for a formula i can solve for dollars per kWh?

This problem is pretty easy, so I don't want to give too much away. Here are the overall steps:

1. Calculate the power delivered to the CD player. You have all the info for that.
2. Calculate the total energy delivered to the CD player. You were given one piece of info for that, and you will calculate the other piece in Step 1.
3. Calculate the cost for that energy. You were given one piece of info for that, and you will calculate the other piece in Step 2.

Give it a shot.

2) The monthly(30days) electric bill included the cost of running a AC unit for 2hr/day at 5000W, and a series connection of ten 60W lightbulbs for 5hr/day. How much did these items contribute to the cost of the monthly electric bill if electricity costs 8.0 per kWh?

1st i added the ten 60W=6KW.
I assume you meant to say 60W+6kW. That's fine, just don't forget to convert kW-->103W.

Next, i took (2hr/d)*(30d) = 60h. Also took (5hr/d)*(30d)=1.5Kh,
You made a mistake. 1.5Kh=1500h, which is wrong.

I am having trouble following you from here. Please correct the above mistake, check your math carefully, and come back if you need more help.