Calculating Charge and Current

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In summary, the conversation discussed the relationship between current and charge, as well as solving problems involving these quantities using the equation I = ΔQ/ΔT. The correct answers were provided for the given questions, and further clarification was given on the concept of amp-hours and its relation to Coulombs. The conversation also briefly touched on the relationship between Watts and Joules.
  • #1
FeDeX_LaTeX
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Hello;

I just wanted to clarify whether I am using this formula correctly. By definition, current is the rate of flow of charge. In other words, its how much charge flows per second. One amp (1 A) is equal to one coulomb per second (1 C/s). Charge and current are related by the equation: I = ΔQ/ΔT.

1) A battery supplies 10 C over a period of 50 seconds. What is the current?

My answer: Given the formula above, I = 10 coulombs / 50 seconds = 0.2 A. Is this correct?

2) Another battery is connected for 2 minutes and provided a current of 0.4 A. How much charge flowed?

My answer;

0.4 = Q/120
Q = 0.4*120 = 48 C

3) A car battery has a capacity of 24 Ah (amp hours). If it provides a current of 48A how long can it be used for? How much charge (in coulombs) does it contain?

My answer: Not completely sure about this question, but I think that the car battery can be used for 2 hours. As for the latter question, I don't understand... how do I convert from amp hours to amps?

Thanks.
 
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  • #2
Your answers to 1 and 2 are correct. An amp-hour is current multiplied by time. What physical quantity do you get when you do that?
 
  • #3
Hello;

Yeah I thought about that but I ended up getting 1 amp per 150 seconds, which isn't much help... but 1 amp is 1 C/s. How can I use that in my formula?

Also, I said that the battery can be used for 2 hours, but is it actually 0.5 hours? If the battery has a capacity of 24 Ah, so if 48*time = 24, 24/48 = 0.5?
 
  • #4
Once more, what physical quantity do you get when you multiply amps with hours?
 
  • #5
Oh... 1 coulomb. How could I not see that...

So is the answer 2 coulombs?
 
  • #6
No. To get coulombs, you need to multiply amps by seconds, not hours.
 
  • #7
2 hours = 3600*2 = 7200 seconds.

So the answer is 7200 coulombs?
 
  • #8
Why are you multiplying by 2? What is the rating of the battery in amp-hrs?
 
  • #9
Whoops. If it provides 48A, then it will be using twice as much current and thus will last for half the time, so 0.5 hours. 0.5 hours in seconds is 1800 seconds, so is it 1800 coulombs?
 
  • #10
FeDeX_LaTeX said:
Whoops. If it provides 48A, then it will be using twice as much current and thus will last for half the time, so 0.5 hours. 0.5 hours in seconds is 1800 seconds, so is it 1800 coulombs?
One more time. To get Coulombs you multiply amps with seconds. You have 1800 s, how many amps does the battery provide during this time?
 
  • #11
48*1800 = 86400 coulombs
 
  • #12
Yup.
 
  • #13
Little bit unrelated, but Watts work in the same way, correct? Watts = Joules / seconds, so to find J I'd have to multiply watts by seconds...?
 
  • #14
Precisely.
 

1. How do you calculate electric charge (Q)?

Electric charge (Q) can be calculated by multiplying the current (I) by the time (t). The formula for calculating charge is Q = I x t.

2. What is the unit of electric charge?

The unit of electric charge is coulomb (C), which is defined as the amount of charge that passes through a point in a conductor in one second when there is a current of one ampere (A).

3. How is electric charge represented in an electric circuit diagram?

Electric charge is represented by the symbol "Q" in an electric circuit diagram.

4. Can electric charge be negative?

Yes, electric charge can be negative. This means that the charge is made up of more electrons than protons. Objects with a negative charge will be attracted to objects with a positive charge.

5. How is electric charge conserved in a closed system?

In a closed system, the total amount of electric charge remains constant. This means that the total amount of positive charge is always equal to the total amount of negative charge. This is known as the law of conservation of charge.

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