Studying: Fundamentals of Electricity Level 3 course

  • 1
  • Question
  • Updated 3 years ago
I'm hopping that someone on here would be able to give me a helping hand with a couple of questions I have.
I have emailed the tutor but heard nothing back yet.

I am going through Assignment four - Unequal resistance in Parallel.
Example 2
In Figure 5,
R = 3 ohms.
R = 4 ohms.
R = 5 ohms.

Find the joint resistance and the total current if the potential difference between the points A and B is 20 volts.

Solution: 1 = 1 + 1 + 1
R 3 4 5

= l = 0.78
R

THEREFORE: R = 1.28 OHMS.

Total current = l = V = 20
R 1.28

= 15.63 amperes.

My question is were does "Therefore:R = 1.28 ohms come from"? I understand the rest it's I just can't see where the 1.28 ohms came from.

Also

Therefore, if we consider batteries B1 and B2 only in Figure 6. Assume that B1 has an emf of 10 volts and an internal resistance of 0.4 ohm, while B2 has an emf of 10.2 volts and an internal resistance of 0.5 ohm.

Suppose that the conditions of the circuit are such that the current taken from B1 is 4 amperes, the terminal pd of B1 is [emf – internal voltage drop], or, as a formula:

Terminal pd of B1

= E1 – l1R
= 10 – 4 × 0.4
= 10 – 1.6
= 8.4 volts.

The terminal pd of B2 must also be at 8.4 volts, because B1 and B2 are connected at points A and B.

Therefore:

Terminal pd of B2 8.4 = 10.2 – l2R2
8.4 = 10.2 – (l2 × 0.5)
but 0.5I2 = 10.2 – 8.4
0.5l2 = 1.8

Therefore: l2 = 1.18 *****

0.5
= 3.6A

That is, for B1, and B2 to have the same terminal potential difference, the current through B2 must be 3.6 amperes.

Where does "Therefore I2 = 1.18 **** come from???
Photo of Doug

Doug

  • 1 Post
  • 0 Reply Likes

Posted 3 years ago

  • 1
Photo of Stonebridge Student Support Team

Stonebridge Student Support Team, Official Rep

  • 1194 Posts
  • 113 Reply Likes
Hi Michael, is there anything we can help you with? Stonebridge
Photo of Doug

Doug

  • 1 Post
  • 0 Reply Likes
Hi there,

I could do with a bit of help with the above questions
Photo of Stonebridge Student Support Team

Stonebridge Student Support Team, Official Rep

  • 1194 Posts
  • 113 Reply Likes
Hi Michael, if you require tutor support for this you can email the question to sarah@stonebridge.uk.com. Stonebridge