Physics: Why do we subtract in one case and in other add?

  • MHB
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In summary, the conversation discussed the concept of effective power in batteries and how it is affected by internal resistance. The speaker also clarified that there is always an internal resistance in the circuit. In addition, the difference between the questions in (b) and (c) was explained.
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  • #2
Yakin, let's try to do this in small steps. Do you understand you have different situations in each one? Can you explain what happens in each?
 
  • #3
Hi yakin! :)

In (b) the battery generates power (plus), but the effective power is reduced by the losses in its internal resistance (minus).

In (c) the applied power is partially stored in the battery (plus) and part is converted to heat in the internal resistance (plus).
 
  • #4
Well i know that current would be constant in the circuit. There would be potential drop across the internal and external resistances. Is it because current flows from positive to negative and in 16.0-V battery, the internal resistance(connected to +ve terminal comes in a way) dissipates some energy therefore we subtract the internal resistance from EMF, and in 8.0-V battery there is no internal resistance after negative terminal so we add the internal resistance. Am i right?
 
  • #5
yakin said:
Well i know that current would be constant in the circuit. There would be potential drop across the internal and external resistances. Is it because current flows from positive to negative and in 16.0-V battery, the internal resistance(connected to +ve terminal comes in a way) dissipates some energy therefore we subtract the internal resistance from EMF,

Yes.
The effective power of the battery, is what is generated outside the battery.
That means that the power dissipated in the internal resistance must be subtracted from the power that is generated from the ideal voltage source.
and in 8.0-V battery there is no internal resistance after negative terminal so we add the internal resistance. Am i right?

That doesn't sound right. There is an internal resistance.

The question in (c) is slightly different from (b). It asks how much power is converted inside the 8 V battery.
That is, how much power is absorbed by the combination of the voltage source and the internal resistance.
 

FAQ: Physics: Why do we subtract in one case and in other add?

Why do we subtract in physics calculations?

In physics, subtraction is used when we want to find the difference between two values. For example, when calculating the change in position or velocity, we subtract the initial value from the final value.

When do we add in physics?

In physics, addition is used when we want to combine or add together two or more quantities. This is often the case when calculating forces, where multiple forces acting on an object must be added together to find the net force.

Why can't we always add or subtract in physics?

In some cases, it is not appropriate to use addition or subtraction in physics calculations. This is because certain quantities in physics, such as vectors, have both magnitude and direction. Adding or subtracting these quantities can result in incorrect answers. Instead, we use vector addition or subtraction, which takes into account both magnitude and direction.

How do we know when to use addition or subtraction in physics?

The use of addition or subtraction in physics calculations depends on the specific problem and the quantities involved. It is important to carefully analyze the problem and determine what operations are necessary to find the correct answer. In some cases, a combination of addition and subtraction may be needed.

Can addition and subtraction be used interchangeably in physics?

No, addition and subtraction cannot be used interchangeably in physics calculations. It is important to use the correct operation based on the physical quantities involved in the problem. Using the wrong operation can lead to incorrect results and a misunderstanding of the underlying physics principles.

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