# Power dissipated in variable resistor?

1. May 1, 2015

### CAH

See photo!

Why does the power dissipated increase and then decrease (see graph)? Is it something to do with emf?
Also can internal resistance change as the variable resistor increases/decreases or is it always constant?

- this isn't one of the questions they ask I'm just wondering

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2. May 1, 2015

### ehild

You can derive the power on the variable resistor in terms its resistance. Keep the emf and internal resistance constant.
It is a very important result that the power is maximum at a certain loading resistance - how is it related to the internal resistance of the source?
Have you heard about impedance matching?

3. May 1, 2015

### Staff: Mentor

Internal resistance can be assumed as being fixed for this exercise, though in practice it can be expected to change slightly.

Can you derive an expression for the power in Y, perhaps assume a particular value for r. Then sketch your graph, power versus value of Y.

4. May 1, 2015

### CAH

I don't understand this still,

Py= VI
V= emf - rI
Py= (emf - Ir)I ....=0

5. May 1, 2015

### ehild

What is the current in the circuit in therms of the emf and the resistors?

6. May 1, 2015

### donpacino

to answer your question, the internal resisitance won't change with respect to the variable resistor

7. May 1, 2015

### donpacino

this is a good start. Your problem is you need to get rid I.

In your second equation, try to get rid of I using Y and r

8. May 2, 2015

### cnh1995

Last edited: May 2, 2015
9. May 2, 2015

### cnh1995

Eliminate current from your equations and see what function you get relating the power with the load Y..Also see whether this power is total power or power consumed by load Y..

10. May 3, 2015

### CWatters

Power = voltage * current.

Consider two cases...

1) The variable resistor is set to a very high value (eg open circuit) => The current is very low or zero => Power is very low.
2) The variable resistor is set to a very low value (eg short circuit) => Voltage is very low or zero => Power is very low.

So at both ends of the range the power is very low. So no surprise it's higher in the middle.