# 2 Questions: V=IR and TemperatureWatch

#1
I was thinking about the equation V=IR and realised it doesn't make a lot of sense. For example I learnt that a higher current in a circuit leads to a higher resistance in a circuit, this is why power lines run on very high voltages to reduce current and hence resistance. However this equation shows that resistance is inversely proportional to current. This then got me thinking that in exam questions we see resistors as fixed things as in their resistance remains constant regardless of the current or voltage across them. Could someone please shed some light on the situation

My second question is about heat transfer, imagine there was an electrical heater surrounded in water inside a perfect insulator. If the electrical heater is giving out a constant power output will the temperature of the water never stop rising?
0
5 years ago
#2
(Original post by FarmerMan)
I was thinking about the equation V=IR and realised it doesn't make a lot of sense. For example I learnt that a higher current in a circuit leads to a higher resistance in a circuit, this is why power lines run on very high voltages to reduce current and hence resistance. However this equation shows that resistance is inversely proportional to current. This then got me thinking that in exam questions we see resistors as fixed things as in their resistance remains constant regardless of the current or voltage across them. Could someone please shed some light on the situation

My second question is about heat transfer, imagine there was an electrical heater surrounded in water inside a perfect insulator. If the electrical heater is giving out a constant power output will the temperature of the water never stop rising?
Well a more obvious example is a filament lightbulb - the resistance you measure when it's at room temperature is a lot lower than the resistance implied by it's rating, say 60w 240v. ohms law isn't wrong; V still equals IR, you made an error thinking that R aught to remain constant.

a lot of the time you'll be thinking about components passing negligible currents which can't change the temperature enough to be noticed. components are usually designed to have low temperature coefficients and these are stated in the data sheets so designers can produce circuits that continue to work at different temperatures.

---
guess not - though the water would eventually boil off unless is was also a perfectly insulated pressure vessel of infinite strength.
0
5 years ago
#3
(Original post by FarmerMan)
However this equation shows that resistance is inversely proportional to current.
Resistance would only be inversely proportional to current if the voltage were constant. But an increase in current can only be caused by an increase in voltage.

My second question is about heat transfer, imagine there was an electrical heater surrounded in water inside a perfect insulator. If the electrical heater is giving out a constant power output will the temperature of the water never stop rising?
Well it wouldn't stay as water- it would become gaseous lol. And then the temperature would continue to rise. But if the heater was constantly giving out power the insulator would break down.
0
5 years ago
#4
(Original post by FarmerMan)
I was thinking about the equation V=IR and realised it doesn't make a lot of sense. For example I learnt that a higher current in a circuit leads to a higher resistance in a circuit, this is why power lines run on very high voltages to reduce current and hence resistance. However this equation shows that resistance is inversely proportional to current. This then got me thinking that in exam questions we see resistors as fixed things as in their resistance remains constant regardless of the current or voltage across them. Could someone please shed some light on the situation
For a resistor, R=V/I applies since increasing the resistance causes a reduction in current (because there are more collisions between the electrons, reducing their rate of flow)--this is essentially what a resistor does.

However, for power transmissions lines, they aren't resistors and a different equation applies here: power=voltagexcurrent. Now, to maintain a certain amount of power, if the voltage increases, the current also has to decrease and vice versa. By reducing the current, the rate of flow of electrons decreases so it causes the surroundings to warm up less (as they are moving more slowly)
0
5 years ago
#5
(Original post by FarmerMan)
I was thinking about the equation V=IR and realised it doesn't make a lot of sense. For example I learnt that a higher current in a circuit leads to a higher resistance in a circuit, this is why power lines run on very high voltages to reduce current and hence resistance. However this equation shows that resistance is inversely proportional to current. This then got me thinking that in exam questions we see resistors as fixed things as in their resistance remains constant regardless of the current or voltage across them. Could someone please shed some light on the situationMy second question is about heat transfer, imagine there was an electrical heater surrounded in water inside a perfect insulator. If the electrical heater is giving out a constant power output will the temperature of the water never stop rising?
V=IR is always true, because R=V/I is really the definition of resistance. As someone has pointed out already, your problem is caused by assuming resistance is always constant. Resistors are designed to have a constant resistance but with some other things, resistance varies with temperature, which is affected by power. Other things, like diodes for example, have strange I vs V curves and the concept of resistance is not very useful for them although for any given applied voltage they have a definite effective resistance given by R=V/I. The short answer to your second question is 'yes'.
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