If the EMF of a power supply were doubled, would the reading on the voltmeter double?

Question

This is one of the questions in a practice exam paper i will be sitting next week, so I'd really appreciate any help-im a little worried at the moment!

The circuit in question has a power supply, a thermistor and a fixed resistor in series. There is also a voltmeter attached in parallel to the fixed resistor - that is the voltmeter the question is referring to.

Thanks for helping out,

Answer 1

The thermistor, unlike the resistor, is designed to vary in resistance with its temperature caused by the heating due to the current flow through it, which heats it. A resistor does this also to a very small degree, but that is a side effect. So you have two parts, one designed to not respond to a current change and one that is supposed to.

When you double the current, the heat in the varistor changes its resistance which then changes the current again and so on until the varistor settles to its final temperature and the current settles to its final value, both occurring simultaneously. The answer is that, because of the thermistor's thermal characteristic which makes the circuit non-linear, the final current won't be double the starting current.

Because the voltmeter is reading the voltage drop across a fixed resistance, it is actually being an ammeter. If the current won't exactly double, then the reading won't either. Note: If the thermistor was replaced with a resistor, then it would double exactly. They are putting in the thermistor to see if you know what a thermistor will do, which is vary resistance by temperature. It is commonly used as an inexpensive temperature sensor.

Answer 2

The short answer is no. If you double the emf, the current will instantaneously double, as will pd across the resistor, but that increased current will dissipate additional power in the thermistor. The thermistor will heat up.

When the thermistor heats up its resistance will change.

There are two sorts of thermistor:

negative temperature coefficient = resistance goes down as temperature goes up.

positive temperature coefficient = resistance goes up as temperature goes up.

Either way, the change in resistance of the thermistor will change the current, and therefore the p.d. across the fixed resistor.

When the new equilibrium is reached, the voltmeter reading will not be double what it was.

Good luck for next week.

Answer 3

At least initially yes, since the current through the circuit doubles and the voltage across the fixed resistor also doubles.

The effect of thermistor will change the picture later and right now I cannot analyse that part with my rather rusty physics.

Answer 4

The current is the same in the series circuit.Emf supply plus thermistor plus resistor are in series. Two Emf are two amperes.This makes more current on thermistor to head and change its value.V=V1+V2 .Voltmeter reads different V1 voltage.

Answer 5

Emf and voltage are the same thing.