What is the difference between a Resistor and a Variable Resistor?

Question

Basically, I am studying OHM's Law for GCSE, and there is an example of a circuit set up to prove the validity of OHM's Law. This is a series circuit that involves an Ammeter, Voltmeter, Resistor AND a Variable Resistor (next to the symbol of the Variable Resistor, my teacher has written 'VARIES CURRENT.'). What I don't understand is this
-Why would you need two Resistors?
-Although a Variable resistor would affect the current, wouldn't it actually be varying the resistance? Why has my teacher put 'VARIES CURRENT' instead?
-What is the difference between the two?
-Do Resistors read the resistance or just alter it?

Please answer these in simple terms as I find physics pretty hard to get my head round! (I'm more of a History/English person.)

Answer 1

"Why would you need two Resistors?"



Probably as a protection. You can set the variable resistor to 0 and you wouldn't want to damage the other components in the circuit by creating a short. So that means there will always be some resistance in the circuit.



"Although a Variable resistor would affect the current, wouldn't it actually be varying the resistance?"

Yes.



"Why has my teacher put 'VARIES CURRENT' instead?"

I don't know why, but if you change the resistance, you're going to change the total resistance of the circuit which will change the current.



"What is the difference between the two?"

Between a variable resistor and an ordinary resistor? The variable resistor can be varied, a normal resistor has a fixed resistance. A variable resistor is normally setup as a dial or slider and used to control the level of something, such as volume.



"Do Resistors read the resistance or just alter it?"

I don't understand that question. A resistor is something that resists the flow of current. Period. It doesn't "read" anything and I don't know what you mean by "alter it". Alter what? If you have a 10 ohm resistor in a circuit, it has 10 ohms of resistance. Period. There's nothing to read.

Answer 2

A resistor can be fixed or variable. It's just the broad category of devices designed to impede current. A variable resistor is just that...variable over a range of resistance.



A fixed resistor will have no dials or means to vary the resistance. A variable one will have such means, which are typically a dial or a slide.



The fixed resistor might be used to limit what the circuit carries when the variable resistor is dialed down to zero ohms. So the fixed resistor would be a soft of protection from too much current and consequent short circuit (burn out).



Consider this: o------WW----w/w-------x where WW = R is fixed and w/w = r is variable.



There is a fixed voltage across the terminals o and x. So when the variable resistance r = 0, the current through the circuit is I = V/(R + r) = V/(R + 0) = V/R. But when the variable is cranked up to, say r = R, the current is i = V/(R + r) = V/(R + R) = V/2R = I/2; it's half what it was when r = 0.



Bottom line; the fixed resistor limits the max current to a safe level and the variable resistor varies the current within those limits.



I don't understand your "Do Resistors read the resistance or just alter it". Resistors resist, duh. They resist the flow of current through them and that's like closing or opening a valve on a water pipe where the flow of water is like the flow of electrical current.

Answer 3

Define Variable Resistor

Answer 4

First question (title)



A fixed resistor (or just a 'resistor' for short) has a single value that can't be change, e.g 5Ω.



A variable resistor is adjustable from zero up to some maximum value. E.g. a 20Ω variable resistor can be adjusted so its resistance is anywhere from zero up to 20Ω.



Your circuit should NOT be series. The voltmeter should be in parallel to the fixed resistor,

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2nd question



I believe you are checking that the fixed resistor obeys Ohm's Law. That's what the experiment is about.



You are trying to measure the voltage across the fixed resistor and the current through it, to see if they are proportional. You need to be able to alter the current through the fixed resistor to get a range of different readings. The variable resistor is needed because it allows you to change the current/voltage.

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3rd and 4th questions



In electrical circuits the voltage, current and resistance are inter-related. In fact by increasing the total resistance in a circuit, you reduce the current. By changing the variable resistor, you are making it easier or harder for current to flow - so it gives you a way of controlling the current.

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5th question



See 1st question

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6th question



Resistors don't 'read 'resistance. 'Reading' resistance needs a special type of meter.

Resistors don't 'alter' resistance - they have a value of resistance.

Think of resistance as 'how hard' it is for a current to pass through something. It is easier for current to pass through a low resistance component than a large resistance one.

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Try watching the video-lesson below - it's not that difficult. In the circuit near the beginning, the 'variable power supply' is equivalent to a fixed power supply and a variable resistor.

Answer 5

Well resistors in simple terms, resist the flow of current. If we look at the equation Voltage=Current*Resistance.

From this we can see if the voltage is constant ie a powerpack that is set at a certain voltage and you increase the resistance the Current has to decrease.



So in some ways it does vary the current but that is an effect of the resistance changing.



I don't know what the circuit you have a diagram of but it might be there just to limit the current to prevent damage to components in the circuit.



If you still don't understand this or I have missed something message me and I will try and explain it another way.

Answer 6

A resistor is a device, normally used in electronic circuits, which presents a measure of resistance to electronic flow (measured in ohms) to the circuit. A variable resistor is very similar, except that it may be adjusted over a range to present an exact electrical resistance to the circuit.

Answer 7

Ohm's law : I = U/R



Variable resistors are used to change the resistance (R). When you change R, I also changes.

For example, if you double the R, I is twice smaller. ( I = U / 2R )

That's why he wrote 'varies current'.



'' -Do Resistors read the resistance or just alter it? ''

- No, they just alter it, usually by changing the length of the circuit.