Understanding Circuit Measurement Techniques and Symbols

[Air]

While doing revision, I've come across aspects on circuits which I cannot get my head around. May I have some help with these questions and ideas.

1. What's a variable voltage and how do we draw it on a circuit? Does it have a different symbol to a voltmeter?
2. Why does touching the leads together allow the voltmeter to be zeroed?
   Can I say that it discharges it. In the mark scheme, it says that it is short-circuited, is that the same as being discharged?
3. When using ohmmeter, the resistance of a resistor should not be measured while it is in a circuit. Is it that this will cause the circuit to short-circuit because there are too many component to the circuit?

I'm getting confused. Please guide me in the right direction and clear my confusion. Thanks in advance.

 

[mikelepore]

When you say "variable voltage", perhaps you mean a time-varying voltage source, which is an active component that makes it become true that the voltage between its two terminals will be a certain function of time. The electrical outlet on your wall is an example of one. The symbol I'm a customed to is the plus and minus signs in a circle, with some expression written next to it to indicate the time-varying function. Or perhaps by the words "variable voltage" you mean the voltage across any two points in a circuit, not measured across a voltage source. You could apply a volt meter across those two points if you wanted to, but it wouldn't be meaningful to say that the voltage "is" a volt meter. A volt meter shows a measurement.

A DC voltmeter placed across a time-varying voltage would measure an effective constant value. That may not be what you wanted.

If you remove a voltmeter from the circuit and then short its terminals together, the short will guarantee that the voltage across the terminals is zero. If the meter displays a number other than zero, you can turn the screw to make it display zero.

You have "calibrated" the meter, but I won't say that you "discharged", it because I'm not aware that a voltmeter stores a significant amount of charge that needs to be discharged.

If you measure a resistor with an ohmmeter, the meter does it by measuring the voltage to current ratio of whatever is between those two terminals, in this case, just one resistor. If you tried to do it while the resistor is part of a circuit, the meter would read some voltage to current ratio of the entire network as seen at those two terminals.

 

[Moetasim]

Hello there,

I'm happy to assist you with your questions about circuits. Let's start with the first one about variable voltage. A variable voltage is a type of power supply that can be adjusted to provide different levels of voltage. This allows for more flexibility in the circuit and can be used for various purposes such as testing different components or controlling the speed of a motor. To draw it on a circuit, you can use a variable voltage source symbol which is usually a battery with an arrow pointing to a curved line representing the adjustable voltage. It may also have a label indicating the range of voltage it can provide.

As for the voltmeter, it does have a different symbol than a regular voltmeter. The symbol for a variable voltmeter usually has a V with a small curved line above it, indicating that it can measure variable voltages.

Touching the leads of a voltmeter together allows it to be zeroed because it creates a closed circuit with zero resistance. This allows the voltmeter to calibrate itself and display a reading of zero. Saying that it discharges the voltmeter is not entirely correct, as the voltmeter does not hold a charge like a battery. Short-circuiting is a more accurate term, as it describes the connection of the two leads which bypasses the rest of the circuit.

When using an ohmmeter, it is important to not measure the resistance of a resistor while it is in a circuit. This is because the other components in the circuit can affect the reading and give an inaccurate measurement. It is not necessarily because there are too many components, but rather that the circuit is not isolated and the resistance of other components can interfere with the measurement.

I hope this clears up your confusion and helps you understand these concepts better. Remember, if you have any further questions, don't hesitate to ask. Keep up the good work with your revision!