# Why are we connecting voltmeter in series with power when we measure voltage across it?

I am a physics teacher and demonstrated in the class that the voltage across at wall socket is around 220V ( in Europe ) and my student asked why we are connecting a voltmeter in series with power outlet ( I stated to my students previously that if we want to measure voltage of the resistor, we make a parallel connection with it.) I explained them that we need to think the power source as a battery and the internal resistance of new opened battery is very small compared to the internal resistance of the voltmeter.

He came out with the second question that the voltage drop across the voltmeter is not the same as voltage in the power source because there is internal resistance of power source of which we don't know. He claimed that the voltage across the power outlets needs to change depending the distance of wall outlet to the power plant.

My question is: what is the internal resistance of the circuit coming from power plant to our home? Does it make too much difference?

• To potential answer writers: circuit questions and answers should include a diagram. To understand the output resistance of a power outlet, you have to remember that there's a transformer. Then again, volt meters have very high input resistance... – DanielSank Jan 21 '17 at 17:11

## 1 Answer

The power company generates "moderately high" voltage (e.g., 10000 V), and runs that power through a step-up transformer, which converts the power to very high voltage (e.g., 300,000 V). Since energy is conserved, when the voltage in the step-up transformer rises, the current drops. This means that the minimum current necessary to support all electrical loads is sent to neighborhoods, which minimizes the power lost to electrical resistance in the transmission lines, because those losses are seen to be $P = I^2 * R$. Thus, when transmission current is minimized, power transmission losses are also minimized.

When the electrical power gets close to the load that is using it (e.g., your neighborhood), it goes through a series of step-down transformers, which converts it to 220 V, causing the transmitted current to rise accordingly. The transmission losses rise due to this, but the transmission distance is short, so the losses are acceptable.

Regarding the measuring of 220 V current with a volt meter, your house is wired in parallel. This means that you are connecting the voltmeter in parallel across the outlet, just as you would in a more familiar circuit. What is substantially different in your case is the fact that you are measuring AC current, which has a continuously varying voltage. Your student's comment implies that he is thinking in DC current terms, so his question is not entirely appropriate for the situation of measuring voltage across a wall outlet. In addition, if voltage drop from the power plant is in any way "substantial", this can easily be dealt with in your neighborhood by selecting the appropriate step-down transformer, in order to transform the high voltage current to 220 V. This means that the internal resistance of the power plant circuit is not particularly relevant regarding the voltage that you see at your power outlet.