# 13.2 Current and measurement  (Page 2/2)

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How much charge flows per second in a circuit with a current of $1,5\phantom{\rule{2pt}{0ex}}\mathrm{A}$ ?

Given: Current $I=1,5\phantom{\rule{2pt}{0ex}}\mathrm{A}$

$\Delta t=1\phantom{\rule{2pt}{0ex}}\mathrm{s}$

2. $I=\frac{Q}{\Delta t}$
3. $\begin{array}{ccc}\hfill Q& =& I×\Delta t\hfill \\ & =& \hfill 1,5\phantom{\rule{2pt}{0ex}}\mathrm{A}×1\phantom{\rule{2pt}{0ex}}\mathrm{s}\\ & =& 1,5\phantom{\rule{2pt}{0ex}}\mathrm{C}\hfill \end{array}$

If $500×{10}^{3}\phantom{\rule{2pt}{0ex}}\mathrm{\mu C}$ of charge flow past a point in a circuit in 1 second, what is the current in the circuit?

Given: charge $Q=500×{10}^{3}\phantom{\rule{2pt}{0ex}}\mathrm{\mu C}$
$\Delta t=1\phantom{\rule{2pt}{0ex}}s$

2. $\begin{array}{}Q=500×{10}^{3}\mathrm{\mu C}\\ \phantom{x}=500×{10}^{3}×{10}^{-6}\mathrm{C}\\ \phantom{x}=0,5\mathrm{C}\end{array}$

3. $I=\frac{Q}{\Delta t}$

4. $\begin{array}{}I=\frac{Q}{\Delta t}\\ \phantom{x}=\frac{0,5\mathrm{C}}{1\mathrm{s}}\\ \phantom{x}=0,5A\end{array}$

I measure the current in a circuit to be $500\mathrm{mA}$ . How much charge is flowing per second in the circuit?

Given: Current $I=500\mathrm{mA}$ ,
$\Delta t=1\mathrm{s}$

2. $500\mathrm{mA}=500×{10}^{-3}\mathrm{A}$

3. $I=\frac{Q}{\Delta t}$

4. $\begin{array}{}Q=I×\Delta t\\ \phantom{x}=500×{10}^{-3}\mathrm{A}×1\mathrm{s}\\ \phantom{x}=0,5\phantom{\rule{2pt}{0ex}}\mathrm{C}\end{array}$

## Measuring voltage and current in circuits

As we have seen in previous sections, an electric circuit is made up of a number of different components such as batteries, resistors andlight bulbs. There are devices to measure the properties of these components. These devices are called meters.

For example, one may be interested in measuring the amount of current flowing through a circuit using an ammeter or measuring the voltage provided by a battery using a voltmeter . In this section we will discuss the practical usage of voltmeters and ammeters.

## Voltmeter

A voltmeter is an instrument for measuring the voltage between two points in an electric circuit. In analogy with a water circuit, avoltmeter is like a meter designed to measure pressure difference. Since one is interested in measuring the voltage between twopoints in a circuit, a voltmeter must be connected in parallel with the portion of the circuit on which the measurement is made.

[link] shows a voltmeter connected in parallel with a battery. The positive lead of the voltmeter must be connected closest to the positive end of the battery and the negative lead closest to the negative end of the battery. The voltmeter may also be used to measure thevoltage across a resistor or any other component of a circuit that has a voltage drop or potential difference.

## Ammeter

An ammeter is an instrument used to measure the flow of electric current in a circuit. Since one is interested in measuring thecurrent flowing through the circuit, the ammeter must be connected in series with the measured circuit component ( [link] ). The positive lead from the ammeter must be connected closest to the positive end of the battery and the negative lead must be connected closest to the negative end of the battery.

## Impact of meters on circuits

A good quality meter used correctly will not significantly change the values it is used to measure. This means that an ammeter hasvery low resistance so as to not slow down the flow of charge. A voltmeter has a very high resistance so that it does not add another parallel pathway to the circuit for the charge to flow along.

## Investigation : using meters

If possible, connect meters in circuits to get used to how to use meters tomeasure electrical quantities. If the meters have more than one scale, always connect to the largest scale first so that the meter will not be damaged by having to measure values that exceed itslimits.

The table below summarises the use of each measuring instrument that we discussed and the way it should be connected to a circuitcomponent.

 Instrument Measured Quantity Proper Connection Voltmeter Voltage In Parallel Ammeter Current In Series

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