To measure the current flowing through an appliance, an ammeter is connected in series with the appliance so that the current it measures should flow through it.

This is possible because ammeters have very low resistances.

Meanwhile to measure the voltage drop across an appliance, the voltmeter is connected in parallel to the appliance.

This is because voltmeters have very high resistances. If connected in series, a voltmeter will obstruct the current flowing in the circuit as seen above.

Electromotive force (e. m. f.) and internal resistance

The electromotive force (e. m. f.) of a voltage source is the amount of other forms of energy it converts to electrical energy for a unit charge.

E. m. f. is got by connecting a voltmeter across the voltage source when it is not supplying current.

Meanwhile the internal resistance:

Fig; parallel (b) and series (a) connection of cells

When connected in series, the combined e. m. f. (driving force) is the sum of the e. m. f. of the individual cells.

The combined internal resistance is also the sum of the individual internal resistances.

When connected in parallel, the combined e. m. f. remains the e. m. f. of one of the cells (Provided that the cells have the same e. m. f.)

However, the combined internal resistance is lower than the internal resistance of each of the cells since they are like resistances in parallel.

In the connection diagram above, the bulb in (a) will shine brighter than that in (b) because of the higher driving force when cells are connected in series.

 However, the cells in (b) will last longer due to the lower combined internal resistance.

Examples related to topic

The electrical power and energy equations containing R (resistance) above are true only when all the electrical energy is converted to thermal energy. These equations do not hold when part of the electrical energy is converted mechanical work as in a motor and chemical work as n a lead accumulator being charged.

A device which converts all the electrical energy supplied to it into heat is called a passive resistor. The heater elements of electric kettles, bulbs, heaters, etc are made up of passive resistant wires. The quantity of heat released or the rate of dissipation of heat (power) can be calculated using the above equations. ·

The heating effect of current is referred to as Joule effect.