State the principle on which a moving-coil galvanometer is based - Leaving Cert Physics - Question b - 2013

To convert a galvanometer to an ammeter, a small resistance (shunt) is connected in parallel with the galvanometer. This allows most of the current to bypass the galvanometer, ensuring it only measures a fraction of the total current.

• Label: R_s (small resistance) connected in parallel.

To convert a galvanometer to a voltmeter, a large resistance (multiplier) is connected in series with the galvanometer. This ensures that only a small fraction of the voltage is applied across the galvanometer.

• Label: R (large resistance) connected in series.

To convert the galvanometer to function as an ammeter reading up to 5 A, use the following formula:

$I_{shunt} = I_{total} - I_{g}$

Where:

• $I_{shunt}$ is the current through the shunt resistor.
• $I_{total}$ is the maximum current (5 A).
• $I_{g}$ is the galvanometer current (2 mA = 0.002 A).

The current through the shunt resistor is: $I_{shunt} = 5 - 0.002 = 4.998 ext{ A}$

Then, apply the formula for resistances:

$V = I_g imes R_g$ $R_{shunt} = \frac{V}{I_{shunt}} = \frac{0.002 imes 100}{4.998} \approx 0.040 ext{ Ω}$

Using a shunt of approximately 0.040 Ω will allow the galvanometer to function correctly as an ammeter.

An example of another device based on the same principle is a d.c. motor or a moving-coil loudspeaker.

Part A: Generates a large emf (electromotive force) when the circuit is activated, causing the magnetic field to induce.

Part B: Produces sparks or electrical discharges used in applications such as igniting a spark plug in a car or activating an electric fence.

One application of the induction coil is to create a spark in the engine of a car (spark plugs) or for use in electric fences.