A moving-coil galvanometer detects and measures small currents - Leaving Cert Physics - Question b - 2020

(a) Ammeter Conversion Diagram:

• Connect a low resistance shunt in parallel with the galvanometer.
• The shunt allows most of the current to bypass the galvanometer, protecting it from high currents while ensuring accurate measurement. Label the diagram with the: galvanometer, shunt resistor, and total current entering the circuit.

Diagram Explanation:

1. Shunt configured in parallel with the galvanometer ensures only a fraction of the total current flows through it.

(b) Ohmmeter Conversion Diagram:

• Connect a variable resistor (potentiometer) in series with the galvanometer. Label the connections as well as the variable resistor.

Diagram Explanation:

1. When the ohmmeter is connected across the resistor being measured, current flows and causes needle deflection. The variable resistor allows for calibration to measure resistance accurately.

To convert the galvanometer into a voltmeter, the resistance required can be calculated using Ohm's law.

1. Calculate the total resistance required (R_total):
   Using the formula:
   R_{total} = rac{V}{I}
   where V = 10 V and I = 0.050 A (50 mA):
   R_{total} = rac{10}{0.050} = 200 \, ext{Ω}

2. Calculate the series resistance (R):
   The resistance needed in series with the galvanometer:
   $R = R_{total} - R_g$
   where R_g = 7.2 Ω:
   $R = 200 - 7.2 = 192.8 \, ext{Ω}$
   Therefore, to convert the galvanometer into a voltmeter, a resistance of approximately 193 Ω is required.

An LED (Light Emitting Diode) is a semiconductor device that emits light when an electric current passes through it. It operates based on the principle of electroluminescence, where charge carriers recombine to release energy in the form of photons. LEDs are known for their efficiency and longevity compared to traditional light sources.

An LED consists of a p-n junction where the p-type material is rich in holes and the n-type material is rich in electrons. When a forward voltage is applied, electrons move from the n-side to the p-side, and holes move to the n-side.

When recombination occurs at the junction, energy is released in the form of light photons.

Diagram Explanation:

1. Indicate the p-region and n-region, showing the direction of current flow.
2. Show the emitted light as arrows diverging from the junction.

To protect LEDs in a circuit, various methods can be employed:

1. Use of a Resistor: Placing a resistor in series with the LED limits the current flowing through it, preventing damage from excess current.
2. In Series with a Fuse: Incorporating a fuse that blows when current exceeds a safe level protects the LED.

These methods ensure that the LED operates within its safe limits.

1. Light Emission: LEDs emit light when current flows through them, while photodiodes absorb light and generate a current in response to incident light.
2. Operating Mode: LEDs operate in forward bias to emit light, whereas photodiodes are typically operated in reverse bias to detect light.