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

To convert a moving-coil galvanometer into an ammeter, a shunt resistor is used in parallel with the galvanometer. This allows a larger current to flow while directing a smaller, measurable portion through the galvanometer.

1. Diagram: Show the galvanometer and the shunt resistor in parallel.

2. Label the galvanometer as 'G' and the shunt resistor as 'R_s'.

3. Low Resistance: The shunt has a low resistance to ensure the vast majority of the current bypasses the galvanometer.

(b) To convert into an ohmmeter, a variable resistor is connected in series with the galvanometer.

1. Diagram: Show the galvanometer with a variable resistor and a power supply connected in series.

2. Label the components: 'G' for the galvanometer, 'R_v' for the variable resistor, and 'V' for the power supply.

3. Black connection: This setup measures the current through the resistor, displaying resistance on the galvanometer's scale.

To convert the galvanometer into a voltmeter, we use the formula:

$R_{total} = R_g + R = \frac{V}{I}$

Where:

• $V$ = full scale deflection of voltmeter = 10 V
• $I$ = full scale deflection of galvanometer = 50 mA = 0.050 A
• $R_g$ = internal resistance of galvanometer = 7.2 Ω

First, calculate the total resistance (R_total):

$R_{total} = \frac{10 ext{ V}}{0.050 ext{ A}} = 200 ext{ Ω}$

Now, subtract the internal resistance:

$R = R_{total} - R_g = 200 ext{ Ω} - 7.2 ext{ Ω} = 192.8 ext{ Ω}$

Thus, the resistance required is approximately 192.8 Ω.

An LED, or Light Emitting Diode, is a semiconductor device that emits light when an electric current passes through it. It consists of a p-n junction that allows current to flow in one direction, producing photons during recombination of charge carriers.

The operation of an LED is based on the principle of electroluminescence. When forward bias is applied to the LED, electrons from the n-region combine with holes in the p-region, releasing energy in the form of photons.

1. Diagram: Show a p-n junction setup with arrows indicating the flow of current from the p-type to the n-type.

2. Label the layers: 'p-type', 'n-type', and indicate forward bias direction.

3. Energy interaction: Highlight the photon emission as a result of electron-hole recombination.

In a circuit, LEDs can be protected using:

1. A current limiting resistor: This ensures that the current flowing through the LED is within safe limits, preventing burnout.
2. Series connection: Connecting LEDs in series with a resistor helps distribute the voltage correctly, further protecting from excess current.

1. Light Emission: LEDs emit light when current flows through them, while photodiodes are designed to absorb light and generate an electrical current.
2. Functionality: LEDs are used primarily for light generation in displays and indicators, while photodiodes are used mainly for light detection and measurement applications.