State Ohm's Law - Leaving Cert Physics - Question 8 - 2005

The metal conductor obeys Ohm's law. This is indicated by the graph (a) where the current (I) increases linearly with the potential difference (V), resulting in a straight line through the origin. In contrast, the filament bulb (graph b) exhibits a non-linear relationship due to its temperature-dependent resistance, which changes as the current changes.

To find the total resistance of the circuit when a 100 Ω resistor is in series with a 500 Ω thermistor, use the formula:

$R_{total} = R_1 + R_2$

where:

• $R_1 = 100 \, \Omega$
• $R_2 = 500 \, \Omega$

Thus:

$R_{total} = 100 \, \Omega + 500 \, \Omega = 600 \, \Omega$

Using Ohm's Law, we can find the current (I) flowing in the circuit:

$I = \frac{V}{R}$

Given that the total voltage (V) is 6 V and the total resistance (R) is 600 Ω:

$I = \frac{6 \, V}{600 \, \Omega} = 0.01 \, A$

To calculate the potential difference (V1) across the 100 Ω resistor, we can use Ohm’s Law again:

$V_1 = I imes R_1$

Using:

• $I = 0.01 \, A$
• $R_1 = 100 \, \Omega$

Thus:

$V_1 = 0.01 \, A \times 100 \, \Omega = 1 \, V$

As the thermistor is heated, its resistance decreases. This is due to the fact that thermistors are temperature-sensitive resistors, which typically lower their resistance with an increase in temperature.

As the thermistor's resistance decreases when heated, the total circuit resistance also decreases. Consequently, this leads to an increase in the current flowing through the circuit, which may change the potential difference across the 100 Ω resistor according to Ohm's law. The potential difference across it might increase depending on the current and the configuration.

Thermistors are commonly used in temperature sensing applications. For instance, they can be found in digital thermometers, where they help in accurately measuring temperature changes.