When heat is transferred to or from an object the temperature of the object changes - Leaving Cert Physics - Question 9 - 2013

• Conduction
• Convection
• Radiation

To demonstrate heat transfer in a liquid:

Apparatus:

• A beaker filled with water
• A heat source (e.g., a heater)
• Food coloring (optional)

Procedure:

1. Place a heater at the bottom of the beaker.
2. Start heating the water while observing the motion.
3. (Optional) Drop food coloring into the water.
4. Observe how the coloring spreads as the water heats up.

Conclusion: This experiment shows convection currents, where warmer, less dense water rises while cooler, denser water sinks.

In a kettle, the heating element must be positioned at the bottom. Heat transfer occurs primarily through convection in liquids, where heated water rises while cooler water sinks. This positioning ensures efficient heating throughout the body of water.

The handle of a kettle is made of insulating material because insulators do not conduct heat well. This design ensures that the handle remains cool to the touch, providing safety and comfort when handling the kettle.

Suitable insulating materials include plastic or rubber.

Using the formula: $Q = mc \Delta T$ we calculate:

• $m = 1.3 \, \text{kg}$
• $c = 4200 \, \text{J kg}^{-1} K^{-1}$
• $\Delta T = 80 \, \text{°C} - 10 \, \text{°C} = 70 \, \text{°C}$

Substituting values: $Q = 1.3 \, \text{kg} \times 4200 \, \text{J kg}^{-1} K^{-1} \times 70 \, \text{K} = 38.8 \, \text{kJ}$

First, we find the time in seconds:

• $3$ minutes = $180$ seconds.

Power ($P$) can be calculated using the formula: $P = \frac{E}{t}$ where $E$ is energy and $t$ is time.

Using the energy calculated: $P = \frac{38.8 \times 10^3 \, \text{J}}{180 \, \text{s}} \approx 215.56 \, \text{W}$

Thus, the power rating is approximately $216 \, \text{W}$.