Define resistivity and give its unit of measurement - Leaving Cert Physics - Question 7 - 2008

Convection is the transfer of heat through the movement of fluids (liquids or gases), where warmer areas of fluid rise while cooler areas sink, creating a circulation pattern. This requires a medium for heat transfer.

Radiation, on the other hand, is the transfer of heat without the need for a medium. It occurs through electromagnetic waves and can happen in a vacuum. Unlike convection, radiation does not involve the movement of matter.

Given:

• Resistance, R = 12 Ω
• Length of the coil, L = 40 m
• Cross-sectional area, A = (\pi \left(\frac{d}{2}\right)^2 = \pi \left(\frac{2.2 \times 10^{-3}}{2}\right)^2)

Calculating the cross-sectional area:

$A = \pi \left(\frac{2.2 \times 10^{-3}}{2}\right)^2 = 3.80 \times 10^{-7} m^2$

Now, using the resistivity formula:

$\rho = \frac{R \cdot A}{L} = \frac{12 \times 3.80 \times 10^{-7}}{40} = 1.14 \times 10^{-6} \Omega m$

The heat generated can be calculated by the formula:

$H = P \cdot t$

Where:

• P = 1050 W (power of the toaster)
• t = 2 minutes = 120 seconds
• Efficiency = 96% = 0.96

First calculate the effective power based on efficiency:

Effective Power = 1050 W (\times 0.96 = 1008 W)

Now, substituting into the formula:

$H = 1008 W \cdot 120 s = 120960 J$

To reduce the risk of electrocution, electrical appliances like toasters often implement safety features such as:

• Double insulation: The electrical components are protected by two layers of insulating material, reducing the risk of electric shock.
• Earth leakage circuit breaker (RCD): This device detects leakage currents and disconnects the circuit, preventing electric shocks.
• Use of non-conductive materials in construction: This ensures that users do not come into contact with electrical components when using the appliance.

When a metal is heated, the thermal energy increases the kinetic energy of the electrons within the metal. This causes the electrons to jump to higher energy states. When they return to their original (lower energy) states, they emit energy in the form of light. The light emitted is characteristic of the material and typically includes various wavelengths, resulting in visible light, such as the red light emitted by the heating coil of the toaster.