Physics: Mass, Volume, and Thermal Conductivity

Determining Object Mass and Volume

• Regular objects: Mass is determined using a balance; volume is calculated by multiplying length, width, and height. Density is mass divided by volume.
• Irregular objects: Mass is determined using a balance. Volume is measured by water displacement in a eureka can and measuring cylinder. Density is mass divided by the displaced water volume. Repeat measurements and calculate the mean for improved accuracy.
• Sources of inaccuracy: Water spillage, balance inaccuracy, incomplete water displacement, and falling hazards. Care should be taken to minimize these.

Thermal Conductivity in Buildings

• Modern houses have two-layered walls (brick and breezeblock with a cavity). High thermal conductivity of these walls leads to energy loss and increased heating costs.
• Cavity wall insulation (low thermal conductivity material) reduces energy loss.
• Double-glazed windows and loft insulation also reduce energy loss by minimizing convection and heat transfer.
• Thicker walls reduce the rate of thermal energy transfer.

Thermal Insulator Practical

• Experiment setup: Two beakers (small inside large), hot water, cardboard lid with thermometer hole, stopwatch, insulating materials.
• Procedure: Measure water temperature at 3-minute intervals for 15 minutes. Repeat with various insulating materials (same mass). Plot a cooling curve.
• Newspaper experiment: Repeat the above, adding layers of newspaper to observe the effect on cooling rate.
• Sources of Inaccuracy: Inconsistent initial water temperature, human error during water transfer, timing errors, and inconsistent insulator mass. Minimizing these errors will increase experimental accuracy.

Efficiency

• Efficiency is calculated as (useful energy output / total energy input) or (useful power output / total power input).
• Efficiency improvements include using wider-based pans with lids, strategically placed heating elements, plastic walls/lids, lubrication (mechanical devices), and insulation.

Electricity and Electrical Appliances

• Power (Watts or J/s) is the rate of energy transfer or work done. Power = Energy transferred/Time = Work done/Time = Voltage x Current = Current² x Resistance.
• Appliances and their energy conversions:
  • Blender, fan, hairdryer: Electrical energy to kinetic energy (with thermal energy waste).
  • Kettle, iron: Electrical energy to thermal energy.
  • Lamp: Electrical energy to light energy (with thermal energy waste).
  • Refrigerator, washing machine, toaster: Various energy conversions depending on the appliance function.

Circuits and Components

• Circuit components: Cell, battery, voltmeter, ammeter, filament bulb/lamp, open/closed switch, fuse, light-dependent resistor (LDR), variable resistor, thermistor, diode, light-emitting diode (LED), resistor.
• Potential difference (voltage): The force driving current flow. 0 volts means no force and no work done.
• Current: The rate of charge flow (flow of electrons). Conventional current is depicted as flowing from positive to negative, despite electrons' actual flow in the opposite direction.
• More energy leads to higher voltage (greater potential for energy transfer).
• Series circuits: Current remains constant throughout the circuit.

Charge, Current and Time

• Relationship between charge (coulombs), current (amperes), and Time (seconds): Current = Charge/Time

Direct and Alternating Current (AC/DC)

• Direct current (DC) flows in one direction only.
• Alternating current (AC) constantly changes direction; in the UK, it does this 50 times per second (50 Hz).
• UK mains AC has a potential difference (PD) of approximately 230V.
• AC is easier to transform to increase or decrease PD.

Transformers

• Transformers only work with AC due to the need for a changing magnetic field to induce a PD.
• They consist of a primary and secondary coil around an iron core.The coils are electrically separate.
• A changing magnetic field in the primary coil induces a PD in the secondary coil.
• Step-up transformers increase PD (more turns in secondary coil).
• Step-down transformers decrease PD (fewer turns in secondary coil).
• PD relationship: V_p/V_s = N_p/N_s (where V is PD and N is number of turns).
• Power is conserved in an ideal transformer: P_p = P_s (power in primary = power in secondary).

Mains Electricity and Safety

• Mains electricity is AC.
• Three-core cables: live wire (230V), neutral wire (0V), and earth wire (0V).
• The earth wire provides a low-resistance path to ground, preventing electric shock.
• The fuse in the plug melts if a large current flows due to a fault, breaking the circuit.
• Even when switched off, the live wire remains at 230V and presents a shock hazard.
• Copper wires are used for their conductivity.
• PVC insulation prevents shock.

The National Grid

• The National Grid uses step-up transformers to increase PD to 400kV for efficient long-distance transmission (reducing current and heat loss).
• Step-down transformers reduce PD to 230V for safe household use.
• Electricity is generated at 25kV.

Experimental Sources of Inaccuracy

• Zero error in measuring instruments is a systematic error, requiring subtraction from all readings.
• Heating effects in wires increase resistance, minimized by using a low PD or applying current only during measurements.

Explore the principles of determining mass and volume of both regular and irregular objects, emphasizing the importance of accuracy and sources of inaccuracy. Additionally, learn about the impact of thermal conductivity in building design and the ways to minimize energy loss through insulation techniques.