Physics 1005 Lecture Summary 4/25

Heat Transfer and Refrigeration

• Heat naturally flows from hot to cold objects/regions when in thermal contact.
  • Example: Heat flows into a cooler house from a warmer outside environment (85°F house, 90°F outside).
• Refrigeration requires energy to force heat to flow from a cold to a hot environment.
• The Vapor-Compression Refrigeration Cycle:
  • Components:
    • Evaporator (Evaporator Coil).
    • Compressor.
    • Condenser (Condenser Coil).
    • Expansion Valve (Metering Device).
  • Process:
    1. Evaporator: Low-pressure cold liquid refrigerant extracts heat from the air, causing the refrigerant to evaporate.
    2. Compressor: Pressurizes the gas, making it hot (hotter than the ambient temperature outside).
    3. Condenser: Heat is released to the outside as the refrigerant gas condenses back into a liquid.
    4. Expansion Valve: Reduces the pressure and temperature of the liquid refrigerant. High-pressure liquid passes through the valve, causing a rapid pressure drop that leads to partial evaporation and cooling.

Electric Charge

• Electric charge is a fundamental quantity, similar to mass.
• Two types of charge: positive and negative.
  • Protons: positive charge.
  • Electrons: negative charge.
• Charge is a property that particles may or may not possess (like magnetism).
• "Charges" often refers to particles that have charge, such as electrons.
• Unit of charge: Coulomb (C).
• Like charges repel; unlike charges attract.
• Electricity and magnetism are related phenomena.
• Coulomb's Law:
  • The force between charged objects is proportional to the magnitude of their charges and inversely proportional to the square of the distance between them.
  • F \propto \frac{\text{(charge on object 1)} \cdot \text{(charge on object 2)}}{\text{(distance between objects)}^2}
  • More charge means a stronger force.
  • Greater distance means a weaker force.

Moving Charges and Electric Potential

• What causes charges to move in one direction (e.g., in a wire)?
• Analogy: A ball on a hill rolls down due to gravitational potential.
• Electric potential (voltage) is the "electric hill" that causes charges to move.
  • Measured in volts.
  • Voltage and electric potential are the same thing.
• Voltage is what causes charges to move.
• Remember the V shape of hills to associate with Voltage.
• Batteries supply the electric potential (voltage) to move charges.

Electric Current

• Electric current: the movement or flow of electric charge per unit time.
  • \text{Electric current} = \frac{\text{flow of charge}}{\text{time}}
• Analogy: Water current in a river is the flow of water.
• Electric current in a wire is the flow of electric charge.
• Units of water flow: Gallons per minute (Gal/min).
• Units of electric current: Coulombs per second (C/s), also known as Ampere (A).
  • 1 \text{ ampere} = \frac{1 \text{ Coulomb}}{\text{second}}
• 10 Coulombs of charge passing a point in a wire every second equals a current of 10 amps.
• Electric potential (voltage) causes current to flow in a wire.
• Important to understand the difference between current and voltage.