Physics Concepts Review

Scalars and Vectors

• Scalars are quantities that possess only size (magnitude) and no direction. Examples of scalar quantities include:

  • Distance
  • Speed
  • Mass
  • Energy

• Vectors are quantities that have both size and direction. Examples of vector quantities include:

  • Displacement
  • Velocity
  • Acceleration
  • Force
  • Weight

• Resultant Force

  • The resultant force is defined as a vector quantity that combines all acting forces on an object.
  • Forces acting in the same direction can be added together to find the total force.
  • Forces acting in opposite directions can be subtracted to determine the net force.

• Resolution of Forces

  • Resultant forces can be resolved into their horizontal and vertical components to analyze motion more effectively.

• Acceleration

  • Defined as the change in velocity, which can involve speeding up, slowing down, or changing direction.
  • Negative acceleration, which refers to slowing down, is commonly termed deceleration.
  • The standard unit of acceleration is measured in meters per second squared, denoted as $m/s^2$.
  • Objects in circular motion experience acceleration since their direction is continuously changing.
  • Near Earth's surface, all objects experience a gravitational acceleration of approximately $9.8 ext{ m/s}^2$.
  • Air resistance (or drag) increases as an object's speed increases.

Newton's Laws

• Newton's Laws of Motion
  • Newton's Third Law states: "For every action, there is an equal and opposite reaction."
  • Newton's First Law indicates that an object will remain in its current state of motion unless acted upon by an unbalanced force.
  • If the resultant force is $0 ext{ N}$, a stationary object will continue to remain stationary.
  • If the resultant force is $0 ext{ N}$, an object in motion will continue moving at a constant velocity.
  • If the resultant force is not $0 ext{ N}$, a stationary object will accelerate in the direction of that resultant force.
  • If the resultant force is not $0 ext{ N}$, an object in motion will also accelerate in the direction of the resultant force.

Acceleration

• Definition: Acceleration is the rate of change of velocity.
• Calculation of Change in Velocity: Change in velocity can be computed as final velocity minus initial velocity.

Velocity-Time Graphs

• Description of Motion Using Graphs: Velocity-time graphs visually represent an object's motion.
  • A horizontal line on such a graph denotes constant velocity.
  • A straight line with a positive gradient represents an object accelerating positively (speeding up).
  • A straight line with a negative gradient depicts negative acceleration or deceleration (slowing down).
  • Acceleration can be calculated by determining the gradient of the velocity-time graph.
  • Distance covered can be interpreted by calculating the area under the graph.
  • A curved line indicates that acceleration is varying.

Internal Energy

• Total Internal Energy is defined as:

  • Internal energy = kinetic energy of the particles in a system + potential energy of particles in a system.

• Kinetic Energy: Particles in solids, liquids, and gases are in constant motion, thus possessing kinetic energy. The hotter a material is, the faster its particles move, leading to increased kinetic energy.

• Potential Energy: Occurs because particles' motion keeps them separated; the larger the distance between particles, the greater the potential energy. Gases have more internal energy due to their higher kinetic and potential energy compared to solids and liquids.

• Heating and Energy Changes:

  • Heating impacts the energy stored in a system by enhancing particle energy levels, thereby raising temperature or causing a change of state.
  • The thermal energy of an object is contingent upon its mass, temperature, and material composition.

Thermal Transfers

• Energy Transfer: Energy flows from hotter to cooler substances.
• Temperature: Represents the kinetic energy and motion of particles.
  • When thermal energy is added to an object through heating, its temperature is influenced by the substance's composition, mass, and amount of transferred energy.
• Conduction: Energy transfer through particle vibration. Metals are effective thermal conductors due to the mobility of delocalised (free) electrons.
• Convection: Occurs as particles in heated fluids rise, creating less dense areas. Fluids, including liquids and gases, expand when heated, increasing particle gaps leading to convection currents.
• Radiation: Transfer of thermal energy via waves; occurs in a vacuum without relying on particles. Surface properties affect radiation absorption and reflection (e.g., shiny surfaces reflect radiation well).

Changes in Energy States

• Heating Effects in States:
  • When temperature raises without change of state, internal energy increases, though potential energy remains static.
  • During boiling, temperature stays constant while internal energy rises through kinetic energy stability and potential energy growth.

Specific Heat Capacity

• Definition: Specific heat capacity is the energy needed to raise the temperature of 1 kg of substance by 1 °C.

• Formula: $AE = mcΔT$ where:

  • $AE$ = energy change (J)
  • $m$ = mass (kg)
  • $c$ = specific heat capacity (J/kg °C)
  • $ΔT$ = temperature change (°C)

• Variation: Different substances require varying energy amounts to heat or change state.

Specific Latent Heat

• Definition: Specific latent heat of a material represents the energy needed to alter the state of 1 kg of substance without temperature change.

• Formula: $E = mL$ where:

  • $E$ = energy for a change of state (J)
  • $m$ = mass (kg)
  • $L$ = specific latent heat (J/kg)

• Types of Latent Heat:

  • Specific latent heat of fusion: change from solid to liquid.
  • Specific latent heat of vaporisation: change from liquid to vapor.

• Energy Impact on State Change: During a state change, kinetic energy does not rise. Energy is absorbed into potential energy to weaken inter-particle forces, allowing a state change. A solid at its melting point has less energy than the same mass of liquid at equivalent temperatures.

Mains Electricity

• Voltage: Also known as potential difference (p.d.), measured in Volts (V) using a voltmeter.

• Simple Circuit Structure: Typically consists of two wires - a live wire and a neutral wire.

  • The live wire (brown): Conducts electricity from the power source to the appliance.
  • The neutral wire (blue): Returns electricity from the appliance to the power source, completing the circuit.

• Switch Placement: Essential for safety, ensuring that when an appliance is off, it does not carry live voltage.

Direct and Alternating Current

• Direct Current (d.c.): Flows in one constant direction, typically supplied by cells and batteries.

• Alternating Current (a.c.): Reverses direction continually; in the UK, has a potential difference of 230V and a frequency of 50Hz.

• Current Representation: Represented graphically with oscillating wave patterns in terms of potential difference over time.

Electrical Safety Features

• Three-Core Cable: Most UK appliances use this type, consisting of:

  • Blue neutral wire
  • Brown live wire
  • Green/yellow earth wire (safety feature)

• Earth Wire Purpose: Connects to the appliance's case to prevent shock in case of wire exposure, while it remains at 0V potential difference.

• Plastic Casing: Reduces shock risk as it is a poor conductor; some plugs may not require an earth wire due to material properties.

Power

• Definition: Power is the rate at which energy is transferred or work is done, measured in Watts (W).

• Formula:

  • $P = \frac{E}{t}$ where:
  • $P$ = power (W)
  • $E$ = energy (J)
  • $t$ = time (s)

• Conversion: 1 Watt of power corresponds to 1 Joule of energy transferred each second.

Cost of Electricity

• Usage of Electricity: Terms associated with using electricity involve energy transferred electrically.
• Electricity Meters: Measure the number of electricity units (energy) consumed leading to billing.
• Billing Calculation: Energy transferred is generally measured in kilowatt-hours (kWh).
• kWh Definition: A unit representing energy transferred.
• Calculations Related to Units:
  • $E = P imes t$
  • $ext{Units (kWh)} = ext{power (kW)} imes ext{time (h)}$
  • Total cost = number of units x cost per unit.

Energy Transfer in Appliances

• Power Calculations: Can also assess power if the current flowing through an appliance and the potential difference across it are known.
• Power Formula:
  • $P = I imes V$ where:
  • $P$ = power (W)
  • $I$ = current (A)
  • $V$ = voltage (V)
• Energy Transfer Calculation:
  • $E = P imes t$
  • $E = Q imes V$, where:
  • $Q$ = charge flow (Coulombs)

Energy Resources

• Fossil Fuels: Non-renewable resources, examples being coal, oil, and natural gas. They release significant energy when burned.

• Nuclear Energy: Derived from atomic nuclei splitting, utilizing materials like uranium and plutonium.

  • Does not produce carbon or sulfur dioxide during reactions but poses health risks if radioactive material is released through accidents.

• Renewable Resources: Resources naturally replenished while in use. Examples include:

  • Biofuels, wind, hydroelectricity, geothermal, tidal, solar, and water waves.

• Comparison of Resources:

  • Advantages: Reliability (to meet demand), lower emissions, sustainable (e.g., new crops for biofuels).
  • Disadvantages: Non-renewable nature of fossil fuels versus reliability issues for renewables (e.g., wind and solar).

Electrical Grid

• National Grid: A system of cables, pylons, and transformers that transmit electrical power from generating stations to end-users.

  • Does not include power plants or homes; solely represents infrastructure for electrical transfer.

• Pylon Usage: Supports overhead power lines in the national grid.

• Transformers:

  • Step-up Transformers: Increase the potential difference for efficient energy transfer.
  • Step-down Transformers: Decrease potential difference for safety in use.