Study guide: Power and Energy Transfers

📋 Course Outline

1. Power and Its Measurement
2. Energy Stores and Conservation
3. Efficiency of Energy Transfers
4. Work and Energy Equations
5. Energy Store Changes in Systems

📖 1. Power and Its Measurement

🔑 Key Concepts & Definitions

• Power: The rate at which energy is transferred or work is done

★ Must-know

• A transfer of 1 joule of energy per second is equal to a power of 1 watt.

Further detail

📌 A higher-power motor lifts the same mass in a shorter time, whereas a lower-power motor takes longer.

📖 2. Energy Stores and Conservation

🔑 Key Concepts & Definitions

• System: An object or group of objects

★ Must-know

📌 In a closed system, energy cannot be created or destroyed, so the total energy remains constant.

• During all system changes, energy is dissipated and stored in less useful ways, usually in the thermal store of the surroundings.

Further detail

• The five listed energy stores are:

  • kinetic
  • gravitational potential
  • chemical
  • thermal
  • elastic potential

• Lubrication can reduce unwanted energy transfers by reducing friction.

• Insulation can reduce unwanted energy transfers by reducing the rate of thermal energy transfer.

💡 Memory Hook

Energy is conserved, but dissipated energy becomes less useful.

📖 3. Efficiency of Energy Transfers

★ Must-know

📐 Formula - Efficiency is the useful energy output divided by the total energy input: $\text{Efficiency} = \frac{\text{Useful energy output}}{\text{Total energy input}}$.

• Efficiency has no unit.

Further detail

📐 Formula - Efficiency can also be calculated using power: $\text{Efficiency} = \frac{\text{Useful power output}}{\text{Total power input}}$.

📖 4. Work and Energy Equations

★ Must-know

📐 Formula - Work done equals force multiplied by distance: $W = F \times s$.

📐 Formula - Kinetic energy equals half the mass multiplied by the square of the speed: $E_k = \frac{1}{2}mv^2$.

📐 Formula - Gravitational potential energy equals mass multiplied by gravitational field strength and height: $E_p = mgh$.

📐 Formula - Energy equals power multiplied by time: $E = Pt$.

📐 Formula - Elastic energy equals half the spring constant multiplied by the square of the extension: $E_e = \frac{1}{2}ke^2$.

Further detail

• Work done and all forms of energy are measured in joules (J).

📖 5. Energy Store Changes in Systems

📝 Essential Points

• Projecting a tennis ball upwards decreases its kinetic energy store and increases its gravitational potential energy store.

• When a vehicle slows down, its kinetic energy store decreases and its thermal energy store increases.

• When a battery-powered drill is turned on, its chemical energy store decreases while its kinetic and thermal energy stores increase.

• When a person slides down a zip-wire, the gravitational potential energy store decreases while the kinetic and thermal energy stores increase.