Physics Definitions: Half-life, Heat and more

• Study notes on key Physics definitions, including: Half-life, Heat, Specific Heat Capacity, Power, Electric Field, Charge, Current, Reflection, Refraction, and Magnification

Half-life

• Half-life is the time required for half of the radioactive nuclei in a sample to decay.
• It measures the rate of radioactive decay and its SI unit is seconds (s).
• Radioactive substances decay over time, emitting radiation (alpha, beta, or gamma), and the activity of a radioactive sample decreases over time.
• The half-life is constant for a given isotope and is not affected by temperature or pressure.
• The formula is: 𝑁 = 𝑁₀ × (1/2)^(𝑡/𝑇), where 𝑁 is remaining undecayed nuclei, 𝑁₀ is the initial number of nuclei, 𝑡 is the elapsed time, and 𝑇 is the half-life.
• For example, a substance with a half-life of 10 minutes will have half of it remaining after 10 minutes, and one-quarter remaining after another 10 minutes.

Heat

• Heat is a form of energy transfer due to a temperature difference between two bodies.
• The SI unit is the Joule (J).
• Heat flows from a hotter object to a colder one, and can be transferred through conduction, convection, or radiation.

Specific Heat Capacity

• Specific heat capacity is the amount of heat required to raise the temperature of 1 kg of a substance by 1°C (or 1K).
• The SI unit is Joules per kilogram per Kelvin (J/kg·K).
• The formula is: 𝑄 = 𝑚𝑐Δ𝑇, where 𝑄 is heat energy (J), 𝑚 is mass (kg), 𝑐 is specific heat capacity (J/kg·K), and Δ𝑇 is the change in temperature (°C or K).
• Different materials absorb heat at different rates.
• Water has a high specific heat capacity (4200 J/kg·K), meaning it takes a lot of heat to warm up or cool down.

Power

• Power is the rate at which work is done or energy is transferred.
• Its SI unit is Watt (W) = Joules per second (J/s).
• The formula is: 𝑃 = 𝑊/𝑡 = 𝐸/𝑡, where 𝑃 is power (W), 𝑊 is work done (J), 𝐸 is energy transferred (J), and 𝑡 is time (s).
• Higher power means more work is done in less time.
• 1 Watt means 1 Joule of energy is transferred per second.

Electric Field

• An electric field denotes the region around a charged object where it exerts a force on other charges.
• The SI unit is Newton per Coulomb (N/C) or Volts per meter (V/m).
• The formula is: 𝐸 = 𝐹/𝑞, where 𝐸 is electric field strength (N/C), 𝐹 is force on charge (N), and 𝑞 is charge (C).
• Electric fields point away from positive charges and toward negative charges.
• The field is stronger near the charge and weaker further away.

Charge

• Charge is a fundamental property of matter causing it to experience an electric force.
• The SI unit is Coulomb (C).
• The formula is: 𝑄 = 𝐼𝑡, where 𝑄 is charge (C), 𝐼 is current (A), and 𝑡 is time (s).
• Protons have a +1.6 × 10⁻¹⁹ C charge.
• Electrons have a -1.6 × 10⁻¹⁹ C charge.

Current

• Current is defined as the rate of flow of charge in a circuit.
• The SI unit is Ampere (A) = Coulomb per second (C/s).
• The formula is: 𝐼 = 𝑄/𝑡, where 𝐼 is current (A), 𝑄 is charge (C), and 𝑡 is time (s).
• Conventional current flows from positive to negative.
• Electron flow is opposite to conventional current.

Reflection

• Reflection is the bouncing back of light when it hits a surface.
• The laws of reflection state that the angle of incidence equals the angle of reflection, and the incident ray, reflected ray, and normal lie in the same plane.
• The formula is: 𝜃ᵢ = 𝜃ᵣ, where 𝜃ᵢ is the angle of incidence and 𝜃ᵣ is the angle of reflection.

Refraction

• Refraction is the bending of light when it passes between two materials of different densities.
• The formula (Snell’s Law) is: 𝑛 = sin 𝑖 / sin 𝑟, where 𝑛 is the refractive index, 𝑖 is the angle of incidence, and 𝑟 is the angle of refraction.
• Light bends toward the normal when entering a denser medium, and bends away from the normal when entering a less dense medium.

Magnification

• Magnification is the ratio of the image size to the object size.
• The formula is: 𝑀 = Image Height / Object Height.
• 𝑀 > 1: Image is enlarged
• 𝑀 < 1: Image is reduced