Electric Physics

Electric Physics Ex...brrr...plained.

⚛ Protons & Electrons

• Protons → Positive (+)

• Electrons → Negative (-)

• Electrons move more easily than protons

⚖ Neutral Atoms vs. Ions

• Neutral Atom: Electrons = Protons

• Ion: Electrons ≠ Protons

  • Anion (-): Extra electrons

  • Cation (+): Fewer electrons

🔋 Elementary Charge & Coulomb

• Elementary charge (e) is defined as the charge of one electron (-e) or one proton (+e):

• 𝑒 = 1.6 × 10⁻¹⁹ C

• Where C = Coulomb

• 1 C = 6.24 × 10¹⁸ protons/electrons

⚡ Conductors vs. Insulators

• Metals (Conductors): Free-moving electrons through the lattice of positive ions.

• Non-metals (Insulators): Tightly bound electrons

⚡ What is Current?

• Rate of flow of electric charge

• Carried by electrons (in wires) or ions (in solution)

• Requires a complete circuit

• All circuits require an energy source, wires, and a circuit element.

🔄 Conventional vs. Electron Flow

• Conventional Current (I, current): + to -

• Electron Flow (electron current): - to +

Note: + = LONG SIDE, - = SHORT SIDE

Representing electric circuits

Voltmeters: don’t allow current to pass through it. Must be put in parallel series.

Ammeters: allow current to pass through it. May be placed in series.

Diode: the triangle shows the direction at which the current flows.

📏 Current Formula

• 𝑰 = 𝑸/𝒕 (Charge per second), I = Current (A), Q = amount of charge and t = time (s)

• 𝑸 = 𝒏ₑ × 𝒒ₑ (Charge = No. of electrons × charge per electron)

• Unit: Amperes (A), 1A = 1 Coulomb/s

📊 Measuring Current - Ammeter v.s. Voltmeter

• Ammeter measures current

• Connected in series with the circuit.

• A voltmeter measures voltage

• Connecting in parallel across components as it doesn’t allow current to pass through.

💧 Water Analogy for Current

• Battery = Water pump

• Current = Water flow

• Light bulb = Turbine (converting energy)

• Charges are not used up, like chain links in a bicycle

🔋 Energy for Electrons

• Electrons need energy to move, provided by the battery.

• When a circuit connects two ends of the battery, the battery transforms chemical energy into electrical potential energy.

⚡ Potential Energy in a Circuit - Potential Difference

• Potential energy is stored as a separation of charge between battery terminals.

• The amount of energy gained when moved from one to another battery.

• This is called potential difference (V), measured in volts (V).

• This can also be referred as: Volts / Electromagnetic Force (EM Force).

💡 Voltage Drop

• Voltage drop is the energy lost by each coulomb of charge.

• Equation: 𝑉 = 𝐸 / 𝑄

  • 𝑉 = potential difference (V)

  • 𝐸 = electric potential energy (J)

  • 𝑄 = charge (C)

⚙ Energy in a Circuit

Energy (E) = 𝑉 × 𝐼 × 𝑡

• 𝐸 = energy (J)

• 𝑉 = potential difference (V)

• 𝐼 = current (A)

• 𝑡 = time (s)

🧮 Power Formula

Power (P) is energy used per second: 𝑃 = 𝐸 / 𝑡

• 𝑃 = power (W)

• 𝐸 = energy (J)

• 𝑡 = time (s)

Power equation: 𝑃 = 𝑉 × 𝐼

• 𝑉 = potential difference (V)

• 𝐼 = current (A)

• Voltage and current are easily measured in a circuit.

We can then derive (from V=IR, Ohm’s Law):

Start with:
P = VI
Use Ohm’s Law: V = IR and I = V/R

1. P = VI = (IR) × I = I²R

2. P = VI = V × (V/R) = V²/R

3. Original: P = VI

So, the three forms are:

• P = VI = I²R = V²/R

Resistance (and to the flow of charge)

Resistance is a measure of how hard it is for current to flow through a material. Conductors have low resistance, while insulators have high resistance. Resistance is measured in ohms (Ω).

Energy required:

• Create an electric current: Separating electrons from atoms.

• Maintain an electric current: Keeping electrons moving through the material.

Note: two resistance in a circuit means faster since more cross-sectional area.

Electron Movement

• Electron movement:

  • Do not move in a straight line but bump into ions.

  • Although they move with great speed, their net speed (drift velocity/average speed) is slow.

• Example:

  • In a 10A current through copper, the electron's net speed is 0.16 mm/s or 1.6 x 10⁻⁴ m/s.

Ohm’s Law

• Georg Ohm's discovery:

  • Current is directly proportional to potential difference (I ∝ V).

• Equation:

  • 𝑅 = 𝑉 / 𝐼 or V = IR

  • Where:

    • V = potential difference in Volts (V)

    • I = current in Amps (A)

    • R = resistance in Ohms (Ω)

Ohmic / Non-ohmic Conductors

• Ohmic conductors:

  • Conductors that obey Ohm's law.

  • Usually called resistors.

  • Often used to control the amount of current and the voltage drop across parts of a circuit.

• Identification:

  • An ohmic conductor is identified by measuring the current that flows through it when different potential differences are applied.

Resistance of ohmic and non-ohmic conductors:

• Can be found from the gradient of the V-I (Ohmic) or I–V (non-Ohmic) graph

• Ohmic:

  • V ↑ → I ↑ → R ↑

  • V ↓ → I ↓ → R ↓

• Non-Ohmic:

  • V ↑ → m ↓ → R ↑

  • V ↓ → m ↓ → R ↓

• Gradient: Ohmic - just use rise/run; Non-ohmic: find avg gradient on a point from start.

Variables that Affect resistance

• Factors affecting resistance:

  • Directly proportional to length (l).

  • Inversely proportional to cross-sectional area (A).

  • Affected by resistivity (𝜌), a property of the material.

• General formula for resistance:

  • 𝑅 = 𝜌(𝐿 / 𝐴)

  • R = Resistance (Ohms, Ω)

  • 𝜌 = Resistivity (Ohm meters, Ω·m; constant)

  • L = Length of the conductor (meters, m)

  • A = Cross-sectional area of the conductor (square meters, m²)

Resistors - Colours Coding

Summary of Series vs Parallel

Aspect	Series Circuit	Parallel Circuit
Connection	Components in a single loop, one break all break.	Components in separate branches, one break others fine
Resistance (Rₜ)	RT=R1+R2+R3+⋯+Rn	1/RT=1/R1+1/R2+⋯+1/Rn 𝑅ₜ = (1/𝑅₁ + 1/𝑅₂ + 1/𝑅₃ + … + 1/𝑅ₙ)-1.
Voltage (V)	VT=V1+V2+V3+⋯+Vn	Same across all branches: VT=V1=V2=⋯=Vn
Current (I)	Same in all components: IT=I1=I2=⋯=In	Total current: IT=I1+I2+I3+⋯+IN
Resistance Note	Increases with more resistors	Always less than the smallest resistor 𝑅ₜ < 𝑅ₛ𝑚𝑎𝑙𝑙𝑒𝑠𝑡 𝑟𝑒𝑠𝑖𝑠𝑡𝑜𝑟.
Effect of Break	Whole circuit stops	Only the affected branch stops

Kirchhoff’s Loop Rule

• Potential difference in a closed-loop circuit:

  • The sum of the potential differences across all elements must be zero.

  • The energy gained (potential gain) equals the energy lost (potential drop) by the charges.

• In a series circuit:

  • The sum of the potential drops (V₁ + V₂) across components like the resistor and the lamp equals the EMF.

• Source of energy:

  • Devices that provide energy are referred to as sources of EMF (electromotive force), measured in volts.

Identical Resistors in Parallel Circuits

Identical resistors in parallel:

• If there are n resistors of equal value R, the equivalent resistance is:

  • 1/𝑅ₜ = 𝑛/𝑅

  • 𝑅ₜ = 𝑅/n.

Kirchhoff’s junction rule

• Kirchhoff’s junction rule:

  • The total current flowing into a junction must be equal to the total current flowing out of the junction.

Resistors and Power Formula

• Resistors in parallel circuits draw more current than in series circuits, leading to higher power consumption.

• Power formula:

  • P = V I, where:

    • P is the power (W)

    • V is the voltage (V)

    • I is the current (A).

Transducers

• Transducer: A device that converts one form of energy into another (e.g., microphone, light bulb, speaker, battery).

  → Input transducers: convert physical signals → electrical (e.g. microphone, temperature sensor)

  → Output transducers: convert electrical signals → physical (e.g. speaker, LED, motor)

Input Transducers

Light Dependent Resistor (LDR):

• A transducer whose resistance varies based on the amount of light falling on it.

• Commonly used in devices like cameras, streetlights, and night lights, as well as in detectors for various applications.

Thermistor:

• A variable resistor that changes with temperature, with types:

• NTC (Negative Temperature Coefficient) — resistance decreases as temperature increases.

• PTC (Positive Temperature Coefficient) — resistance increases as temperature increases.

• WARNING: NO SLOPE = NOT THERMISTOR

• Applications: Thermistors are used in temperature-sensitive devices (e.g., refrigerators, toasters).

Potentiometer:

• A variable resistor with three terminals, used to adjust current/voltage or divide voltage. The wiper's position controls resistance.

Voltage Divider

• Definition: A circuit that reduces an input voltage to a desired output voltage.

• Components: Typically uses two resistors in series.

• Function: Divides the voltage in proportion to the resistances.

• Not a transducer: It doesn't convert energy; just modifies voltage.

AC / DC

AC (Alternating Current):
Electrons oscillate back and forth in the circuit. Household energy is AC with a voltage of -340V to +340V, equivalent to 240V rms at 50 Hz.

DC (Direct Current):
Electrons flow in one direction only. Batteries, such as a car battery, provide DC, usually at 12V.

Root-Mean-Square (RMS)

RMS (Root Mean Square) is the effective value of an alternating current (AC) or voltage, calculated by squaring all values, averaging them, and then taking the square root, which gives the equivalent DC value that would produce the same power.

• Square each value of the signal: First, you take all the values in the signal and square them (make them positive).

• Find the average: Next, you calculate the average of all these squared values.

• Take the square root: Finally, you take the square root of that average value.

Home Electricity

• Three-pin plug color code:

  • Brown (or red): Active

  • Blue (or black): Neutral

  • Green/Yellow: Earth

• Active wire:

  • Connected to 240V, oscillates between +340V and -340V.

  • Has a root-mean-square (RMS) voltage of 240V.

• Neutral wire:

  • Connected to the neutral link at the switchboard and earth.

  • Always at 0V.

• Energy Flow:

  • Current flows back and forth between active and neutral wires, providing energy to the appliance.

• Electricity bills:

  • Measured in kilowatt-hours (kWh).

  • 1 kWh = 3.6 × 10⁶ J

  • E = P × t

  • Cost = Energy × Rate

• Energy Measurement:

  • Meters measure electrical energy supplied before distribution to various parts of the house.

Electrical Safety Devices: Fuses and Circuit Breakers

• Function: Protect circuits by interrupting current if it exceeds a safe level.

  • Fuses: Contain a metal wire that melts when current is too high, breaking the circuit.

  • Circuit Breakers: Use a bimetallic strip that bends when heated, tripping a switch to break the circuit. These can be reset after activation, unlike fuses, which need replacing.

• Appliance Placement: High-power appliances like ovens and air-conditioners are put on separate circuits from lights and power points.

• Power Boards:

  • Limit: Most power boards can carry a maximum of 10 A of current.

  • Overuse: Overloading power boards with high-current devices can cause overheating, melting insulation, and potentially starting a fire.

Short Circuit

Short circuits occur when a low-resistance path allows current to bypass resistors, leading to excessive current flow. Overloads happen when too many devices draw current from a single circuit. Both can cause wires to heat up, potentially melting insulation and increasing the risk of fire. Faulty appliances and damaged wires can also lead to short circuits.

A short circuit is present in the branch parallel to R₂ and R₃, therefore no current will be measured across R₂ and R_3:

Electrical Safety and Protection Mechanisms

If the active wire inside an appliance touches the metal case, the case becomes live. An earth wire directs the current to the ground to prevent danger. A short circuit causes a large current, which trips the fuse or circuit breaker.

Double-insulated appliances have two layers of protection and don’t need an earth wire. They often have two-pin plugs.

RCDs (Residual Current Devices) detect current differences between the active and neutral wires. If there’s a leak, the RCD turns off power in 20 milliseconds.

Electrical Safety

• 50 Australians die from electrical accidents yearly.

• Shock impact depends on current, duration, and path through the body.

• 240V source can cause deadly current flow if contact is good (wet hands make it worse).

• Electrical energy turns to heat, causing burns.

• 80mA current can cause heart fibrillation, often leading to death.

Current: LDR / Light bulb example

• Bright light → LDR has low resistance → current bypasses bulb → bulb off

• Darkness → LDR has high resistance → current flows through bulb → bulb on

💡 Current takes the path of least resistance in parallel circuits.