Electricity Study Notes

Electric charge and basic properties

Electricity is the movement of energy through charged particles, primarily electrons (a type of charged particle).
Electric charge exists in two types: positive and negative.
Electrons carry negative charge; other charged particles can carry positive charge.
In most materials, electrons are tightly bound to atoms, but in electrical conductors like metals they can move freely between atoms.
Conductors: materials through which electricity flows easily due to free-moving charges (e.g., copper metal).
Insulators: materials in which electrons/ions are held tightly to the atoms, so there are few or no free charged particles (e.g., cotton fabric). Oils are liquids that do not conduct well.
Two main ways electricity can work:
- Static electricity: buildup of electric charge in one place that doesn’t move (static means not moving).
- Current electricity: electrons move through a pathway; electrons may move in one direction or switch directions back and forth.
Triboelectric effect: some insulating materials can transfer electrons when rubbed together; the object gaining electrons becomes more negative, the object losing electrons becomes more positive.
Static electricity can produce an electrical spark when the repulsive force pushes electrons through air to a nearby object or to the ground, neutralizing the charge on the object.
Charge induction: when a charged object comes near a neutral object, it can induce a charge distribution in the neutral object by attracting or repelling electrons; this causes a temporary uneven distribution of charges within the neutral object.
Even when discussing electrons flowing from negative to positive, historically current is described as flowing from positive to negative due to naming tradition (before electrons were discovered).

Conductors, insulators, and practical examples

Conductors: usually metals with free, delocalised electrons; allow easy electron flow.
Example of conductors: copper metal; metals generally.
Liquids with free-moving ions can conduct electricity too (electrolytes).
Insulators: materials that do not allow electricity to flow easily; electrons/ions are held tightly.
Example insulators: cotton fabric; oils (liquids with no free ions).
Metals are good conductors due to free delocalised electrons.

Basic circuit concepts

Electric circuit: a path that transmits electricity; typically includes:
- A power supply to provide electrical energy (e.g., battery).
- A load (or loads) where electrical energy is converted to other forms (light, heat, sound).
- A conducting path that allows electric charge to flow around the circuit.
In a basic circuit, electrons move from the negative electrode (terminal) of a power source toward the positive electrode.
There is typically a single loop in a simple circuit, meaning there is only one path for current to flow.

Electric current, voltage, and resistance

Electric current: the flow of electric charge through a conducting path; measured as the number of electrons passing a point per second.
Unit of current: ampere, abbreviated A (also called amp).
The current is the amount of electric charge passing a point per second; in powered circuits, electrons flow from negative to positive (electron flow).
Convention vs. electron flow: although electrons move from negative to positive, current is often described as flowing from positive to negative due to historical convention.
Voltage (electric potential difference): the energy that pushes electrons around a circuit; provided by the power source (battery, power pack).
Unit of voltage: volt, abbreviated V; measured with a voltmeter.
Resistance: a measure of how much an object resists the flow of electrons; it controls the ease of current flow.
Unit of resistance: ohm, symbol Ω.
Resistance can be varied by changing physical properties (e.g., length of the wire): longer length → higher resistance → lower current.
Practical uses of resistance variation: volume controls in radios, dimmers in cinemas, speed control of toy cars.

Current in series and parallel circuits

Series circuits:
- Components are connected one after another in a single loop.
- Current is the same at each device in series (I is constant).
- Voltage is shared (divided) among devices according to each device’s resistance.
Parallel circuits:
- There are multiple branching paths (branches) with their own components.
- The current is split between the branches according to each branch’s resistance.
- The voltage across each branch is the same (V is constant across all branches).
- Adding more branches means the battery must provide more total current (even though individual branch currents depend on branch resistance).

Measurements and key equations

Current: measured as the number of electrons passing a point per second; unit is A.
Voltage: energy pushing electrons around the circuit; measured in V.
Resistance: measured in Ω (ohms).
Ohm’s Law: relationship between voltage, current, and resistance in a circuit:
- V = R\cdot I
- Where V is in volts (V), I is in amperes (A), and R is in ohms (Ω).
Alternative form derived from Ohm’s Law:
- I = \frac{V}{R}
The electric potential difference between two points on a circuit is denoted as \Delta V and is defined as:
- \Delta V = V{2} - V{1}
Notation in a circuit context:
- I = current between two points
- R = total resistance of all devices between those two points
Instruments:
- Ammet er measures current (device: ammeter).
- Voltmeter measures voltage (device: voltmeter).

Practical implications and conceptual notes

Direction conventions: current direction is traditionally shown as flowing from positive to negative, even though electrons (negative charge) flow from negative to positive.
Real circuits follow Ohm’s Law, but networks can be more complex with multiple components; series and parallel rules help analyze simple circuits.
Resistance can be used to control devices (volume, lighting brightness, speed) by altering current flow through components.
Static discharge (spark) is a common observable outcome when accumulated charge finds a path to neutralize (to ground or another object).
Charge induction explains how a nearby charged object can influence a neutral object without direct contact, leading to polarization effects.