1. Electric Current

An electric current exists when charge carriers move through a complete circuit. In metals, the charge carriers are delocalised electrons. In electrolytes, the charge carriers are positive and negative ions.

Current tells you how quickly charge passes a point. If a large amount of charge passes each second, the current is large. If only a small amount passes each second, the current is small.

Here, $I$ is current in amperes, $\Delta Q$ is charge in coulombs, and $\Delta t$ is time in seconds. Rearranging gives $\Delta Q = I \Delta t$. A charge of 1 coulomb passing each second is a current of 1 ampere.

The direction of conventional current is defined as from positive to negative around the circuit. In a metal wire, the electrons actually move in the opposite direction, but exam questions use conventional current unless they explicitly ask about electron flow.

The circuit below makes that contrast clear; notice that the conventional current arrow runs from the positive terminal round the loop while the electron-flow arrow points the other way through the metal wire.

[DIAGRAM: asset_name: 5.1.1 - Basics of Electricity - Diagram 1; asset_slug: 5.1.1 - Basics of Electricity - Diagram 1; recommended_method: retained_png; description: A simple series circuit containing a cell, a lamp, and an ammeter. Show conventional current flowing from the positive terminal to the negative terminal, and electron flow in the opposite direction through the wire.]

2. Potential Difference

A source such as a battery transfers energy to charge carriers. As those charge carriers move through a component, they transfer energy to it. Potential difference tells you how much energy is transferred for each coulomb of charge.

The volt is equivalent to one joule per coulomb, so $1 \text{ V} = 1 \text{ J C}^{-1}$. A larger potential difference means more energy is transferred to each coulomb of charge passing through the component.

In this equation, $V$ is potential difference in volts, $W$ is work done or energy transferred in joules, and $Q$ is charge in coulombs. Rearranging gives $W = QV$, which is useful when you need the energy transferred by a known charge.

For example, if 3.0 C of charge passes through a lamp with a potential difference of 12 V across it, the energy transferred is $W = QV = 3.0 \times 12 = 36$ J.

3. Resistance

When charge carriers move through a component, they collide with particles in the material. These collisions oppose the movement of charge and transfer energy to the material. Resistance is the quantity that measures this opposition to current.

This is a definition, not a special law. It applies to any component, whether or not its resistance stays constant.

Here, $R$ is resistance in ohms, $V$ is potential difference in volts, and $I$ is current in amperes. Rearranging gives $V = IR$ and $I = \frac{V}{R}$.

To measure the resistance of a component, place an ammeter in series with the component to measure the current through it, and place a voltmeter in parallel across the component to measure the potential difference across it.

In the circuit below, notice that the ammeter sits in series with the whole loop while the voltmeter is connected in parallel across the test resistor only.

[DIAGRAM: asset_name: 5.1.1 - Basics of Electricity - Diagram 2; asset_slug: 5.1.1 - Basics of Electricity - Diagram 2; recommended_method: retained_png; description: A circuit with a cell, switch, variable resistor, ammeter, and test resistor all in series. A voltmeter is connected in parallel across the test resistor.]

4. Connecting the Three Quantities

These three equations are often used together. Current tells you how quickly charge moves, potential difference tells you how much energy each coulomb transfers, and resistance compares the potential difference across a component with the current through it.

In a multi-step problem, it helps to decide which quantity you can calculate first. For example, if you know current and time you can find charge, then use that charge with potential difference to find energy transferred.

If you can move confidently between these three equations, you have the core language needed for the rest of the electricity topic.