Electrical quantities

At the heart of electricity is electric charge. There are two types: positive and negative. Protons in an atom's nucleus are positive, and electrons orbiting the nucleus are negative. Objects become charged by gaining or losing electrons. Gaining electrons makes an object negatively charged; losing them makes it positively charged. Like charges repel each other (positive repels positive), while opposite charges attract (positive attracts negative). Materials that allow electrons to move freely are called electrical conductors (e.g., metals), while materials that don't are electrical insulators (e.g., plastic, rubber). An electric field is a region of space around a charged object where another charge would experience a force. For the Extended syllabus, you need to know the shape of these fields: they radiate outwards from positive charges and inwards towards negative charges.

Electric current is the rate of flow of electric charge. Think of it like the amount of water flowing through a pipe per second. We measure current in amperes (A) using an ammeter connected in series. The charge itself is measured in coulombs (C). For current to flow, there needs to be a 'push'. This electrical push is called the potential difference (p.d.) or voltage, measured in volts (V) using a voltmeter connected in parallel. Potential difference is the energy transferred per unit charge. A battery or power supply provides an electromotive force (e.m.f.), also measured in volts. E.m.f. is the total energy supplied per coulomb of charge by the source to the whole circuit, while p.d. is the energy used per coulomb by a specific component.

Resistance is a measure of how difficult it is for current to flow through a component. It's measured in ohms (Ω). A high resistance means a small current will flow for a given potential difference. The relationship between p.d. (V), current (I), and resistance (R) is given by the equation R = V/I. Ohm's Law states that for a conductor at a constant temperature, the current is directly proportional to the potential difference across it. This means its resistance is constant. A graph of current against p.d. (an I-V graph) for an 'ohmic' resistor is a straight line through the origin. However, many components are 'non-ohmic'. For a filament lamp, the I-V graph curves because as it gets hotter, its resistance increases. For a diode, current flows easily in one direction (forward bias) but not at all in the other (reverse bias).

There are two basic ways to connect components in a circuit: series and parallel. In a series circuit, components are connected end-to-end in a single loop. The current is the same at all points in a series circuit. The total potential difference from the supply is shared between the components. The total resistance is the sum of the individual resistances (R_total = R1 + R2 + ...). In a parallel circuit, components are connected on separate branches. The potential difference across each branch is the same. The total current from the supply is the sum of the currents in each branch (it splits up). The total resistance of a parallel circuit is always less than the smallest individual resistor.

Power is the rate at which energy is transferred. In electrical circuits, power is the amount of energy an appliance uses per second. It is measured in watts (W). The power of a component can be calculated by multiplying the potential difference across it by the current flowing through it. Since energy is power multiplied by time, the electrical energy transferred can also be calculated directly. The standard unit for energy is the joule (J). However, electricity companies measure energy in a larger unit called the kilowatt-hour (kWh). One kilowatt-hour is the energy used by a 1 kW appliance running for 1 hour.