Electricity is a form of energy. It is the phenomenon that describes the flow of charged particles (particularly electrons) from one place to another. Everything in the world is made up of atoms. Every atom is composed of a nucleus surrounded by negatively charged electrons. The nucleus contains particles called neutrons (which have no charge) and protons (which have a positive charge). The number of protons and electrons is the same in a stable atom to balance the overall neutral charge.
In conductors (e.g., metals like copper or silver), the movement of electrons known as free electrons is responsible for moving the charge. The moving charge is what we call an electric current.
The phenomenon of electricity and its applications are studied in more detail in the field of electrical engineering.
Defining electric current
We can define the electric current as the amount of charge that moves during a specific time period. The formula for calculating electrical current and the units used are as follows:
- The SI base unit for electric current is the amperes (A).
- Current (I) is measured in amperes (A).
- Q is measured in coulombs (C).
- Time (t) is measured in seconds (s).
- Charge, current, and time are related to each other as \(Q = I \cdot t\).
- Change in charge is denoted as ΔQ.
- Similarly, the change in time is denoted as Δt.
Another interesting point is that electrical current produces a magnetic field, while a magnetic field can also produce an electrical current.
Batch variation
When two charged objects are connected using a conductive wire, a charge flows through them, producing a current. The current flows because the charge difference causes a voltage difference.
Figure 1. Flow of charge in a conductor. Source: StudySmarter.
The equation for current flow, therefore, is:
\[\Delta Q = \Delta I \cdot \Delta t\]
Conventional current flow
In a circuit, current is the flow of electrons across the circuit. Electrons, which are negatively charged, move away from the negatively charged terminal and towards the positively charged terminal, following the basic rule that like charges repel each other while opposite charges attract each other.
Conventional current is described as the flow of positive charge from the source’s positive terminal to its negative terminal. This is opposite to the flow of electrons, as it was stated before the direction of current was understood.
Figure 2. Conventional flow vs electron flow. Source: StudySmarter.
An important point to make is that the flow of current has a direction and magnitude given in amperes. However, it is not a vector quantity.
How to measure current
Current can be measured using a device called an ammeter. Ammeters should always be connected in series with the part of the circuit where you wish to measure the current, as shown in the figure below.
This is because current has to flow through the ammeter in order for it to read the value. The ideal internal resistance of an ammeter is zero in order to avoid any voltage being on the ammeter because it can affect the circuit.
Figure 3. Arrangement to measure current using Ammeter - StudySmarter Originals
Q: In which of the options below does 8 mA of current pass through the electrical circuit?
A. When a charge of 4C passes in 500s.
B. When a charge of 8C passes in 100s.
C. When a charge of 1C passes in 8s.
Solution. Using the equation:
\(I = \frac{Q}{t}\)
\(I = \frac{4}{500} = 8 \cdot 10-3 = 8 mA\)
\(I = \frac{8}{100} = 80 \cdot 10-3 = 80 mA\)
\(I = \frac{1}{8} = 125 \cdot 10-3 = 125 mA\)
Option A is correct: 8 mA of current will pass through the circuit.
Quantisation of charge
The charge on the charge carriers is quantised, which can be defined as follows:
A single proton has a positive charge, and a single electron has a negative charge. This positive and negative charge has a fixed minimum magnitude and always occurs in multiples of that magnitude.
Therefore, the quantity of charge may be quantised based on the number of protons or electrons present.
This means that a charge on any particle is a multiple of the magnitude of the charge of the electron. For example, the charge of an electron is -1.60 · 10-19 C, and the charge of a proton, by comparison, is 1.60 · 10-19 C. We can represent the charge of any particle as a multiple of this.
Calculating current in a current-carrying conductor
In a current-carrying conductor, a current is generated when the charge carriers move around freely. The charge on the charge carriers can either be positive or negative, and the current is considered to travel in one direction across the conductor. The current in a conductor has several characteristics:
- The charge carriers are mostly free electrons.
- Although the current flows in a particular direction in each conductor, the charge carriers move in opposite directions with a drift speed v.
- The first image in Figure 2 has positive charge carriers. Here, drift speed and charge carriers move in the same direction. The second image has negative charge carriers, and drift speed and charge carriers move in the opposite direction.
- The charge carriers’ drift speed is the average speed at which they travel through the conductor.
- The current in a current-carrying conductor can be mathematically expressed as:\(I = A \cdot n \cdot q \cdot v\)
- Where A is the area of the cross-section, in units of area.n is number density (the number of charge carriers per m3).v is drift velocity in m/s.q is the charge in Coulombs.I is the current in Amperes.
Electric Current - Key takeaways
- Electricity is a form of energy. It is the phenomenon that describes the flow of charged particles (particularly electrons) from one place to another.
- The SI base unit of electric current is amperes (A).
- Conventional current is described as the flow of positive charge from the cell’s positive terminal to its negative terminal.
- The charge on the charge carriers is quantised.
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