OCR Specification focus:
‘Electric current is the rate of flow of charge; I = Q ÷ t.’
Electric current forms the foundation of electrical physics, describing how electric charge moves through conductors and circuits. Understanding current as the rate of flow of charge provides essential insight into how electrical systems operate, from simple circuits to complex electronic devices.
The Concept of Electric Current
Electric current represents the movement of electric charge through a material. When charge carriers—typically electrons—move in a consistent direction through a conductor, they constitute an electric current. The study of current is crucial in understanding the behaviour of electric circuits, power transfer, and electromagnetic interactions.
In everyday electrical systems, charges move due to an applied potential difference that creates an electric field, forcing charge carriers to drift through a medium such as a metal wire. The amount of charge passing a given point per second determines the magnitude of the current.
Defining Current as the Rate of Flow of Charge
Electric Current: The rate of flow of electric charge through a point or component in a circuit....
This definition indicates that current depends on how much charge passes through a given cross-section in a given time.
A labelled sketch showing charges moving through a conductor’s cross-section in a time interval Δt. The diagram highlights that current measures how much charge crosses a surface per unit time, directly illustrating I = Q ÷ t at the microscopic level. Source
EQUATION
—-----------------------------------------------------------------
Current (I) = Q ÷ t
I = Electric current in amperes (A)
Q = Electric charge in coulombs (C)
t = Time in seconds (s)
—-----------------------------------------------------------------
This equation shows that current is proportional to the total charge that flows and inversely proportional to the time interval during which it moves. If more charge passes in less time, the current is higher.
The Unit of Current – The Ampere
The ampere (A) is the SI unit of electric current. One ampere corresponds to a flow of one coulomb of charge per second.
A clear schematic defining the ampere as coulombs per second, illustrating electrons crossing a wire’s cross-section each second. This makes the unit interpretation of I = Q ÷ t explicit. The graphic may also mention the approximate number of electrons per coulomb (6.24 × 10¹⁸), which is supportive but not required by the syllabus. Source
In practical terms, this means that when a current of 1 A flows, approximately 6.25 × 10¹⁸ electrons pass a point each second, since each electron carries a charge of 1.6 × 10⁻¹⁹ C.
The ampere is a base unit in the International System of Units (SI) because it serves as a fundamental measure for defining other electrical quantities such as voltage, resistance, and power.
Understanding Electric Charge
Electric Charge: A fundamental property of matter that causes it to experience a force when placed in an electric field.
Charge can be positive (as in protons) or negative (as in electrons). The elementary charge, symbolised by e, equals 1.6 × 10⁻¹⁹ C. All observable electric charge is an integer multiple of this value, making charge a quantised property.
The coulomb (C) is the unit of charge and can be expressed through current:
1 coulomb = charge transferred by a current of 1 ampere in 1 second.
Thus, if a current of 2 A flows for 3 seconds, the total charge that moves is 6 C, reinforcing the relationship between charge, current, and time.
Flow of Charge in Conductors
In metallic conductors, electrons act as the primary charge carriers. Metals contain a lattice of positive ions surrounded by delocalised electrons that are free to move. When a potential difference is applied across the metal, the electric field exerts a force on these electrons, causing them to drift opposite to the field direction.
Although each electron moves randomly due to collisions with ions, the net movement of many electrons constitutes a steady flow, measured as current.
The rate of flow remains constant throughout a uniform section of a circuit if the system is in steady state, meaning that the number of electrons entering a section equals the number leaving it per unit time.
Direction of Current Flow
Two conventions exist for current direction:
Conventional current direction is defined as the flow of positive charge from the positive terminal to the negative terminal of a power supply.
Electron flow direction is the actual motion of negatively charged electrons, moving from negative to positive.
Even though electrons move in the opposite direction, circuit analysis and diagrams typically use the conventional current direction for consistency.
Current in Different Media
Charge carriers differ depending on the type of material:
In metals, current is due to the movement of free electrons.
In electrolytes, current arises from the movement of positive and negative ions.
In semiconductors, both electrons and positive “holes” act as charge carriers, depending on the type of doping.
Despite these differences, the definition of current as the rate of flow of charge remains universal.
Relating Microscopic and Macroscopic Views
At a microscopic level, current results from the collective motion of countless charge carriers. Although each electron moves relatively slowly (a few millimetres per second), the overall current appears instantaneous because the electric field propagates at nearly the speed of light through the conductor.
The macroscopic measurement of current therefore reflects the aggregate behaviour of these microscopic carriers, rather than individual particle velocities.
Practical Importance of I = Q ÷ t
The relationship I = Q ÷ t serves as a foundation for understanding electrical behaviour in circuits:
It enables calculation of the total charge that moves when a known current flows for a given time.
It provides a way to determine current when measuring how much charge passes through a circuit component.
It links directly to energy transfer, since moving charge through a potential difference performs electrical work.
This fundamental principle appears throughout circuit analysis, electromagnetism, and even particle physics, making it one of the most essential relationships in the study of electricity.
FAQ
If the total charge passing a point (Q) and the time interval (t) remain constant, the current (I = Q ÷ t) does not change — even if more charge carriers are available.
However, if there are more charge carriers, each individual carrier might carry less charge or move more slowly. The macroscopic current depends only on the total charge transferred per second, not on how many individual carriers contribute to it.
In a series circuit, there is only one continuous path for charge flow. Therefore, the rate of charge flow (current) must be identical through all components.
Since no charge is lost or stored between components, the amount of charge entering a section of the circuit each second equals the amount leaving it. This directly follows from the conservation of charge, which underpins Kirchhoff’s first law.
Electrons in metals are delocalised, meaning they are free to move throughout the lattice of fixed positive ions. These mobile electrons allow current to flow when a potential difference is applied.
The positive metal ions, however, remain in fixed positions within the solid lattice structure and can only vibrate about equilibrium points, so they do not contribute to the current.
The current begins almost instantaneously, but not because electrons themselves move that fast.
When a switch is closed, the electric field generated by the potential difference propagates at nearly the speed of light through the circuit. This field causes electrons everywhere in the circuit to start drifting simultaneously, creating the immediate appearance of current flow even though individual electrons move slowly.
As temperature increases, the metal’s resistivity rises because the ions in the lattice vibrate more vigorously. These vibrations cause more frequent collisions between electrons and ions.
Increased collisions reduce the average drift velocity of electrons for a given potential difference.
Consequently, the current decreases even though the applied voltage remains the same.
This effect demonstrates how current depends not only on the rate of charge flow but also on the conditions that influence that flow.
Practice Questions
Question 1 (2 marks)
Define electric current and state its unit.
Mark Scheme:
1 mark for stating that electric current is the rate of flow of charge through a point or component in a circuit.
1 mark for stating the unit of current as the ampere (A), or that 1 A = 1 C s⁻¹.
Question 2 (5 marks)
An electric current of 2.5 A flows through a wire for 4.0 minutes.
(a) Calculate the total charge that passes through the wire during this time.
(b) Explain, in terms of the motion of electrons, what this current represents.
Mark Scheme:
(a) Calculation (2 marks)
1 mark for correctly using the equation Q = I × t.
1 mark for correct numerical answer:
t = 4.0 min = 240 s
Q = 2.5 × 240 = 600 C.
(b) Explanation (3 marks)
1 mark for recognising that current is due to the movement of charge carriers (electrons) through the wire.
1 mark for stating that electrons move in the opposite direction to conventional current.
1 mark for explaining that a steady current means electrons pass a point in the wire at a constant rate, so 600 C of charge has moved through the cross-section in 240 s.
