Electric current Revision Notes

Electric current is defined as the flow of electric charge through a conductor or circuit. It represents how much charge passes a given point in the circuit per unit time.

The SI unit of electric current is the ampere (A). One ampere corresponds to one coulomb of charge passing a point every second.

Mathematically, electric current (I) is expressed as: I = Q / t, where Q is the electric charge in coulombs (C) and t is the time in seconds (s).

• Electric current is the rate of flow of electric charge.
• Unit of current: ampere (A).
• 1 A = 1 C/s.

Electric charge is a fundamental property of particles such as electrons and protons. In electrical circuits, the charge carriers are usually electrons, which carry a negative charge.

In metallic conductors, electrons are free to move through the lattice of metal ions. When a potential difference is applied, these free electrons drift in a specific direction, creating an electric current.

The movement of these charges constitutes the electric current. Without the movement of charge, there can be no current.

• Electric charge is carried by electrons in metals.
• Electrons move through the conductor when a potential difference is applied.
• Current is the result of this charge movement.

A potential difference (voltage) across a conductor creates an electric field inside it. This field exerts a force on the free electrons, causing them to move and produce an electric current.

The greater the potential difference, the greater the force on the electrons and the higher the current, assuming the conductor's resistance remains constant.

This movement of electrons is slow on average (drift velocity), but the effect of current flow is almost instantaneous throughout the circuit.

• Potential difference causes electrons to move.
• Electric field inside conductor drives charge flow.
• Current depends on potential difference and resistance.

Electric current is measured using an ammeter connected in series with the circuit component.

Ammeters are calibrated in amperes and can measure current ranging from milliamperes (mA) to several amperes (A).

The reading on the ammeter indicates the amount of charge passing through the circuit per second.

• Ammeter measures current in amperes.
• Connected in series to measure current through a component.
• Reading shows charge flow rate.

Electric current is the flow of electric charge, primarily electrons in metals.

It is measured in amperes, where 1 ampere equals 1 coulomb of charge passing per second.

A potential difference across a conductor causes electrons to move, producing current.

• Current = flow of electric charge.
• Unit: ampere (A) = coulomb per second (C/s).
• Charge movement driven by potential difference.

An ammeter is an instrument used to measure the electric current flowing through a circuit. Electric current is the rate of flow of electric charge, measured in amperes (A). To measure current accurately, an ammeter must be connected in series with the component or section of the circuit where the current is to be measured. This ensures that the same current flows through both the circuit and the ammeter, allowing the device to register the correct value.

Connecting an ammeter in parallel is incorrect and can damage the instrument or affect the circuit operation because ammeters have very low internal resistance to avoid voltage drop across them.

• Ammeters measure current in amperes (A).
• Must be connected in series with the circuit element.
• Have very low resistance to minimise voltage drop.

Analogue ammeters use a moving coil galvanometer mechanism. Inside the device, a coil of wire is placed in a magnetic field. When current flows through the coil, it experiences a torque that causes a pointer (needle) to move across a calibrated scale. The position of the needle indicates the current value.

The scale is usually marked in amperes or milliamperes, depending on the range of the ammeter. Analogue ammeters provide a continuous scale, allowing estimation between marked values. However, they can be less precise than digital meters and are subject to parallax error if not read at eye level.

• Use a moving coil and needle to indicate current.
• Display current on a graduated scale in amperes or milliamperes.
• Can be less precise and prone to parallax error.

Digital ammeters use electronic circuits to convert the current into a digital numerical display. They often employ analogue-to-digital converters (ADCs) to measure the current and show the value on an LCD or LED screen.

Digital ammeters provide direct numerical readings, which are easier to read accurately and reduce human error. They often have a wider range of measurement and can display current in microamperes (μA), milliamperes (mA), or amperes (A) depending on the selected range.

Some digital ammeters have auto-ranging features that automatically select the appropriate measurement range for the current being measured.

• Use electronic circuits and ADCs to display current numerically.
• Provide precise and easy-to-read measurements.
• May include auto-ranging for convenience.

Ammeters are designed for different current ranges to ensure accuracy and protect the instrument. For example, some ammeters measure small currents in microamperes (μA), others measure milliamperes (mA), and others measure larger currents in amperes (A).

Selecting an ammeter with an appropriate range is important. Using an ammeter with a range too low for the current can damage the meter or cause it to give incorrect readings. Conversely, using a meter with a range too high may reduce measurement resolution and accuracy.

When measuring an unknown current, it is good practice to start with the highest range and then switch to lower ranges for a more precise reading if the current is within those limits.

• Ammeters have different ranges: μA, mA, A.
• Choose a range suitable for the expected current.
• Start with the highest range to avoid damage, then adjust for precision.

When using ammeters, ensure the circuit is switched off before connecting or disconnecting the meter to avoid electric shock or damage.

Because ammeters have very low resistance, connecting them incorrectly (e.g., in parallel) can cause a short circuit, potentially damaging the meter and the circuit.

Always observe the polarity markings on the ammeter, especially for analogue meters, to ensure correct current direction and accurate readings.

• Switch off circuit before connecting ammeter.
• Never connect ammeters in parallel.
• Observe polarity markings for correct connection.

Metals consist of atoms arranged in a regular lattice structure. Each metal atom contributes some of its outer electrons to a 'sea' of free electrons that are not bound to any particular atom. These free electrons can move freely throughout the metal lattice, unlike electrons in insulators or semiconductors.

The metal lattice itself is made up of positively charged metal ions fixed in position. The free electrons move among these ions, enabling electrical conduction.

• Metal atoms form a fixed lattice of positive ions.
• Outer electrons become free electrons, delocalised from atoms.
• Free electrons can move throughout the metal.

When a potential difference (voltage) is applied across a metal, an electric field is established inside it. This field exerts a force on the free electrons, causing them to drift slowly towards the positive terminal.

Although free electrons move randomly at high speeds due to thermal energy, the electric field causes a net drift in one direction. This drift of free electrons constitutes the electric current in metals.

As electrons move, they collide with the fixed metal ions, which impedes their motion and causes electrical resistance.

• Voltage creates an electric field inside the metal.
• Free electrons drift towards the positive terminal.
• Electron collisions with ions cause resistance.

Free electrons are the charge carriers responsible for conduction in metals. Their ability to move freely allows metals to conduct electricity efficiently.

The metal ions remain fixed in the lattice and do not move. They provide the structure and act as scattering centres for electrons, affecting resistance.

The number of free electrons per unit volume influences the conductivity of the metal. Metals like copper and silver have many free electrons and thus high conductivity.

• Free electrons carry charge through the metal.
• Metal ions are fixed and cause electron scattering.
• Higher free electron density means better conductivity.

Increasing temperature causes the metal ions to vibrate more intensely. These vibrations increase the frequency of collisions between free electrons and ions, reducing the average drift velocity of electrons for a given electric field.

As a result, the electrical resistance of metals increases with temperature, and their conductivity decreases.

This explains why metals conduct less efficiently at higher temperatures.

• Higher temperature increases ion vibrations.
• More collisions reduce electron drift velocity.
• Resistance increases and conductivity decreases with temperature.

Electric current is the rate of flow of electric charge through a conductor, measured in amperes (A). It is caused by the movement of charged particles, usually electrons in metals.

Current can flow in two main ways: direct current (d.c.) and alternating current (a.c.).

• Electric current (I) is the amount of charge (Q) passing a point per unit time (t): I = Q / t.
• Unit of current: ampere (A), where 1 A = 1 coulomb per second (1 C/s).

Direct current is the flow of electric charge in one constant direction only. The magnitude of the current may be steady or vary with time, but the direction does not change.

In d.c., electrons move steadily from the negative terminal to the positive terminal of the power source.

Common sources of d.c. include batteries, cells, and solar panels.

• D.C. flows in one direction only.
• Electrons flow from negative to positive terminal.
• Used in electronic devices, torch batteries, and some motors.

Alternating current periodically reverses its direction of flow. The current changes magnitude and direction in a sinusoidal pattern, typically many times per second.

In most countries, the mains electricity supply is a.c., with a frequency of 50 Hz or 60 Hz, meaning the current changes direction 50 or 60 times per second.

A.c. is generated by power stations using alternators.

• A.C. changes direction periodically, typically 50 or 60 times per second (50 Hz or 60 Hz).
• Electrons oscillate back and forth rather than moving steadily in one direction.
• Used for household power supply and large-scale power distribution.

To draw a diagram representing d.c., sketch a horizontal straight arrow along a wire indicating current flow in one direction only.

For a.c., draw a sinusoidal wave along the wire to represent the current varying with time, with arrows showing the current direction reversing periodically.

Label the time axis (horizontal) in seconds (s) and current axis (vertical) in amperes (A).

• D.C. current vs time graph: a straight horizontal line above zero.
• A.C. current vs time graph: a sine wave crossing zero, alternating positive and negative values.

D.C. is used in devices that require a steady voltage and current, such as electronic circuits, battery-powered devices, and some motors.

A.C. is preferred for power distribution because it is easier to transform between voltages using transformers, reducing energy loss over long distances.

Understanding the difference is essential for safely using electrical devices and for designing circuits.

• D.C. powers small electronic devices and batteries.
• A.C. powers homes, industries, and large appliances.
• Transformers only work with a.c., enabling efficient voltage changes.

Electric current is the rate at which electric charge flows through a conductor or circuit element. It quantifies how much charge passes a given point in the circuit per unit time.

The standard symbol for electric current is I, measured in amperes (A). One ampere corresponds to one coulomb of charge passing a point every second.

Mathematically, electric current is defined as: I = Q / t, where Q is the electric charge in coulombs (C) and t is the time in seconds (s).

• Electric current (I) is a scalar quantity measured in amperes (A).
• Charge (Q) is measured in coulombs (C).
• Time (t) is measured in seconds (s).

The equation I = Q / t expresses that current is the amount of charge flowing per unit time.

Rearranging the formula gives Q = I × t, which calculates the total charge passing through a conductor over a time interval t when the current I is known.

This relationship is fundamental in analysing electric circuits and understanding charge flow dynamics.

• I = Q / t (current equals charge divided by time).
• Q = I × t (charge equals current multiplied by time).

The ampere (A) is the SI unit of electric current. It is defined as one coulomb of charge passing a point in one second.

The coulomb (C) is the SI unit of electric charge. One coulomb corresponds to approximately 6.24 × 10¹⁸ elementary charges (electrons or protons).

Time is measured in seconds (s), the SI base unit for time.

• 1 A = 1 C/s (one ampere equals one coulomb per second).
• Charge Q in coulombs represents the total quantity of electric charge transferred.

A current of 3.0 A flows through a wire for 10 seconds. Calculate the total charge that passes through the wire.

Using the formula Q = I × t: Substitute I = 3.0 A and t = 10 s.

Q = 3.0 A × 10 s = 30 C. Therefore, 30 coulombs of charge pass through the wire in 10 seconds.

• Formula: Q = I × t
• Substitution: Q = 3.0 × 10
• Answer: Q = 30 C
• Common error: forgetting to multiply current by time or mixing units.

To draw a simple diagram illustrating current and charge flow:

1. Draw a horizontal straight line representing a conductor or wire.

2. Mark a point on the wire to indicate the location where current is measured (label it 'Point P'). 3. Draw arrows along the wire to show the direction of conventional current flow (from positive to negative). 4. Label the current as I (in amperes) and indicate charge Q passing point P over time t. This diagram helps visualise that current is the amount of charge passing a point per unit time.

• Conventional current direction is from positive to negative.
• Charge Q passes point P in time t, producing current I = Q/t.

Electric current is the rate of flow of electric charge through a conductor or circuit element. It is measured in amperes (A), where 1 ampere corresponds to 1 coulomb of charge passing a point per second (I = Q/t).

In metals, the charge carriers responsible for current are free electrons, which are negatively charged particles that can move through the metal lattice.

• Electric current (I) is charge flow per unit time: I = Q/t, measured in amperes (A).
• Charge (Q) is measured in coulombs (C).
• In metallic conductors, free electrons carry the charge.

Conventional current is defined as the flow of positive charge from the positive terminal of a power supply to the negative terminal through the external circuit. This convention was established historically before the discovery of the electron.

By this definition, the direction of conventional current is from higher electric potential (positive terminal) to lower electric potential (negative terminal).

This direction is used universally in circuit diagrams, analysis, and calculations regardless of the actual charge carriers.

• Conventional current flows from positive to negative terminal.
• It represents the direction positive charges would move.
• Used as the standard direction in circuit analysis and diagrams.

In reality, in metallic conductors, the charge carriers are free electrons, which have a negative charge.

Electrons move from the negative terminal (where there is an excess of electrons) to the positive terminal (where there is a deficit of electrons).

Thus, electron flow is opposite to the direction of conventional current.

• Electrons carry negative charge and move from negative to positive terminal.
• Electron flow is opposite to conventional current direction.
• Electron movement causes the observed current in metal wires.

When a potential difference (voltage) is applied across a conductor, it creates an electric field inside the conductor.

This electric field exerts a force on free electrons, causing them to drift slowly opposite to the field direction (from negative to positive).

The drift velocity of electrons is much slower than the speed of the electric field propagation, which causes the current to establish almost instantaneously.

• Voltage creates an electric field inside the conductor.
• Electric field pushes electrons from negative to positive terminal.
• Electron drift velocity is slow; current flow is effectively instantaneous.

Despite electrons moving from negative to positive, conventional current direction is used in all circuit diagrams and calculations.

This convention simplifies the analysis and is consistent with the direction of positive charge flow assumed in many components and laws (e.g., Ohm's law, Kirchhoff's laws).

Understanding the difference prevents confusion when interpreting circuit behaviour and when relating physical electron movement to conventional current.

• Conventional current direction is standard in circuit analysis.
• It aligns with positive charge flow assumptions in laws and components.
• Knowing electron flow direction aids conceptual understanding but does not change calculations.