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Physics Fundamentals
Electric Current: Definition, Formula, Units, Ohm’s Law and Examples
Electric current is the movement of electric charge through a material or circuit. It is the idea behind lighting, motors, chargers, computers and many other electrical devices.
What Is Electric Current?
Definition
Electric current is the rate at which electric charge passes through a point in a circuit.
In a metal wire, the moving charges are usually electrons. A complete conducting path is needed for a continuous current. If the circuit is open, charges cannot move all the way around the path, so the current stops.
Current and charge
Electric charge is measured in coulombs (C). Current tells us how quickly that charge is moving past a point.
SI unit
The SI unit of electric current is the ampere (A), commonly called an amp.
The Formula for Electric Current
I = current in amperes (A) | Q = charge in coulombs (C) | t = time in seconds (s)
This formula means that current increases when more charge passes through a point in the same amount of time. It also means that the same amount of charge produces a smaller current if it takes longer to pass.
Rearranging the formula gives:
Use the form that places the unknown quantity on its own.
Understanding the Ampere
One ampere is equal to one coulomb of charge passing a point each second:
Because an electron has a very small charge, one coulomb represents a very large number of elementary charges. In simple circuit calculations, however, it is usually enough to work with coulombs, amperes and seconds.
Direction of Electric Current
There are two directions to keep separate:
- Conventional current: defined as flowing from the positive terminal toward the negative terminal through the external circuit.
- Electron flow in a metal: electrons move from the negative terminal toward the positive terminal.
Remember
In most circuit diagrams and equations, the arrow for current shows the direction of conventional current. This convention is used even though electrons in metal wires move in the opposite direction.
How Does Current Flow in a Circuit?
A simple circuit normally contains a source, conducting wires and a load such as a lamp or resistor. The source provides a potential difference, often called voltage, that establishes an electric field in the circuit. The moving charges transfer energy to components as they pass through them.
Source
A cell, battery or power supply provides the electrical energy needed to drive charge around the circuit.
Conducting path
Wires and connections provide a continuous route for charge to move.
Load
A lamp, motor, heater or other device uses electrical energy and offers resistance to current.
Switch
A closed switch completes the path. An open switch breaks the path and stops the current in that branch.
Voltage, Resistance and Ohm’s Law
Voltage is the potential difference between two points. It is measured in volts (V). It indicates how much energy is transferred per unit of charge.
Resistance is the opposition a component or material presents to the flow of charge. It is measured in ohms (Ω).
For an ohmic component under conditions where its resistance remains constant, the relationship between voltage, current and resistance is given by Ohm’s law:
V = voltage in volts (V) | I = current in amperes (A) | R = resistance in ohms (Ω)
The same relationship can be rearranged in two useful ways:
For a fixed resistance, increasing the voltage increases the current. For a fixed voltage, increasing the resistance decreases the current.
Numerical Example: Finding Current
Question
A charge of 12 C passes through a wire in 4 s. What is the current?
- Write the formula: I = Q ÷ t
- Substitute the values: I = 12 C ÷ 4 s
- Calculate: I = 3 A
Answer: The current is 3 amperes.
Numerical Example: Using Ohm’s Law
Question
A resistor has a resistance of 6 Ω and is connected across a potential difference of 12 V. Find the current.
- Choose the formula: I = V ÷ R
- Substitute the values: I = 12 V ÷ 6 Ω
- Calculate: I = 2 A
Answer: The current through the resistor is 2 amperes.
Factors That Affect Resistance
The resistance of a wire depends on several physical factors:
- Material: different materials oppose charge flow by different amounts.
- Length: a longer wire generally has greater resistance.
- Cross-sectional area: a thicker wire generally has lower resistance than a thinner wire of the same material and length.
- Temperature: for many metal conductors, resistance increases as temperature rises.
For a uniform wire, resistance can be described by:
R = resistance; ρ = resistivity of the material; L = length; A = cross-sectional area
Resistivity is a property of the material. Its SI unit is the ohm metre (Ω·m).
Direct Current and Alternating Current
Direct current (DC)
In direct current, the current has a constant direction. Batteries provide a common example of a DC source.
Alternating current (AC)
In alternating current, the direction and usually the size of the current vary repeatedly with time. Household electrical supply is an example of AC.
Measuring Electric Current
An ammeter measures current. It is connected in series so that the same current flowing through the circuit element also flows through the meter.
Important safety point
An ammeter should not normally be connected directly across the terminals of a source. Doing so can create a very large current and may damage the meter or the circuit. Electrical measurements should be made with suitable equipment and safe voltage levels.
Current in Series and Parallel Circuits
| Circuit arrangement | What happens to current? | Key idea |
|---|---|---|
| Series | The current is the same through each component in a single unbranched path. | There is only one route for charge to follow. |
| Parallel | Current divides between branches and the total current is the sum of the branch currents. | There are multiple routes for charge to follow. |
For parallel branches, the total current equals the sum of the currents in the branches.
Electric Power and Energy
Electric current transfers energy through a circuit. The rate of electrical energy transfer is called power, measured in watts (W).
P = power in watts (W) | V = voltage in volts (V) | I = current in amperes (A)
Using Ohm’s law, two additional forms are:
Electrical energy transferred over time can be found using:
E = energy in joules (J), P = power in watts, and t = time in seconds
Common Mistakes and Misconceptions
- Confusing current with voltage: current is the rate of charge flow, while voltage is potential difference.
- Forgetting the time unit: in I = Q ÷ t, time must be in seconds when the answer is required in amperes.
- Using the wrong current direction: circuit diagrams normally use conventional current, not electron-flow direction.
- Thinking current is used up: in a simple series circuit, charge is not consumed by a lamp or resistor. Electrical energy is transferred to other forms.
- Applying Ohm’s law to every device without checking: the relationship V = IR is directly applicable when the component behaves as an ohmic conductor under the stated conditions.
- Connecting an ammeter incorrectly: an ammeter is placed in series, not directly across a voltage source.
Important Points to Remember
- Electric current is the rate of flow of electric charge.
- The SI unit of current is the ampere, and 1 A = 1 C/s.
- The basic current formula is I = Q ÷ t.
- Conventional current is taken to flow from positive to negative in the external circuit.
- Ohm’s law is V = IR for an ohmic component under suitable conditions.
- Current requires a complete conducting path.
- An ammeter is connected in series.
- In a parallel circuit, total current is the sum of the branch currents.
Quick Recap
- Electric current describes how quickly charge flows.
- Formula: I = Q ÷ t.
- Unit: ampere (A), where 1 A = 1 C/s.
- Ohm’s law: V = IR.
- Power: P = VI.
- Electrons move opposite to the direction of conventional current in metal wires.
- A closed circuit provides a complete path for current.