Electric Current and Electric Charge (Leaving Cert Physics): Revision Notes
Electric Current and Electric Charge
What is electric charge?
Electric charge is a fundamental property of matter that exists in atoms. All matter consists of atoms, which contain three types of particles in their structure.
The nucleus sits at the centre of each atom and contains:
- Protons - particles with a positive electric charge
- Neutrons - particles with no electric charge (neutral)
Surrounding the nucleus are electrons - particles with a negative electric charge that orbit in the space around the nucleus.
Under normal circumstances, atoms have equal numbers of protons and electrons, making them electrically neutral. However, electrons can be gained or lost, creating charged particles.
Key properties of electric charge
Understanding Electric Forces
The forces between electric charges are much more significant than you might expect:
- Opposite charges attract each other (positive attracts negative)
- Like charges repel each other (positive repels positive, negative repels negative)
- The forces between charges are called electrostatic forces
- These electrostatic forces are much stronger than gravitational forces
SI unit of electric charge
The SI unit for measuring electric charge is the coulomb (C). This unit represents the amount of charge carried by approximately 6.24 × 10¹⁸ electrons.
Each individual electron carries a charge of 1.6 × 10⁻¹⁹ coulombs, which is an extremely small amount of charge.
Conductors and insulators
Materials can be classified based on their ability to allow electric charge to flow through them.
Electrical conductors are substances that allow electric charge to flow easily through them. Examples include:
- Metals like copper and aluminium
- The human body
- Acids and other ionic solutions
Electrical insulators are substances that do not allow electric charge to flow through them easily. Examples include:
- Plastic materials
- Glass
- Rubber
Why Some Materials Conduct
The difference lies in the structure of these materials. In conductors, some electrons are free to move when connected to a battery or other energy source. In insulators, electrons are tightly bound to their atoms and cannot move freely.
Three effects of electric current
When an electric current flows through a circuit, three observable effects can occur. These effects can be demonstrated using a simple circuit setup.
1. Heating effect
When current flows through any conductor, heat energy is produced. This heat comes from the resistance the conductor offers to the flow of current.
Universal Heating Effect
While some heat is always produced when current flows, the amount varies depending on the conductor and the current strength. This heating effect occurs in ALL current-carrying conductors - it cannot be avoided.
2. Magnetic effect
A conductor carrying electric current creates a magnetic field around itself. This can be detected using a magnetic compass placed near the current-carrying wire. The compass needle will deflect from its normal north-south alignment, indicating the presence of the magnetic field.
3. Chemical effect
When electric current passes through certain solutions (like acids), chemical reactions occur. This process is called electrolysis. In the demonstration circuit, when current flows through the acidic solution, bubbles of gas can be observed forming at the electrodes immersed in the solution.
Size of electric current
Electric current is defined as the flow of electric charge. More specifically, it measures how much electric charge flows past a given point in a conductor per second.
Current formula
The relationship between current, charge, and time is expressed as:
Where:
- I = electric current (measured in amperes)
- Q = electric charge (measured in coulombs)
- t = time (measured in seconds)
The ampere
The SI unit of electric current is the ampere (A). One ampere is defined as one coulomb of charge passing through a point in one second.
Calculating current
To find the current in a circuit, you need to know how much charge passes through it and over what time period.
Worked Example: Calculating Electric Current
If a charge of 10 coulombs passes through a point in a circuit in 5 seconds, the current would be:
Therefore, the current flowing through the circuit is 2 A.
Conventional current
Understanding the direction of current flow requires knowledge of historical conventions in electrical science.
Historical background
Scientists discovered electric current effects long before they understood that electrons were the actual moving particles. They established a convention that current flows from the positive terminal of a battery to the negative terminal through the external circuit.
Electron flow vs conventional current
Critical Concept: Current Direction
In reality, electrons (which carry negative charge) actually move from the negative terminal towards the positive terminal through the conductor. However, the established convention describes current as flowing in the opposite direction to electron movement.
Conventional current is defined as flowing from positive to negative terminals, even though electrons move in the opposite direction.
Practical implications
When analysing circuits or describing current direction, we always use conventional current direction (positive to negative) unless specifically stated otherwise. This standardised approach ensures consistency in electrical engineering and physics.
Direct current (DC) and alternating current (AC)
Electric currents can be classified into two main types based on their flow characteristics.
Direct current (DC)
Direct current always flows in one direction through a conductor. Sources of DC include:
- Batteries
- Solar cells
- DC power supplies
In DC circuits, the current maintains a constant direction from the positive to negative terminal of the voltage source.
Alternating current (AC)
Alternating current periodically reverses its direction of flow. The most common example is mains electricity supply, which reverses direction 100 times per second (50 Hz frequency in the UK).
Why AC for Power Distribution
AC current offers advantages for power distribution over long distances, which is why it's used for domestic and industrial electricity supply. The ability to easily change voltage levels using transformers makes AC more efficient for power transmission.
Measuring electric current
Electric current is measured using an instrument called an ammeter.
Types of ammeters
- Analogue ammeters use a needle and scale to display current readings
- Digital ammeters provide numerical displays of current values
Ammeter ranges
Ammeters are available in different ranges to measure various current levels:
- Milliammeters measure currents in milliamperes (mA)
- Microammeters measure currents in microamperes (μA)
- Standard ammeters measure currents in amperes (A)
Conversion between units
Understanding the relationships between different current units is essential:
Circuit connection
Proper Ammeter Connection
To measure current in a circuit, the ammeter must be connected in series with the component through which you want to measure the current flow. This ensures all the current passes through the ammeter for accurate measurement.
Connecting an ammeter in parallel (across a component) can damage the instrument and give incorrect readings.
Key Points to Remember:
- Electric charge is a fundamental property of matter carried by protons (+) and electrons (-)
- Electric current is the flow of electric charge, calculated using
- Three effects of electric current are heating, magnetic, and chemical effects
- Conventional current flows from positive to negative, opposite to actual electron movement
- Direct current (DC) flows in one direction, while alternating current (AC) periodically reverses direction
- Current is measured using an ammeter connected in series with the circuit