Physics · Year 12 · Charge and Current · Autumn Term

Electric Fields and Coulomb's Law

Students will define electric fields and calculate forces between point charges using Coulomb's Law.

• National Curriculum Attainment Targets
• A-Level: Physics - Electricity
• A-Level: Physics - Electric Fields

About This Topic

Current, Potential Difference, and Resistance moves students from the basic circuit rules of GCSE to a sophisticated understanding of charge flow and energy transfer. The topic covers Kirchhoff’s Laws, which are the fundamental conservation laws for charge and energy in DC circuits. Students also explore the behavior of non-ohmic components like thermistors and LDRs, which are essential for modern sensor technology.

A key challenge at this level is moving away from the 'water pipe' analogy toward a more rigorous model of potential dividers and internal resistance. Students must understand why the voltage across a battery drops when a current is drawn. Students grasp this concept faster through structured discussion and peer explanation of circuit diagrams, where they can troubleshoot 'broken' circuits together.

Key Questions

1. Explain how electric current is defined as the rate of flow of charge and derive the relationship I = ΔQ/Δt from first principles.
2. Analyse how the mean drift velocity of free electrons determines current magnitude using I = nqvA, comparing values in metals and semiconductors.
3. Evaluate how the electron model of conduction accounts for the differences in electrical conductivity between conductors, semiconductors, and insulators.

Learning Objectives

• Define electric field strength and state its unit.
• Calculate the magnitude and direction of the electrostatic force between two point charges using Coulomb's Law.
• Compare the electric forces acting on charges in different configurations.
• Explain the concept of an electric field as a region of space where a charge experiences a force.

Before You Start

Introduction to Electric Charge

Why: Students need to understand the basic properties of electric charge, including positive and negative types and the concept of charge conservation, before exploring forces between charges.

Basic Algebra and Proportionality

Why: Coulomb's Law involves inverse square relationships and calculations, requiring students to be comfortable with algebraic manipulation and understanding proportionalities.

Key Vocabulary

Electric Field	A region around an electrically charged object where another charged object will experience a force. It is represented by field lines indicating direction and strength.
Coulomb's Law	A law stating that the electrostatic force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them.
Point Charge	An idealized electric charge located at a single point in space, with no spatial extent.
Electrostatic Force	The attractive or repulsive force between two electrically charged objects. Like charges repel, and opposite charges attract.

Watch Out for These Misconceptions

Common Misconception

Current is used up as it goes around a circuit.

What to Teach Instead

Current is the rate of flow of charge, and charge is conserved (Kirchhoff’s First Law). Use ammeters at multiple points in a series circuit to show students that the current remains identical throughout, reinforcing the conservation of matter.

Common Misconception

Potential difference and electromotive force (emf) are the same thing.

What to Teach Instead

Emf is the total energy supplied per unit charge by the source, while terminal PD is the energy delivered to the external circuit. Active troubleshooting of circuits with high internal resistance helps students see the energy 'lost' inside the battery itself.

Active Learning Ideas

Stations Rotation: Component Characteristics

Students move between stations to measure I-V characteristics for a filament lamp, a diode, and a fixed resistor. They must use their data to explain why some components do not follow Ohm's Law as they heat up.

60 min·Small Groups

Inquiry Circle: The Potential Divider Challenge

Groups are given a sensor (LDR or thermistor) and must design a circuit that turns on an LED when it gets dark or cold. They must calculate the required fixed resistor values before building and testing their design.

50 min·Small Groups

Think-Pair-Share: Internal Resistance Mystery

Show a battery measuring 9V when 'open' but 8.5V when connected to a bulb. Students work in pairs to hypothesize where the 'lost volts' went, leading to a class discussion on internal resistance and emf.

20 min·Pairs

Real-World Connections

• Engineers use Coulomb's Law to design electrostatic precipitators, which remove particulate matter from industrial emissions by charging the particles and then attracting them to oppositely charged plates.
• In the development of touch screen technology, the principles of electric fields and charge interaction are fundamental to how the screen detects the position of a finger or stylus.

Assessment Ideas

Quick Check

Present students with a diagram showing two point charges, one positive and one negative, separated by a distance. Ask them to draw the electric field lines around each charge and indicate the direction of the force on the negative charge. Then, ask them to write the formula for Coulomb's Law.

Exit Ticket

Provide students with two charges, +2.0 µC and -3.0 µC, separated by 0.10 m. Ask them to calculate the magnitude of the force between them using Coulomb's Law and state whether the force is attractive or repulsive. Include the value of the Coulomb constant, k = 9.0 x 10^9 Nm^2/C^2.

Discussion Prompt

Pose the question: 'How does the electric field strength change as you move further away from a single point charge? Use Coulomb's Law to justify your answer and explain what this means for the force experienced by a test charge.' Facilitate a brief class discussion on their responses.

Frequently Asked Questions

What are Kirchhoff's Laws?

Kirchhoff's First Law states that the total current entering a junction equals the total current leaving it (conservation of charge). The Second Law states that the sum of the emfs in any closed loop equals the sum of the potential drops (conservation of energy). These laws are the foundation for all circuit analysis.

How can active learning help students master DC circuits?

Circuits are often taught as abstract diagrams. Active learning turns these into puzzles. By building circuits and using multimeters to 'hunt' for lost volts or unexpected currents, students develop a mental model of energy transfer. Peer teaching during circuit assembly also helps students correct common errors in parallel wiring.

Why does the resistance of a filament lamp increase with temperature?

As the metal filament gets hotter, the positive ions in the lattice vibrate more vigorously. This increases the frequency of collisions between the flowing electrons and the ions, making it harder for the current to pass through, which we measure as increased resistance.

What is a potential divider?

A potential divider is a simple circuit that turns a large voltage into a smaller one. By using two resistors in series, the input voltage is split in proportion to their resistances. This is the basis for most electronic sensors, where one resistor changes its value based on external conditions.