Preview
IB DP Physics — Unit D: Fields | SL & HL | D.2 Electric and Magnetic Fields
📋 What You Will Learn
- Observe and describe how electric and magnetic fields are represented using field lines, and interpret what these lines tell us about the field's strength and direction
- Define electric field strength \( E \) and magnetic flux density \( B \) as vector quantities, with correct units and physical meaning
- Derive and apply the expression for electric field strength due to a point charge using Coulomb's law
- Analyse field line diagrams for point charges, parallel plates, bar magnets, and current-carrying conductors
- Apply the equations \( E = \frac{F}{q} \), \( E = \frac{kQ}{r^2} \), and \( F = qvB\sin\theta \) to solve figure-based and graph-based problems
- Evaluate common misconceptions about field lines and connect electric and magnetic field concepts to the wider IB Physics course
🌍 Real-World Context When a bolt of lightning strikes, it follows an invisible path carved by the electric field between a thundercloud and the ground. That path — jagged, branching, and always directed from regions of high potential to low — is a physical trace of the electric field lines that existed in the air before the strike. Similarly, the aurora borealis forms when charged particles from the Sun spiral along Earth's magnetic field lines toward the poles, painting the sky in curtains of light. Both phenomena reveal something profound: invisible fields exert real, measurable forces on charged particles, and the geometry of those fields determines everything about how those forces act.
🔍 Guiding Question How can we represent and quantify the strength and direction of electric and magnetic fields, and what do these fields tell us about the forces experienced by charges and currents placed within them?
Physical Setup: What Is a Field?
Imagine holding two magnets close together — you can feel a force even though they are not touching. This force acts across empty space, mediated by something we call a field . A field is a region of space in which a physical quantity (such as force) has a value at every point. In physics, we encounter gravitational fields, electric fields, and magnetic fields — each one invisible, yet each one capable of exerting measurable forces on objects placed within it.
To make these invisible fields visible, physicists use the concept of field lines — imaginary lines drawn in space that show, at every point, the direction a positive test charge (for electric fields) or a north magnetic pole (for magnetic fields) would experience a force. The closer together the field lines are drawn, the stronger the field at that location. This simple visual tool turns an abstract mathematical concept into something we can sketch, analyse, and use to predict physical behaviour.
Notice in Diagram 1 that the field lines for a positive charge radiate outward — this tells us that a positive test charge placed anywhere near +Q would be pushed away. For a negative charge, the lines converge inward, telling us a positive test charge would be pulled toward −Q. The spacing between lines encodes the field strength: tightly packed lines near the charge indicate a strong field, while widely spaced lines far away indicate a weaker field.
Electric Field Lines — Rules and Conventions
Before we can use field line diagrams to solve problems, we need to establish the precise rules that govern how they are drawn. These are not arbitrary conventions — each rule follows directly from the physics of how electric forces behave.
📖 Key Definition — Electric Field Line An electric field line is a line drawn in an electric field such that the tangent to the line at any point gives the direction of the electric field (and hence the force on a positive test charge) at that point. Field lines originate on positive charges and terminate on negative charges, or