CIE AS/A Level Physics 18.1 Electric fields and field lines Study Notes- 2025-2027 Syllabus
CIE AS/A Level Physics 18.1 Electric fields and field lines Study Notes – New Syllabus
CIE AS/A Level Physics 18.1 Electric fields and field lines Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on AS/A Level Physics latest syllabus with Candidates should be able to:
- understand that an electric field is an example of a field of force and define electric field as force per unit positive charge
- recall and use F = qE for the force on a charge in an electric field
- represent an electric field by means of field lines
Electric Field as a Field of Force
An electric field is an example of a field of force, meaning it is a region in which a force is experienced by an object without direct contact. In an electric field, this force acts on any object that carries electric charge.
Definition of Electric Field
\( \mathrm{E = \dfrac{F}{q}} \)
- \( \mathrm{E} \) = electric field strength (N C⁻¹)
- \( \mathrm{F} \) = force experienced
- \( \mathrm{q} \) = charge (positive test charge)
Meaning:
The electric field at a point is the force per unit positive charge placed at that point.
Key Points:
- A positive charge experiences a force in the direction of the field.
- A negative charge experiences a force opposite to the field direction.
- The electric field exists even if no charge is present—any charge placed there will feel a force.
Example
A positive test charge of \( \mathrm{2.0 \times 10^{-6}\ C} \) experiences a force of \( \mathrm{0.04\ N} \) at a point. Find the electric field strength at that point.
▶️ Answer / Explanation
Use:
\( \mathrm{E = \dfrac{F}{q}} \)
\( \mathrm{E = \dfrac{0.04}{2.0\times 10^{-6}} = 2.0\times 10^{4}\ N\,C^{-1}} \)
Electric field strength = \( \mathrm{2.0\times 10^{4}\ N\,C^{-1}} \)
Example
A negative charge is placed in a uniform electric field. It experiences a force of \( \mathrm{5.0\times 10^{-3}\ N} \) to the left. If the charge is \( \mathrm{-2.0\times 10^{-6}\ C} \), determine the direction of the electric field.
▶️ Answer / Explanation
A negative charge experiences a force opposite to the electric field.
Given force is to the left, so the electric field must be to the right.
Electric field direction = rightwards.
Example
An unknown charge is placed in an electric field of strength \( \mathrm{3000\ N\,C^{-1}} \). It experiences a force of \( \mathrm{-0.12\ N} \). Calculate the charge and determine whether it is positive or negative.
▶️ Answer / Explanation
Use:
\( \mathrm{F = qE} \Rightarrow q = \dfrac{F}{E} \)
\( \mathrm{q = \dfrac{-0.12}{3000} = -4.0\times 10^{-5}\ C} \)
The charge is negative because the force is opposite to the field.
Charge = \( \mathrm{-4.0\times 10^{-5}\ C} \)
Force on a Charge in an Electric Field
When a charge is placed in an electric field, it experiences a force. This force is given by the equation:
\( \mathrm{F = qE} \)
- \( \mathrm{F} \) = force on the charge (N)
- \( \mathrm{q} \) = charge (C)
- \( \mathrm{E} \) = electric field strength (N C⁻¹)
Key Points:
- A positive charge experiences a force in the direction of the electric field.
- A negative charge experiences a force in the opposite direction.
- Stronger fields or larger charges produce larger forces.
- This equation applies to both uniform and non-uniform fields.
Physical Meaning:
The electric field tells you how much force each coulomb of charge would feel. Multiplying by the actual charge gives the total force.
Example
A charge of \( \mathrm{3.0\times 10^{-6}\ C} \) is placed in a uniform electric field of strength \( \mathrm{1500\ N\,C^{-1}} \). Find the force acting on the charge.
▶️ Answer / Explanation
Use \( \mathrm{F = qE} \):
\( \mathrm{F = (3.0\times 10^{-6})(1500)} \)
\( \mathrm{F = 4.5\times 10^{-3}\ N} \)
Force = \( \mathrm{4.5\times 10^{-3}\ N} \)
Example
A negative charge of \( \mathrm{-2.0\times 10^{-6}\ C} \) is placed in an electric field of \( \mathrm{800\ N\,C^{-1}} \) directed to the right. State the magnitude and direction of the force.
▶️ Answer / Explanation
Magnitude:
\( \mathrm{F = |q|E = (2.0\times 10^{-6})(800) = 1.6\times 10^{-3}\ N} \)
Direction:
- The field is to the right.
- A negative charge feels force opposite to the field → to the left.
Force = \( \mathrm{1.6\times 10^{-3}\ N} \) to the left.
Example
An oil drop carries a charge of \( \mathrm{1.6\times 10^{-19}\ C} \). It floats motionless between two horizontal plates because the upward electric force balances its weight. If the electric field strength between the plates is \( \mathrm{2.0\times 10^{5}\ N\,C^{-1}} \), find the mass of the oil drop.
▶️ Answer / Explanation
At equilibrium:
Electric force = Weight
\( \mathrm{qE = mg} \)
Rearrange:
\( \mathrm{m = \dfrac{qE}{g}} \)
Substitute values:
\( \mathrm{m = \dfrac{(1.6\times 10^{-19})(2.0\times 10^{5})}{9.8}} \)
\( \mathrm{m = \dfrac{3.2\times 10^{-14}}{9.8}} \)
\( \mathrm{m = 3.27\times 10^{-15}\ kg} \)
Mass ≈ \( \mathrm{3.3\times 10^{-15}\ kg} \)
Representing an Electric Field Using Field Lines
An electric field can be visualised using electric field lines (also called lines of force). These lines give a clear picture of both the direction and the strength of the field in a region.
Key Features of Electric Field Lines
- Direction: Field lines show the direction of the force on a positive test charge.
- Origin & Termination:
- Lines start on positive charges
- Lines end on negative charges
- Density:
- Closely spaced lines = strong field
- Widely spaced lines = weak field
- Field lines never cross (because the field has only one direction at a point).
- Lines are smooth and continuous—no sharp bends.
- Uniform electric field: represented by parallel, equally spaced lines.
| Configuration | Electric Field Line Diagram |
|---|---|
| Single Point Charge: Radial lines (outward for +Q, inward for –Q). |  |
| Two Like Charges: Field lines repel each other (symmetrical pattern, lines bend away). |  |
| Two Unlike Charges: Field lines connect from + to – (dipole pattern). |  |
Purpose of Field Lines:
They help visualise invisible electric forces and understand how charges interact in space.
Example
Describe the field-line pattern around a single positive point charge.
▶️ Answer / Explanation
- Field lines radiate outward in all directions.
- Lines are evenly spaced symmetrically.
- Closer to the charge, lines are dense → field is strong.
- Further away, lines spread → field is weaker.
Example
Sketch or describe the electric field pattern between two parallel metal plates, one positively charged and one negatively charged.
▶️ Answer / Explanation
- Field lines run straight from the positive plate to the negative plate.
- Lines are parallel and equally spaced.
- This represents a uniform electric field.
- Near the edges, lines begin to curve slightly (edge effect), but ignored in ideal diagrams.
Example
Describe and interpret the electric field-line pattern around an electric dipole consisting of a +4 μC charge and a –4 μC charge separated by a short distance.
▶️ Answer / Explanation
- Field lines originate at the +4 μC charge and terminate at the –4 μC charge.
- Close to the charges, lines are dense → field is strong.
- Lines curve smoothly from + to – producing a characteristic dipole pattern.
- Along the perpendicular bisector, lines are symmetrical and the field is strong between charges.
- No two field lines cross, and lines become sparse far from the dipole.
The pattern shows how the two charges interact, producing a combined field.