Electric Fields
This lesson covers electric fields — as required by the Edexcel GCSE Physics specification (1PH0). You need to understand what electric fields are, how to draw and interpret field line diagrams, and the difference between uniform and non-uniform fields.
What Is an Electric Field?
An electric field is a region around a charged object where another charged object experiences a force. Any charged object creates an electric field in the space around it.
Key points:
- The electric field is invisible — but we can represent it using field lines.
- The field exists whether or not another charged object is present to feel the force.
- A stronger electric field exerts a greater force on a charged object placed in it.
Exam Tip: The definition of an electric field is very specific: "a region where a charged object experiences a force." You must mention both "charged object" and "force" for full marks.
Electric Field Lines
We represent electric fields using field lines (also called lines of force). These are arrows that show the direction of the force that would act on a positive test charge placed in the field.
Rules for Drawing Electric Field Lines
- Field lines point away from positive charges and towards negative charges.
- Field lines never cross each other.
- The closer together the field lines, the stronger the field.
- Field lines are perpendicular to the surface of a charged conductor.
Field Around a Single Positive Charge
The field lines point radially outward (away from the charge in all directions).
graph TD
A["Positive<br/>Charge (+)"] --> B["Field lines point<br/>OUTWARD in all<br/>directions"]
B --> C["Stronger field<br/>near the charge<br/>(lines closer together)"]
B --> D["Weaker field<br/>further away<br/>(lines further apart)"]
style A fill:#e74c3c,color:#fff
style B fill:#2980b9,color:#fff
style C fill:#27ae60,color:#fff
style D fill:#7f8c8d,color:#fff
Key Features
- Lines radiate outward from the positive charge.
- The field is non-uniform — it gets weaker as you move further from the charge.
- Lines are closer together near the charge (stronger field) and further apart at a distance (weaker field).
Field Around a Single Negative Charge
The field lines point radially inward (towards the charge from all directions).
Key Features
- Lines point towards the negative charge.
- The field is non-uniform — it gets weaker with distance.
- The pattern is identical to the positive charge pattern, but with arrows reversed.
Field Between Two Opposite Charges
When a positive charge and a negative charge are placed near each other, the field lines go from the positive charge to the negative charge.
graph LR
A["Positive<br/>Charge (+)"] --> B["Field lines<br/>from + to −"]
B --> C["Negative<br/>Charge (−)"]
style A fill:#e74c3c,color:#fff
style B fill:#2980b9,color:#fff
style C fill:#3498db,color:#fff
Key Features
- Field lines curve from the positive charge to the negative charge.
- The field is strongest in the region between the two charges (lines are closest together).
- The field is non-uniform — the strength varies from point to point.
Field Between Two Like Charges
When two charges of the same sign are placed near each other:
- The field lines point away from both (if both are positive) or towards both (if both are negative).
- There is a neutral point between them where the fields cancel out and the resultant field is zero.
- The charges repel each other.
Uniform Electric Field
A uniform electric field has the same strength and direction at every point. This is achieved by placing two parallel charged plates opposite each other — one positive and one negative.
Key Features of a Uniform Field
| Feature | Detail |
|---|
| Field lines | Straight, parallel, equally spaced |
| Direction | From the positive plate to the negative plate |
| Strength | Same at every point between the plates |
| Where it occurs | Between parallel charged plates |
Field Line Diagram
Between two parallel plates:
- The positive plate is on one side, the negative plate on the other.
- Field lines go straight across from + to −.
- The lines are equally spaced (showing uniform strength).
- At the edges, the field lines curve outward slightly (edge effects), but at GCSE level, you can ignore this.
Exam Tip: A uniform field is identified by parallel, equally spaced field lines. A non-uniform field has lines that are closer together in some regions (stronger) and further apart in others (weaker). The exam may ask you to distinguish between them.
Effect of an Electric Field on Charged Particles
A charged particle placed in an electric field experiences a force.
| Particle Charge | Direction of Force |
|---|
| Positive | In the same direction as the field lines (from + to −) |
| Negative | In the opposite direction to the field lines (from − to +) |
In a Uniform Field (Between Parallel Plates)
- A positive charge is pushed from the positive plate towards the negative plate (in the direction of the field).
- A negative charge (e.g. an electron) is pushed from the negative plate towards the positive plate (against the field direction).
- The force on the charged particle is constant throughout the uniform field (because the field strength is the same everywhere).
Voltage and Electric Fields
There is a direct link between voltage (potential difference) and electric fields:
- A potential difference between two points creates an electric field between them.
- The greater the potential difference between two points (for a given distance), the stronger the electric field.
- Between parallel plates, increasing the voltage between the plates increases the field strength.
- Decreasing the distance between the plates (for the same voltage) also increases the field strength.
Electric Field Strength (Higher Tier Awareness)
For a uniform field between parallel plates:
E=dV
Where:
- E = electric field strength (V/m or N/C)
- V = potential difference between the plates (V)
- d = distance between the plates (m)
This shows that field strength increases when the voltage increases or the distance decreases.
Sparking and Discharge Revisited
Linking to the previous lesson on static electricity:
- A large potential difference creates a strong electric field.
- If the field is strong enough, it can ionise air molecules (strip electrons from them).
- This creates a conducting path of ions and free electrons through the air.
- Charge flows rapidly through this path — we see this as a spark.
- Lightning is a dramatic natural example of this process.
When Does Sparking Occur?
- The electric field strength in air must reach approximately 3 × 10⁶ V/m (3 million volts per metre) to cause the air to break down and conduct.
- Pointed objects concentrate the electric field (field lines are closer at the tip), making sparking more likely. This is why lightning conductors have pointed tips.