A charged object reaches out into the space around it, ready to push or pull any other charge that wanders close, even across an empty gap. We map that invisible reach with field lines, and the very same idea, scaled up to a thundercloud, explains the most dramatic display of static electricity on Earth: the lightning strike.
Hold a charged balloon near small pieces of paper and they jump up to it without being touched.
Before reading: write one sentence describing what you think is in the space between the balloon and the paper that lets one pull on the other.
A charge changes the space around it so other charges feel a force
A charged object does not need to touch another charge to push or pull it, it alters the space around itself, and any charge placed in that altered space feels a force.
That altered region is the electric field. We define its direction at any point as the direction of the force that a tiny positive test charge would feel if placed there. This gives the key characteristics of the field around a single point charge:
An electric field is a region in which a charged object experiences a force. Its direction at a point is the direction of the force on a small positive test charge: radially outward from a positive point charge, radially inward toward a negative one.
Pause, copy the highlighted definition into your book before moving on.
Did you get this? True or false: the electric field around a positive point charge points radially inward, toward the charge.
A set of simple rules turns the field into a picture you can read
We just saw that a field has a direction and a strength at every point. That raises a question: how do we picture all of that at once? This card answers it → with field-line diagrams that follow a fixed set of rules.
Field lines are a way of drawing the field so that you can see, at a glance, both which way it points and where it is strong.
The rules for electric field lines are:
A diagram shows field lines running radially inward toward an isolated charge. The lines are much closer together at point P, near the charge, than at point Q, further away. State the sign of the charge, the direction of the force on an electron placed at P, and where the field is stronger.
Field lines start on positive charge and end on negative charge, their arrows give the field direction (force on a positive test charge), and their spacing shows strength: closer lines = stronger field. Lines never cross.
Add the highlighted field-line rules to your notes before the check below.
Quick check: In a field-line diagram, two regions show lines that are very close together (region X) and lines that are far apart (region Y). Where is the field stronger?
Pattern A, Draw then reason
In your book: (1) draw the field-line diagram for a single negative point charge; (2) draw the field-line diagram for two equal positive charges placed side by side; (3) in one sentence each, explain how your diagrams show field direction and field strength.
Fill the blanks: Complete the description of the field itself (not its diagram).
The electric field around an isolated point charge is , pointing straight out from or straight in towards the charge, and its strength as you move further away.
Dot point: Outline characteristics of an electric field surrounding a point charge
A thundercloud is a giant exercise in charge separation
We just learned that charges set up fields in the space around them. That raises a question: what happens when an enormous amount of charge builds up? This card answers it → it creates a field strong enough to tear electrons off air molecules, producing lightning.
Lightning is static electricity on a huge scale: charge separates inside a storm cloud until the electric field becomes so strong that the air itself suddenly conducts.
Inside a thundercloud, violent updraughts cause ice crystals and water droplets to collide and rub against each other. This friction separates charge: the top of the cloud becomes positively charged and the base becomes strongly negative. The negative base then repels electrons in the ground below, leaving the ground surface positively charged.
This charge separation sets up a very strong electric field between the cloud base and the ground. When that field becomes strong enough, it rips electrons from the air molecules in the gap (the air "breaks down" and becomes a conductor), and a huge surge of charge flows along that path, the lightning strike, neutralising the separated charge.
In a lightning strike, friction inside a storm cloud separates charge so the cloud base is strongly negative. This creates a very strong electric field to the ground; when the field is strong enough the air breaks down and conducts, and the built-up static charge discharges as a lightning bolt.
Pause, copy the highlighted lightning explanation into your book before the check below.
Last check: Decide if the statement is true or false.
A lightning strike begins because charge separation in a cloud produces an electric field strong enough to make the air between the cloud and ground conduct.
Two truths, one lie: three of these statements about electric field LINE DIAGRAMS are correct. Which one is the lie?
Dot point: Use field line diagrams to describe the electric field around point charges and between charged particles
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 4(4 marks) 1. Explain the role of static electricity in producing a lightning strike. In your answer, refer to charge separation and the electric field.
1 mark: friction in the cloud separates charge · 1 mark: cloud base becomes strongly negatively charged · 1 mark: this sets up a strong electric field to the ground · 1 mark: field becomes strong enough to make air conduct, discharging as lightning
UnderstandBand 4(3 marks) 2. Outline three characteristics of the electric field surrounding an isolated positive point charge. Your answer should refer to direction, to how the strength varies, and to whether the field depends on a test charge being present.
1 mark: the field is radial, directed away from a positive charge · 1 mark: the strength decreases with distance from the charge · 1 mark: the field exists whether or not a test charge is placed in it, because it is a property of the source charge
Dot point: Outline characteristics of an electric field surrounding a point charge
ExplainBand 4(3 marks) 3. Explain why two electric field lines can never cross, and state what the spacing of field lines tells you about the field.
1 mark: a field line gives the direction of the field at that point · 1 mark: crossing would give two directions at one point, which is impossible · 1 mark: closer spacing means a stronger field
Dot point: Use field line diagrams to describe the electric field around point charges and between charged particles
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): Inside a thundercloud, colliding ice crystals and water droplets rub together, and this friction separates charge so that the base of the cloud becomes strongly negatively charged while the top becomes positive. The strongly negative cloud base repels electrons in the ground, leaving the ground positively charged, and this charge separation sets up a very strong electric field in the gap between the cloud and the ground. When that field becomes strong enough, it strips electrons from air molecules so the air breaks down and conducts, and the accumulated static charge flows along this conducting path as a lightning strike, neutralising the separated charge.
Q2 (3 marks): First, the field is radial: at every point it points directly away from the charge along the line joining that point to the charge, because that is the direction of the force a positive test charge would feel. Second, the strength is not constant. It is greatest close to the charge and decreases as the distance increases, falling as the inverse square of that distance. Third, the field is a property of the source charge alone and exists in the surrounding space whether or not anything is placed there to feel it. Removing a test charge does not remove the field; the test charge only reveals it.
Q3 (3 marks): A field line shows the direction of the electric field, and therefore of the force on a positive test charge, at every point along it. If two lines crossed, the field at the crossing point would have two different directions at once, which is impossible because a test charge placed there can only be pushed one way. The spacing of the lines carries the second piece of information: where the lines are drawn close together the field is strong, and where they spread apart the field is weak. That is why the lines around a point charge crowd together near the charge and spread out with distance.
You can now describe the electric field around a point charge, draw and read field-line diagrams, and explain a lightning strike. Next lesson makes the field strength quantitative with the inverse-square law $E = \dfrac{kq}{r^2}$.
↩ Back to focus-area hubAt the start you described what fills the space between a charged balloon and the paper it attracts. That space contains an electric field, the balloon's charge sets up a region in which any other charge feels a force, with no contact needed. The same idea, scaled up to a thundercloud, explains lightning.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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