Ways To Charge Conductors
Key idea: G3 Physics and O-Level Physics static electricity: how to charge conductors by conduction and induction (with earthing).
By the end, you can
- Describe an experiment that charges a conductor by induction without contact.
- Track electron flow during earthing and use the correct removal sequence.
1. Definition
A. Charging by conduction (contact)
Charging by conduction happens when a charged object touches a conductor and electrons transfer.
B. Charging by induction
Charging by induction charges a conductor without contact. A charged object is brought near, and with earthing, electrons enter/leave the conductor so it gains a net charge.
O Level expects you to describe experiments to show electrostatic charging by induction.
2. Key Ideas
- In a conductor, electrons can move freely, so charge spreads out (mainly on the surface).
- Earthing (grounding) provides a path for electrons to flow to/from the Earth.
- In induction, the order matters:
- remove the earth connection first
- then remove the charged rod
- After induction, the conductor ends with a net charge opposite to the charged rod.
3. Detailed Explanations
A. Charging by conduction (contact)
When a charged object touches a neutral conductor, electrons move until charge redistributes.
Result: the conductor ends up with the same sign as the object that touched it.
The current syllabus outcome specifically assesses an induction experiment. Contact charging is useful supporting comparison, but induction is the core method to describe precisely.
B. Charging by induction (with earthing)
Example: negatively charged rod
- Bring a negatively charged rod near (not touching) a neutral metal sphere.
- Electrons in the sphere move away from the rod (charge separation).
- While the rod is still near, connect the sphere to earth:
- electrons flow from the sphere into Earth
- Disconnect the earth first (rod still near).
- Remove the rod. The sphere is left positively charged.
If the rod is positively charged
The steps are the same, but the electron flow reverses:
- electrons flow from Earth into the sphere
- the sphere is left negatively charged
Static electricity can cause sparks. For real-world hazards and prevention, see Electrostatic Hazards.
4. Common Mistakes
- Saying protons move (in solids, only electrons move).
- Charging by induction but removing the rod before disconnecting earth (the charge will leak away).
- Saying induced charge is the same sign as the rod (it is the opposite sign after earthing).
5. Exam Tips
- Write the induction sequence clearly:
- rod near → 2) earth on → 3) earth off → 4) rod away
- Use the “electron story”:
- positive rod attracts electrons toward it
- earthing supplies electrons
- remove earth first so electrons can’t return
- If asked “why no contact?”, write: “induction charges without contact”.
6. Worked Examples
Example 1: Contact vs inductionCore
State one difference between charging by conduction and charging by induction.
Show Answer
Conduction needs contact between the charged object and the conductor. Induction charges the conductor without contact (using a nearby charged object and earthing).
Example 2: Induction signCore
A positively charged rod is used to charge a metal sphere by induction (with earthing). What is the final charge on the sphere?
Show Answer
Negative. Electrons flow from Earth onto the sphere while the positive rod is near. After removing earth first, the extra electrons remain.
Example 3: Why remove earth first?Core
In induction, why must you disconnect the earth connection before removing the rod?
Show Answer
If you remove the rod first, the charge separation disappears and electrons can flow back through the earth wire, leaving the sphere neutral. Removing the earth first traps the net charge on the sphere.
Example 4: Charging by contact (sign)Core
A negatively charged rod touches a neutral metal sphere, then is removed. What is the final charge on the sphere?
Show Answer
Negative. By contact, electrons transfer onto the sphere and it ends up with the same sign as the rod.
Example 5: Induction with a negative rod (electron story)Core
A negatively charged rod is brought near a neutral metal sphere. The sphere is earthed and then the earth is removed first. What is the final charge on the sphere, and which way did electrons flow during earthing?
Show Answer
The sphere becomes positive. The negative rod repels electrons in the sphere; when earthed, electrons flow from the sphere into Earth. After removing the earth first, the sphere has a deficit of electrons (positive).
7. Mind Stretchers
Mind stretcher 1: Induction without earthingExtension
A charged rod is brought near a neutral isolated metal sphere, but there is no earthing and no contact. After the rod is removed, is the sphere charged? Explain.
Show Answer
No. The charges separate while the rod is nearby, but without earthing there is no net gain/loss of electrons. When the rod is removed, charges redistribute and the sphere becomes neutral again.
Mind stretcher 2: Two different rodsExtension
Using induction, which rod would leave the sphere positive: a positive rod or a negative rod?
Show Answer
A negative rod. Electrons are repelled from the sphere to Earth, so the sphere is left with a deficit of electrons (positive).
8. Practice and next step
Practise writing the four-stage sequence—rod near, Earth on, Earth off, rod away—for both rod signs in Static Electricity structured practice. Then continue to Electric Fields.
Recommended next step
Charging by induction: concept check
Why this will help: Use one focused question set to check that you can apply the lesson without prompts.
About 10 minutes