Topic 3 of 5
Charging by induction
A nearby charged object can move electrons within a conductor without touching it. To leave a net charge afterwards, separate the charge while that influence remains.
Electrons are negative and can move through a metal. The positive ions remain in its structure. Earth can act as a large reservoir that accepts or supplies electrons through a conducting connection.
Redistribution is not yet a change of total charge
Start with a neutral metal sphere on an insulating stand. Hold a negative rod near its left side without contact. The rod repels mobile electrons towards the far side. The near side has an electron deficit, and the far side an excess.
The sphere is still neutral overall: electrons have moved within it, but none have entered or left. Its nearer positive region is attracted more strongly than the farther negative region is repelled in this arrangement, so the neutral sphere can be attracted towards the rod.
If the rod is simply taken away, the electrons redistribute and the sphere remains neutral. A separated charge pattern while a rod is present is not proof of a retained net charge.
Use Earth to leave the sphere positive
Disconnect Earth before removing the negative rod
Signs on the sphere show local surface charge imbalance, not individual protons moving. The unmarked neutral sphere still contains charged particles. Blue arrows identify electron movement.
1. Start with an isolated neutral sphere
Net charge: zero. The metal sphere stands on an insulating support. There is no nearby rod and no conducting connection to Earth. Neutral means balanced charge, not an absence of charged particles.
2. Bring the negative rod close
Net charge: still zero. The negative rod does not touch the sphere. Some mobile electrons shift towards the far, right-hand side. The near side has an electron deficit. No electron leaves the isolated sphere, so its total charge remains zero.
3. Keep the rod close and connect Earth
Net charge: positive. A conducting wire connects the sphere to Earth at the right. Some electrons leave through the wire while the negative rod remains in place. The sphere has a net electron deficit. The blue arrow shows electron movement, not conventional current or positive particles entering.
4. Disconnect Earth first
Net charge: positive and isolated. The Earth connection is opened while the negative rod is still close. The gap prevents further charge exchange along that path. The sphere retains its net positive charge. No electron-flow arrow crosses the open gap.
5. Remove the rod
Net charge: remains positive. The charged rod is removed after Earth has been disconnected. The isolated sphere remains positively charged. In the ideal model with no other nearby objects, its excess positive surface charge is distributed symmetrically. This is a surface electron deficit, not protons moving through the metal.
- Start neutral and isolated. Place the neutral metal sphere on an insulating stand, with no rod nearby and no Earth connection.
- Bring the negative rod close without touching. Electrons move towards the far side. The total charge of the sphere is still zero.
- Keep the rod still and connect the sphere to Earth. Some electrons leave the sphere through the conducting path. The sphere is left with an electron deficit; positive particles do not flow in from Earth.
- Disconnect Earth first, with the rod still close. The sphere is now isolated. Its net positive charge is retained because the electrons that left no longer have that return path.
- Remove the rod. Charge redistributes over the sphere, which stays positive. An ideal isolated spherical conductor with no other nearby objects has a symmetric final surface-charge distribution.
The inducing rod never touches the sphere and retains its own charge in this ideal model. The electrons lost by the sphere go to Earth, so the wider charge account includes Earth.
Explain the order
Why disconnect Earth before removing the rod?
While the negative rod is close, it repels electrons from the sphere. Disconnecting Earth at that point isolates an electron deficit.
If the rod is removed while Earth is still connected, electrons can return through the available path. The sphere becomes neutral in this model. Disconnecting Earth then simply isolates a neutral sphere.
A positive inducing rod reverses the electron movement: electrons enter the sphere from Earth. Disconnect Earth while that rod is still nearby, then remove the rod, and the sphere retains a negative charge. The order stays the same even though the signs reverse.
See the sequence with a gold-leaf electroscope
A gold-leaf electroscope has a conducting cap connected to a metal stem and a thin conducting leaf. When the leaf and the adjacent stem acquire like charge, their repulsion makes the leaf rise away from the stem. Its insulating support helps prevent unintended leakage.
Use an initially uncharged instrument as the conductor in the negative-rod experiment. Bring the rod near the cap without touching: electrons move away from the cap towards the stem and leaf, and the leaf rises. With the rod held in place, briefly connect the cap to Earth; electrons leave and the leaf falls in this standard arrangement.
Disconnect Earth while keeping the rod close. Then remove the rod: the remaining positive charge redistributes onto the stem and leaf, and the leaf rises again. This retained divergence after the rod has gone indicates that charge remains. Leaf divergence alone does not identify its sign; the electron account explains why it is positive here.
| Sequence | Final observation and explanation |
|---|---|
| Disconnect Earth, then remove the rod | The leaf remains diverged after removal. The instrument was isolated with an electron deficit, so it retains positive charge. |
| Remove the rod, then disconnect Earth | The leaf ends down in the ideal arrangement. Electrons could return while the instrument was still earthed, leaving it neutral. |
| Approach and remove the rod without earthing | The leaf rises temporarily and returns down. There was redistribution while the rod was close, but no net charge transfer. |
Discharge the instrument before each new trial and keep the rod's sign, approximate position and support arrangement comparable. Do not touch the rod to the cap when testing induction. Small residual movement, leakage or limited sensitivity can affect what is seen; leaf angle is not a direct measurement in coulombs.
Induction without an Earth connection
Two initially neutral conducting spheres A and B touch each other on insulating supports. Place the negative rod close to A. Electrons are repelled from A into B through their contact, so A has a positive charge balance and B a negative one while the pair remains neutral overall.
Induction can also separate charge without Earth
A and B start neutral. The large signs label each sphere's net charge, not individual particles or a detailed surface distribution. Assume no charge escapes.
1. Keep the spheres touching; bring the rod close
Two initially neutral conducting spheres A and B touch on insulating supports. A negative rod is brought near A without contact. Electrons move through the contact from A to B. A develops a net positive charge and B a net negative charge, while the combined charge remains zero.
2. Separate A and B with the rod still close
The spheres are moved apart while the negative rod is kept near A. There is now an insulating gap between them, so electrons cannot move back through the former contact. A remains positive and B negative. Neither sphere is earthed.
3. Remove the rod after separation
After the spheres have been separated, the negative rod is removed. A retains positive charge and B negative charge. If no charge escaped, their net charges have equal magnitudes and opposite signs, so the combined charge is still zero. Large signs label net charge, not a surface distribution.
- Bring the negative rod near A without touching either sphere. Keep A and B in contact while electrons redistribute.
- Separate the spheres using their insulating supports, with the rod still held nearby.
- Remove the rod. A retains a positive net charge and B a negative net charge.
No electrons had to travel to Earth. They moved from A to B, and separating the spheres removed the path between them before the inducing influence was removed. If the rod is removed before the spheres are separated, electrons redistribute through their contact and both are left neutral in this model.
A charge indicator can show that each isolated sphere retains charge afterwards. Their signs follow from the electron account, or can be checked with suitable known-charge comparisons. Attraction between the spheres by itself would not establish every part of that charge account.
Ask where electrons can move at each stage. A metal contact or Earth lead provides a path; an insulating gap removes it. The correct order isolates a charge imbalance before removing the nearby rod.