Electrostatics
Senior 1 · Physics
Electrostatics
Lesson 3 of 5
25 min
Lay an empty metal drinks can on its side on a smooth table and bring a balloon, freshly rubbed on dry hair, close to it. The can rolls towards the balloon and follows it across the table. Nothing has touched the can, yet it behaves as if it is charged. Metals let electrons move freely through them, and that makes two more ways of charging possible: by contact and by induction. To watch them happen we use a gold-leaf electroscope.
Figure 3.1: A gold-leaf electroscope, here carrying a negative charge. Charge on the plate and on the leaf is of the same sign, so the leaf is pushed away from the plate.
The cap, rod, plate and leaf are all metal and are joined together, so any charge given to the cap spreads down to the plate and the leaf. The leaf and the plate then carry the same kind of charge and repel each other: the thin, light leaf rises. The more charge, the wider the leaf opens. The insulating plug stops the charge escaping to the earthed case, and the case shields the leaf from draughts.
Definition
A gold-leaf electroscope is an instrument for detecting charge. When it is charged, its leaf diverges (opens out); the bigger the charge, the bigger the divergence. It can show whether an object is charged, compare the sign of two charges, and show whether a material is a conductor or an insulator.
Figure 3.2: Charging an electroscope by contact with a negative rod. Some of the rod's extra electrons move onto the electroscope, which keeps them after the rod is removed.
When a charged polythene rod touches the cap, some of its extra electrons move onto the metal and spread over the cap, the rod, the plate and the leaf, pushed apart by their repulsion. The leaf rises and stays up after the rod is taken away. A positive rod works the other way: electrons move from the electroscope onto the rod, leaving the electroscope positive.
Definition
Charging by contact (by conduction) means touching a neutral conductor with a charged object, so that the charge is shared between them. The conductor gets charge of the same sign as the charged object, and the charged object loses some of its charge.
Worked example 1: sharing charge between identical spheres
Two identical metal spheres on insulating stands carry charges of and . They are touched together, then separated. What is the charge on each?
Charge is conserved, so the total stays the same, and identical spheres share it equally:
In fact electrons moved: the negative sphere gave electrons to the positive one until the two were equal.
Figure 3.3: Charging a metal sphere by induction with a negative rod. The rod never touches the sphere, and the sphere ends up with the opposite charge to the rod.
Definition
Charging by induction means charging a conductor by bringing a charged object NEAR it (without touching) and earthing the conductor for a moment. The conductor gets charge of the opposite sign to the charged object, which loses none of its own charge.
Earthing means connecting a conductor to the ground, which can take or give any number of electrons.
Important
Order matters. Always break the earth connection BEFORE you remove the charged rod. If the rod goes first, the electrons flow straight back from the earth and the sphere ends up neutral.
Worked example 2: electrons to earth
A metal sphere charged by induction with a negative rod ends up with . How many electrons flowed to earth?
| Friction | Contact | Induction | |
|---|---|---|---|
| What is charged | two different insulators | usually a conductor | a conductor |
| Do they touch? | yes, rubbed together | yes | no |
| Sign of the new charge | opposite on the two materials | SAME as the charged object | OPPOSITE to the charged object |
| Does the charging object lose charge? | (both become charged) | yes, it shares its charge | no |
First charge the electroscope with a known sign, say negative, so its leaf is part-way open. Then bring the object near the cap, without touching:
| What the leaf does | What it tells you |
|---|---|
| opens wider | the object has the SAME charge as the electroscope (negative) |
| closes a little | the object is positive, OR it is neutral |
| does not move | the object is uncharged, or its charge is too small to notice |
As with the rods in lesson 2, only the increase is a sure test. Touching the cap with an object held in the hand is a test of conductors: if the leaf falls at once, the object is a conductor and the charge has run through it and through you to earth.
Worked example 3: reading the leaf
An electroscope is charged positively. A rod brought near the cap makes the leaf rise further. A second rod makes the leaf fall. What can you say about each rod?
Warning
Common mistakes. (1) Giving the induced charge the same sign as the rod: induction gives the OPPOSITE sign. (2) Removing the rod before breaking the earth connection. (3) Thinking that positive charges flow to or from the earth: in metals only electrons move.
Tip
Contact copies the sign; induction gives the opposite sign. "Contact copies, induction inverts."
When a charged thundercloud passes over Rwanda's hills, it repels electrons in the ground and in tall objects below it, so their tops gain the opposite charge by induction; that is where lightning is most likely to strike (lesson 5). In PhET: John Travoltage (version 1.6.22), John's foot rubs on the carpet and picks up electrons by friction; as his hand approaches the metal doorknob, the doorknob's charges separate by induction, and a spark jumps. Try moving his hand slowly and watch where the charges sit.
Charging by induction (The Organic Chemistry Tutor). Watch the charges separate in two metal spheres and in a single earthed sphere, and notice the order: earth broken first, rod removed last.
Practice
Key points
Sources and credits
The diagrams, the worked examples, the comparison tables and the practice exercises are written by Ganzaa.
