Electric charge: charging, conservation and quantisation
Rubbing, contact and induction only move electrons between bodies. Charge comes in two kinds and is conserved and quantised: q = Ne.
Try it: two kinds of charge
- 1Rub a hard rubber rod (or a plastic comb) with wool or fur. Rub a glass rod with silk.
- 2Hang each rod from a silk or nylon thread so that it can turn freely.
- 3Bring a second rubbed rubber rod near the hanging rubber rod. Observe: they repel.
- 4Bring the rubbed rubber rod near the hanging glass rod. Observe: they attract.
- 5Repeat with a second rubbed glass rod. It repels the glass rod and attracts the rubber rod.
- 6★So, there are two kinds of charge. Like charges repel and unlike charges attract. Benjamin Franklin called the charge on glass positive and the charge on rubber negative.
★ key step
What rubbing really does
Every atom has a positive nucleus with electrons around it. A neutral atom has as many electrons as protons. Rubbing two different materials moves some loosely held electrons from one to the other. When glass is rubbed with silk, electrons move from the glass to the silk. The glass is left short of electrons, so it is positive. The silk gains the same number, so it is equally negative. When rubber is rubbed with fur, electrons move from the fur to the rubber. So the rubber becomes negative and the fur positive. Rubbing never makes charge: it only moves electrons.
Three ways to charge a body
| Method | What you do | Charge the body gets |
|---|---|---|
| Rubbing (friction) | Rub two different materials together. | Equal and opposite charges on the two bodies. |
| Contact (conduction) | Touch a charged body to an uncharged conductor. | The same kind of charge as the charged body. Two identical spheres share the total charge equally. |
| Induction | Bring a charged body near a conductor, earth the conductor, remove the earth, then remove the charged body. | The opposite kind of charge. The charged body loses none of its own charge. |
How to charge a sphere by induction (and draw it)
A metal sphere stands on an insulating stand. You have a negatively charged rubber rod.
- 1Bring a negatively charged rod near the sphere, without touching. Free electrons in the sphere are pushed to the far side, so the near side is left positive.
- 2Draw: a circle on a stand, with the rod and its − signs on the left. Put + signs on the near half of the sphere and − signs on the far half.
- 3Touch the far side with an earth wire (or your finger). The pushed electrons flow away to the earth. Draw the wire ending in the earth symbol, three short lines.
- 4Remove the earth wire first, while the rod is still near. Your NIOS lesson says both are removed together. Remove the earth first: if the rod goes first, the electrons flow back and the sphere is left uncharged.
- 5Now take the rod away. The positive charge spreads over the whole sphere. Draw + signs all round it.
- 6★So, induction gives the sphere a charge opposite to that of the rod, and the rod keeps all its charge.
★ key step
Conservation and quantisation of charge
| Property | What it means | Example |
|---|---|---|
| Only two kinds | Charge is either positive or negative. No other kind has ever been found. | Glass rubbed with silk is positive; rubber rubbed with fur is negative. |
| Conservation | The total charge of an isolated system stays the same. Charge moves from one body to another, but is never created or destroyed. | The glass gets +q and the silk gets −q, so the total stays zero. |
| Quantisation | Charge always comes in whole-number multiples of e: q = Ne, where N is an integer. Millikan showed this by experiment in 1909. | No body can carry 2.5e or 6.4e of charge. |
Worked example: electrons transferred
A polythene strip rubbed with wool gains a charge of −3.2 × 10⁻⁷ C. (a) How many electrons were transferred, and in which direction? (b) Is there a transfer of mass? (Charge of an electron = 1.6 × 10⁻¹⁹ C, mass of an electron = 9.1 × 10⁻³¹ kg)
- Given: q = −3.2 × 10⁻⁷ C, e = 1.6 × 10⁻¹⁹ C.
- Formula: q = ne, so n = |q|/e.
- n = 3.2 × 10⁻⁷ C ÷ 1.6 × 10⁻¹⁹ C = 2 × 10¹².
- The strip is negative, so electrons moved from the wool to the polythene.
- (b) Mass moved = n × mass of one electron = 2 × 10¹² × 9.1 × 10⁻³¹ kg = 1.82 × 10⁻¹⁸ kg.
(a) 2 × 10¹² electrons, from the wool to the polythene; (b) yes, about 1.8 × 10⁻¹⁸ kg, far too small to measure
In dry winter weather a woollen sweater crackles and a car door can give a small shock: rubbing has moved electrons. Fuel tankers are earthed before unloading so that charge built up by friction drains away safely.
1 marks: “What is the charge on a body that has lost 5 × 10¹⁸ electrons?” q = Ne = 5 × 10¹⁸ × 1.6 × 10⁻¹⁹ C = 0.8 C. It is positive, because electrons were lost.
2 marks: “Explain, with a diagram, how a metal sphere on an insulating stand can be given a negative charge by induction.” Bring a positively charged glass rod near: electrons gather on the near side. Earth the sphere: electrons flow up from the earth. Remove the earth wire, then the rod. The sphere is left negative. Diagram: sphere, stand, rod, earth wire.
2 marks: “State two properties of electric charge.” (1) Conservation: the total charge of an isolated system stays constant. (2) Quantisation: q = Ne, where N is a whole number and e = 1.6 × 10⁻¹⁹ C.
Glass gives, rubber grabs: glass gives away electrons and turns +, rubber grabs them and turns −. Charge is moved, never made, and always comes in lumps of e.