Embedded Files
objectives
objectives
Attraction of Uncharged Objects
Attraction of Uncharged Objects
- A charged object will attract any uncharged object close to it. For example, the charged screen of a TV will attract dust.
- The diagram on the right shows what happens if a positively charged rod is brought near a small piece of aluminium foil. Electrons in the foil are pulled towards the rod, which leaves the bottom of the foil with a net positive charge.
- As a result, the top of the foil is attracted to the rod, while the bottom is repelled. However, the attraction is stronger because the attracting charges are closer than the repelling ones.
Earthing
Earthing
- If enough charge builds up on something, electrons may be pulled through the air and cause sparks - which can be dangerous.
- To prevent charge building up, objects can be earthed: they can be connected to the ground by a conducting material so that the unwanted charge flows away.
- An aircraft and its tanker must be earthed during refuelling, otherwise charge might build up as the fuel 'rubs' along the pipe. One spark could be enough to ignite the fuel vapour.
Induced Charges
Induced Charges
- Charges that appear’ on an uncharged object because of a charged object nearby are called induced charges.
- In the diagram below, a metal sphere is being charged by induction.
- The sphere ends up with an opposite charge to that on the rod, which never actually touches the sphere.
Detecting Charge
Detecting Charge
- Electrostatic charge can be detected using a leaf electroscope as right.
- If a charged object is placed near the cap, charges are induced in the electroscope.
- Those in the gold leaf and metal plate repel, so the leaf rises.
8.01---The-Gold-Leaf-Electroscope.mp4Unit of Charge
Unit of Charge
- The SI unit of charge is the coulomb (C). It is equal to the charge on about 6 million, million, million electrons, although it is not defined in this way.
- One coulomb is a relatively large quantity of charge, and it is often more convenient to measure charge in microcoulombs:
- 1 microcoulomb (μC) = 1x10-6C (one millionth of a coulomb)
- The charge on a rubbed polythene rod is, typically, only about 0.005 μC.
Using Electrostatic Charge
Using Electrostatic Charge
- In the following examples, the charge comes from an electricity supply rather than from rubbing.
- Electrostatic precipitators are fitted to the chimneys of some power stations and factories. They reduce pollution by removing tiny bits of ash from the waste gases.
- Inside the chamber of a precipitator (see right), the ash is charged by wires, and then attracted to the metal plates by an opposite charge. When shaken from the plates, the ash collects in the tray at the bottom.
- Photocopiers work using the principle shown in the diagrams below.
- Laser printers use the same idea except that, at stage 2, a computer- controlled laser scans the plate strip by strip to create the required image.
electric charge questions
electric charge questions
Fill in the worksheet below and hand in on google classroom.
8.02---Electric-Charge.docxPage updated
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