9.5
Electrostatic forces are non-contact forces In this topic, you will learn that:
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Van de Graaff generators A Van de Graaff generator works in the same way as rubbing a balloon on hair. In the long shaft of the machine, two long belts rub against each other, making the rounded dome of the machine positively charged. Negative charges are attracted to the dome. If anything comes close enough to the dome, the negative charges are attracted and jump through the air. You might see this as a spark (Figure 2).
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causes the hair to become positively charged and the balloon to become negatively charged. When the (negative charged) balloon moves away, the (positively charged) hair is still attracted to it. The hair lifts up and tries to cling to the balloon (Figure 1).
AF T
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• charged objects attract uncharged objects.
Have you ever rubbed a balloon on your hair and seen the hair cling to the balloon? This is a result of a force called an electrostatic force. When two objects rub against each other, a small electrical charge builds up. One object becomes positively (+) charged and the other becomes negatively (–) charged. These two charges act like the north and south poles of a magnet. The positively charged objects are pulled, or attracted, to the negatively charged objects. The unlike charges attract each other. Because the objects do not need to touch each other to attract, electrostatic forces are a noncontact force. Rubbing the balloon on hair
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• unlike charges attract
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• like (two positive or two negative) charges repel
What causes electrostatic force?
electrostatic force the force between two objects caused by a build-up of negative charges
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• friction causes an electrostatic force
Figure 1 When the negatively charged balloon moves away, the positively charged hair is still attracted to it.
Figure 2 Sparks jump across to a Van de Graaff generator when negative charges come close enough.
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OXFORD UNIVERSITY PRESS OXFORD SCIENCE 7: VICTORIAN CURRICULUM No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means.
