copyright © 2010 mcgraw-hill ryerson ltd. how can objects become charged and discharged? if you...
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Copyright © 2010 McGraw-Hill Ryerson Ltd.
How can objects become charged and discharged?How can objects become charged and discharged?
If you walk across a carpet wearing wool socks or rubber-soled shoes, charges build up on your body with each step you take.
The charge you have built up on your body can easily be discharged (released) into objects such as a metal doorknob.
Why do you think the spark jumps to the doorknob?
Charging by FrictionCharging by Friction
Rubbing 2 objects together causes one to lose electrons and the other to gain electrons.
Figure (a) shows an ebonite rod and Figure (b) shows a glass rod.
As you can see, after rubbing, the ebonite rod is negative and the glass rod is positive.
Copyright © 2010 McGraw-Hill Ryerson Ltd.
Charging by FrictionCharging by Friction
To determine which object will be positive/negative, we use the:
Triboelectric Series
(see text pg. 398 Table 10.1)
Did you now“tribos” is a Greek word meaning to rub?
Copyright © 2010 McGraw-Hill Ryerson Ltd.
How can objects become charged and discharged?How can objects become charged and discharged?
Lightning, the most spectacular example of static discharge you can observe, looks like a giant spark.
Scientists know that the bottoms of clouds are negatively charged and the tops of clouds are positively charged.
Why do you think the lightning bolts reach Earth?
Electroscopes Electroscopes
Pith Ball – little ball made of styrofoam that is very light
Stand – the stand is usually made of a sturdy material
String – the pith ball hangs from the end of the stand by a thin piece of string
Electroscopes are devices that can test an object’s charge.
Pith-Ball Electroscopes Pith-Ball Electroscopes
Metal Leaf ElectroscopeMetal Leaf Electroscope
Container – prevents breeze in the air from moving the leaves
Knob – metal round to prevent leakage of electrons
Metal Rod – provide the electrons with a path to and from the leaves
Leaves – made of metal so that the electrons can easily move through them
Copyright © 2010 McGraw-Hill Ryerson Ltd.
Charging by contact occurs when you give a neutral object a charge by touching it with a charged object.
The image on the left shows an electroscope being charged by contact. The leaves of the electroscope repel each other when they have the same charge.
Objects can become charged by contact and by induction.Objects can become charged by contact and by induction.
*Note – only excess charges are shown in diagram, go to next slide to see all charges on
rod and electroscope
Charging by contact occurs when you give a neutral object a charge by touching it with a charged object.
Objects can become charged by contact and by induction.Objects can become charged by contact and by induction.
Neutral*Note: It is important to remember that positive charges do not move, only negative charges (electrons) can move in the conductor
Charging by induction occurs when a neutral object becomes charged by a charged object that is brought near to it but does not touch it.
The negatively charged rod repels the negative charges in the ball, and they move to the leaves. This leaves the ball positively charged. No charges are transferred from the rod.
Charging by Induction: Objects Don’t TouchCharging by Induction: Objects Don’t Touch
*Note – only excess charges are shown in diagram, go to next slide to see all charges on
rod and electroscope
Charging by Induction: Objects Don’t TouchCharging by Induction: Objects Don’t Touch
Charging by induction occurs when a neutral object becomes charged by a charged object that is brought near to it but does not touch it.
Copyright © 2010 McGraw-Hill Ryerson Ltd.
The negative bottoms of the clouds induce a positive charge on the ground and objects on the ground.
Charged objects can be discharged by sparking and by grounding.Charged objects can be discharged by sparking and by grounding.
When the attraction between charges on the bottom of the cloud and the charges on the ground are great enough, charges jump between the cloud and the ground, creating a lightning bolt.
Copyright © 2010 McGraw-Hill Ryerson Ltd.
Grounding involves connecting a conductor to Earth’s surface so that charges can flow safely to the ground.
How Grounding Discharges an ObjectHow Grounding Discharges an Object
Why are fuel trucks always grounded before they deliver their gasoline to the gas station fuel tanks?
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How Grounding Discharges an ObjectHow Grounding Discharges an Object
What do lightning rods prevent from happening?
Metal lightning rods connect houses (usually in country (rural) areas) to the ground. When lightning hits the rod, the electric charges are carried through a cable connected to the rod down to the ground.
Why are lightning rods rarely found on houses located in cities (urban areas)?
Copyright © 2010 McGraw-Hill Ryerson Ltd.
Key Concepts to be reviewed:
• Objects can become charged by contact and by induction.
• Charged objects can be discharged by sparking and by grounding.
ReviewReview
Copyright © 2010 McGraw-Hill Ryerson Ltd.
P. 409 #1-5
P. 415 # 6-9
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