1. More on Electric Charges!
Detecting Electric Charge & Charging by Induction

2. Detecting Electric Charge
How do you know when an object is charged? Rather than testing whether the object sticks to something else, you can use an electroscope. An electroscope is a device that can be used to detect the presence of an _____________________ ________________ .

3. Detecting Electric Charge
ELECTROSCOPES
device used to detect the presence of an electric charge
two types:
metal-leaf pith-ball

4. Detecting Electric Charge
A pith-ball electroscope is a light, metal-coated ball suspended on an insulating thread. If the ball is charged, it will be repelled by a ________________ charged object and attracted to either a _____________ object or an _______________ charged object.

5. Understanding Check – Get out your MC Cards!
PRACTICE
1. Assume that an object is brought near (but not touching) a neutral pith ball. What are the possible outcomes if the object is:
I] positively charged
a) attraction b) repulsion
c) nothing d) explosion
II] negatively charged
III] negatively charged

6. Detecting Electric Charge
A metal-leaf electroscope consists of two thin _____________ __________ suspended from a conductive metal rod (with a metal knob at the top) in a glass container. Any charge spreads out over the entire electroscope. Since the leaves are then charged similarly, they ____________ one another, thus indicating the presence of a charge. The farther apart they move, the _____________ the charges they are carrying.

7. Detecting Electric Charge
For example, when a _____________ charged rod is brought near a neutral metal-leaf electroscope, the electrons in the electroscope are ____________ by the rod. The electrons move down into the leaves of the electroscope. Since the leaves are now both negatively charged, they __________ each other and move ____________.

8. Detecting Electric Charge
When the negatively charged rod is taken away, the negative charges in the electroscope are no longer repelled, so they move back throughout the leaves, stem, and knob. The leaves are then ______________ again. And since the leaves are no longer negatively charged, they drop back down.

9. Detecting Electric Charge
PRACTICE 2. Assume that a positively charged object is brought near (but not touching) a neutral metal-leaf electroscope. (i) explain what will happen, and (ii) with the aid of a diagram(s): show the induced charge separation that occurs on the electroscope. Explanation: Diagram:

10. Check Your Learning – Get Out your MC Cards!
3. What would change about the way an electroscope worked if its metal knob were replaced with a plastic knob? a) It would no longer work – plastic is an insulator so the electrons would not move. b) The metal leaves would move farther apart c) The metal leaves would fall off d) The electroscope would explode

11. Check Your Learning – Get out your MC Cards!
4. Why do the leaves of a positively charged electroscope move farther apart if a rod with the same charge is brought near? (a) electrons in the knob are repelled by the rod and move down to the metal leaves. This would cause the leaves to become negatively charged, thereby separating more. (b) electrons are attracted upwards to the positively charged rod leaving an even greater positive charge on the leaves (causing them to repel even more)

12. Check Your Learning – Get your MC cards!
5. A rod and electroscope have opposite charges. Why would the leaves move closer together if the rod was brought close to the electroscope? (a) electrons in the electroscope knob would be forced down into the leaves reducing the overall positive charge on the leaves (causing them to move closer together) (b) electrons from the rod would jump from the rod to the knob and be forced down into the leaves reducing the overall positive charge on the leaves (causing them to move closer together) (c) protons would move up the electroscope towards a negatively charged rod, thereby neutralizing the metal leaves.

13. Induced Charge Separation
A charged object will attract some neutral objects.
When a ______________________________ object is brought near a _______________object, it causes (____________) the electrons to shift in position – called an induced charge separation.
The movement of electrons occurs according to the Law of Electric Charges.
In this diagram, the rod is _____charged and the pith ball is ____.
The negative charge on the rod ____________ the ____________ in the ball.
The ______________ on the ball will be “pushed” away while the protons stay in place.
This results in a net ____charge on the area of the ball closest to the rod, and the ___charged rod will then _____________ the ball.
Charge: _____
Charge: _____

14. Induced Charge Separation
NOTE! Although there is a shift in the positions of the electrons in the neutral object, it is ________ ___________– it did not gain or lose _____________. And once the charged object is moved away from the neutral object, the electrons ______________ to their original positions.

15. Induced Charge Separation
INDUCED CHARGE SEPARATION (TEMPORARY)
occurs when a charged object is brought near a neutral object
electrons in neutral object shift position while protons stay fixed in place (object remains neutral)
near side becomes oppositely charged and attraction occurs

16. Understanding Check! PRACTICE
6. Complete the following diagrams of A) a rod with a charge of +2 and neutral pith ball; B) the induced charge created by bringing the two objects together.
A B

17. Charging by Induction
NOTE! Objects can be temporarily or permanently charged by induction.

18. Charging by Induction – Temporary
When a charged object is brought near a neutral object (a) it causes (induces) the electrons to shift in position, resulting in an uneven distribution of charges (b).
This will only be temporary as the electrons will move back to their original positions once the charged object is taken away.

19. Charging by Induction – Permanent
When you charge an object permanently by induction, you use a ________________ object to induce a charge in a _________________object and then ____________ the charged object so it retains the charge. This newly charged object has the ______________ charge to the charge on the charging object.
CHARGING BY INDUCTION (PERMANENT)
charged object induces a charge separation in the neutral object
the neutral object is grounded (i.e. electrons move to/from ground)
object now has the opposite charge to the charged object used

20. Charging by Induction – Permanent
Like we’ve seen, when a negatively charged object comes near a neutral electroscope, it repels the electrons in the neutral electroscope downwards. This causes the leaves to repel one another.

21. Charging by Induction – Permanent
When the neutral electroscope is grounded, its electrons are provided with a path away from the repulsive influence. As a result, some electrons leave the electroscope. The leaves now return to their neutral position.

22. Charging by Induction – Permanent
When the ground and charged object are removed, the electroscope is left with a _____________ charge because it has lost electrons. The leaves once again repel each other.

23. Charging by Induction – Permanent
The opposite is true for a positively charged object. When a positively charged object comes near a neutral electroscope, it _____________ the electrons in the neutral electroscope. This causes the leaves to repel one another.

24. Charging by Induction – Permanent
When the neutral electroscope is grounded, a path for electrons to go toward the positive influence is provided. As a result, some electrons ___________ the electroscope. The leaves now return to their neutral position.

25. Charging by Induction – Permanent
When the ground and charged object are removed, the electroscope is left with a negative charge because it has gained electrons. The leaves once again ___________ each other.