# More on Electric Charges Detecting Electric Charge Charging

• Slides: 25

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

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 ___________.

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

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.

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 a) attraction b) repulsion c) nothing d) explosion III] negatively charged a) attraction b) repulsion c) nothing d) explosion

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.

Detecting Electric Charge For example, when a _______ charged rod is brought near a neutral metalleaf 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 ______.

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.

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:

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

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)

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.

Induced Charge Separation A charged object will attract some neutral objects. n When a _______________ object is brought near a ________object, it causes (______) the electrons to shift in position – called an induced charge separation. n 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. Charge: _____ • 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 Charge: _________ the ball. n

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.

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

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

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

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.

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) v charged object induces a charge separation in the neutral object v the neutral object is grounded (i. e. electrons move to/from ground) v object now has the opposite charge to the charged object used

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.

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.

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.

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.

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.

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.