DETECTING ELECTRIC CHARGE

KEY TERMS Electroscope Induced charge separation Grounding Induction

Learning Goals To understand how to tell if a metal leaf electroscope is charged To learn what is meant by the term “induced charge separation” To learn how to charge an electroscope by induction

ELECTROSCOPES Electroscopes are devices that detect electric charge We have looked at two types 1. A pith ball electroscope 2. A metal leaf electroscope

SIMILARITIES & DIFFERENCES Similarities: They can both detect when there is an electric charge on an object;  A pith ball will move toward a charged object  The leaves on a metal leaf electroscope will spread apart when a charged object comes near Differences: It is not possible to tell if a pith ball has a charge on it if it is standing alone If a metal leaf electroscope is charged it is easy to see because the leaves spread apart

DETECTING CHARGE Question: If you have a pith ball that is charged, how do you know if it’s positive or negative?

DETECTING CHARGE Answer: Simple! Charge a rod so you know it’s charge. For example, take an ebonite rod and rub it with fur and it will become negative (see page 473). Next, place the rod close to the ball. If the ball is attracted to the rod the ball is positive. If it is repelled, then it’s negative.

HOW DOES A METAL LEAF ELECTROSCOPE WORK? It is made of metal (a conductor) and has two very light metallic leaves at the bottom. Let’s say that a negatively charged object comes close to it Electrons in the electroscope will try to get as far away from the object as possible. So they go down to the bottom (where the leaves are). Now the leaves have extra electrons so they repel each other

INDUCED CHARGE SEPARATION If the object does not touch the electroscope then the electroscope is neutral. However, the top part of the electroscope is positive and the bottom half is negative. This is called Induced Charge Separation If the object actually touches the electroscope then electrons will jump over to it from the object and the electroscope will be charged

GROUNDING Question: How do you discharge a charged object? Answer: Simple. Just touch it! If it has extra electrons (negatively charged) then the electrons will move on to you and then travel through your body into the earth. If the object has too few electrons (positively charged) then electrons will move from your body to the object. This is called grounding

CHARGING BY INDUCTION Question: How can you charge an electroscope using a charged rod that never touches it?

CHARGING BY INDUCTION Answer: Let’s say the rod is positively charged this time Step 1 – Bring the rod close to (but not touching the electroscope). This will bring the electrons from the bottom of the electroscope up to the top (induced charge separation)

CHARGING BY INDUCTION Step 2 – Put your finger on the electroscope. This is called grounding. Since the positive rod is close, electrons will come up from the ground, through your finger and into the electroscope because they want to get close to the rod. Step 3 – Remove your finger. Now the electrons are trapped in the electroscope. Step 4 – Remove the rod

CHARGING BY INDUCTION Question: What charge does the electroscope have? Answer: Electrons came into the electroscope so it has a negative charge. What would happen if you did the same thing with a negative rod? What kind of charge would the electroscope have?

CHARGING BY INDUCTION Summary With a positive rod With a negative. rod

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