Purpose:

To determine the kind of charge induced on a neutral object when it is approached by a charged object.

Materials:

  • metal-leaf electroscope
  • ebonite rod
  • fur
  • glass rod
  • silk
  • human finger (with body)

Procedure:

  1. The metal-leaf electroscope was approached, but not touched, by a negatively charged ebonite rod. The rod was moved toward and away from the metal ball on the electroscope several times. Observations were recorded using diagrams.
  2. Step 1 was repeated, using a positively charged glass rod.

    Observations were recorded using diagrams.

  3. The metal-leaf electroscope was grounded by a human finger. A negatively charged ebonite rod was brought near the ball on the metal-leaf electroscope.

    While the charged rod was near the metal ball, the finger was removed. The ebonite rod was then taken away. Observations were recorded.

  4. The kind of charge that was induced on the electroscope was predicted and tested. The electroscope was then discharged by touching the ball. Observations were recorded.
  5. Predictions were made about what would happen if step 3 was repeated using a positively charged glass rod.

    The predictions were then tested, and observations were recorded.

Observations:

  1. (refer to procedure 1) When the negatively charged ebonite rod approached the metal-leaf electroscope, the metal leaves moved away from each other.
  2. (refer to procedure 2) When the positively charged glass rod approached the metal-leaf electroscope, the metal leaves moved away from each other.
  3. (refer to procedure 3) While the finger was still on the metal ball, the negatively charged ebonite rod was brought near the same ball on the metal-leaf electroscope. As a result, nothing happened. The finger was then removed, followed by the removal of the ebonite rod. The metal leaves on the electroscope moved away from each other and remained apart until the electroscope was discharged.

  4. (refer to procedure 4) I predicted that a positive charge had been induced on the metal-leaf electroscope when steps 3 and 4 were executed with a negatively charged ebonite rod. To test my prediction, I again brought the ebonite rod near the metal ball on the electroscope. The metal leaves on the electroscope moved a little bit closer together, but did not dangle down freely.
  5. (refer to procedure 5) I predicted that if step 3 was repeated using a positively charged glass rod, the metal leaves would have moved away from each other and remained apart until the electroscope was discharged. I predicted that a negative charge would have been induced on the metal-leaf electroscope. To test my prediction, I again brought the glass rod near the metal ball on the electroscope. The metal leaves on the electroscope moved a little bit closer together, but did not dangle freely.

Analysis:

a) In step 1, I can infer that the metal leaves on the electroscope have the same charge (negative), because they repel each other. In step 2, I can also infer that the metal leaves on the electroscope have the same charge (except positive this time), because they repel each other as well.b) In step 1, a negative charge appears to be induced on the leaves of the metal-leaf electroscope. The electrons in the electroscope are repelled by the negatively charged ebonite rod held near the metal ball, so the electrons travel downwards into the metal leaves, giving the leaves a negative charge. When the ebonite rod was taken away, the leaves dangled down freely. In step 2, a positive charge appears to be induced on the leaves of the electroscope.

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The electrons in the electroscope are attracted to the positively charged glass rod held near the metal ball, so the electrons travel upwards into the ball, leaving the metal leaves positively charged. When the glass rod was taken away, the metal leaves dangled freely.c) It is possible for an uncharged object to appear charged even though no charge has been transferred to it, because electrons can move around. For example, if you bring a negatively charged ebonite rod near an uncharged metal-leaf electroscope, the electrons in the electroscope are repelled into the metal leaves, making the leaves spread apart. This makes them appear charged.d) In step 3, a positive charge was induced on the metal-leaf electroscope. I think my predictions and testing procedures in step 4 are valid, because when the negatively charged ebonite rod approached the assumedly positive electroscope again, the metal leaves moved closer together. The ebonite rod repelled electrons into the leaves, therefore giving the leaves a lesser positive charge.

The leaves moved a little bit closer to each other because the charge was not as strong as it was previously. If the electroscope was negative, the leaves would have moved farther apart due to more electrons being present. This proves that a positive charge was induced on the metal-leaf electroscope.e) In step 3, a positive charge was induced on the metal-leaf electroscope. When the negatively charged ebonite rod was brought close to the metal ball, the electrons in the electroscope were repelled away from the rod. Some of these repelled electrons moved onto the finger that was placed on the metal ball. After the finger was taken away, the electroscope lost some electrons and became positively charged, but the ebonite rod still repelled the remaining electrons into the metal leaves, keeping the metal leaves uncharged. Then the ebonite rod was taken away from the electroscope.

The electrons spread out evenly along the electroscope again, with fewer electrons than protons, making the metal leaves positive. Because the metal leaves on the electroscope became the same charge, they spread apart.f) In step 5, a negative charge was induced on the metal-leaf electroscope.

I think my predictions and testing procedures are valid, because when the positively charged glass rod approached the assumedly negative electroscope again, the metal leaves moved closer together. The glass rod attracted electrons into the metal ball, therefore leaving the leaves with a lesser negative charge. The leaves moved a little bit closer to each other because the charge was not as strong as it was previously. If the electroscope was positive, the leaves would have moved farther apart due to fewer electrons being present. This proves that a negative charge was induced on the metal-leaf electroscope.

g) In step 5, a negative charge was induced on the metal-leaf electroscope. When the positively charged glass rod was brought close to the metal ball, the electrons in the electroscope were attracted to the rod. These attracted electrons moved towards the glass rod. The glass rod also attracted electrons from the finger that was placed on the metal ball. After the finger was taken away, the electroscope gained electrons and became negatively charged. Then the glass rod was taken away from the electroscope. The electrons spread out evenly along the electroscope again, with more electrons than protons, making the metal leaves negative.

Because the metal leaves on the electroscope became the same charge, they spread apart.

Communication:

a) A charged object temporarily induces the opposite charge on the side of a neutral object closest to itself. For example, if a negatively charged ebonite rod was brought close to a metal rod, the electrons in the metal rod would be repelled by the ebonite rod. The side closest to the ebonite rod would become positive.Similarly, if a positively charged glass rod was brought near to the metal rod, the side closest to the glass rod would become negative.

b) The uncharged object always becomes the opposite charge of the charged object. For example, if the charged object is negative, it will repel electrons in the uncharged object. Some of these electrons will travel onto the grounded object. When the charged and grounded objects are removed, the uncharged object is left with more protons than electrons, thereby making the uncharged object positive.If the charged object is positive, then the electrons from the uncharged object would be attracted to the positive object. Electrons will also travel from the grounded object to the uncharged object.

When the charged and grounded objects are removed, the uncharged object is left with more electrons than protons, thereby making the uncharged object negative.