How do objects become “charged”?

(3) Main methods of charging an object

Method 1: Charging by Friction

- When (2) neutral materials are rubbed together, they may become charged.

“Rubbing removes more loosely held electrons from one material and deposits them on the other material.”

- Electrons are transferred. One material becomes negative, the other positive.

Accepts electrons

Loses electrons

** Materials higher on the electrostatic series become more positive (lose electrons).

Ex.: Rubbing a balloon on Human Hair

*** Hair has a weaker hold on electrons so they are transferred to the balloon.

So what’s going on?

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Rub (Friction)

Initial Net Charge = Neutral

Initial Net Charge = Neutral

Resulting Net Charge = Negative

Resulting Net Charge = Positive

Method 2: Charging by Contact

- One material is already charged. The other material is either neutral or charged.

“Electrons are transferred from a negatively charged object to either a neutral or positively charged object.”

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Neutral

Negative

Ex.: Dragging your feet on a carpet builds up static electric charge.

When you touch a door knob, the charge (electrons) jump to the door knob all at once (static discharge).

Charging By Contact

• A neutral object gains the same type of charge as the object that touched it because the electrons move from one object to the other

Induction (definition)

• The movement of electrons within a substance caused by a nearby charged object, without direct contact between the substance and the object.
• Example:

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Charging by Induction

• A charged object is used to induce a charge in a neutral object and then the neutral object is grounded so that it retains the charge.
• This newly charged object has the opposite charge to the charge on the charging object.

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Charging by Induction

• Grounding is the process of connecting a charged object to Earth’s surface.
• When you connect a charged object to the ground, you provide a path for charges to travel to or from the ground.
• The ground is always considered neutral

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Method 3: Charging by Induction

- A neutral object can behave like it has a charge when a charged object comes near it.

- These induced charges are temporary.

- Contact is not needed.

1. Negative rod brought near neutral object.

2. Electrons in rod repel electrons to opposite end

of object, causing one end to become positive.

3. If object is grounded (connected to ground),

electrons from negative end of object travel

into the ground.

4. Now object has more protons than electrons

so it becomes negative.

The Electroscope

An instrument used to detect static charge by either induction or contact.

• Has a metal sphere, rod and leaves to conduct charge
• If there is no charge on the electroscope, the leaves stay together
• If there is a charge on the electroscope, the metal leaves move away from each other because they have the same charge

The Electroscope

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Neutral

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Repel

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Positive Rod

Attracts negative charges

And positive charges remain

leaves.

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Repel

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Negative Rod

Repels negative charges

down to leaves and positive charges remain at top.

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Can be done by induction, or contact!

Questions

• 1. In the contact method of charging, what charge does a neutral substance gain compared to the object that touched it?
• 2. In induction, what charge does a neutral substance gain compared to the object brought near it?
• 3. What is the difference between charging by contact and charging by induction in terms of electron transfer?
• 4. What is grounding?

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