Chapter 20

Electricity

Section 20-1

• Electric Charge and Static Electricity:– An electric charge exerts a force through the

electric field that surrounds the charge. An electric field extends outward from every charged particle, if the charged particles are placed in the electric field of another charged particle.

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–If they are the same, the charges push away.

– ( + ) ( + )

( - ) ( - )

If they are different, the charges pull towards each other:

(+) (-) or (-) (+)

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• Electrons can sometimes leave their atoms. When an object has gained electrons the object is negatively charged. When an object has lost electrons the object is positively charged.

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• Static electricity is the build-up of charges on an object. Charges build up, but do not flow. For charges to build-up, charges must be transferred from one object to another by one of three ways:

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1. Friction: electrons are transferred from one object to another by rubbing:

• Ex: a balloon against your hair.

• Ex: comb rubbed on the carpet.

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2. Conduction: electrons are transferred from one object to another by direct contact.

3. Induction: Charges are moved to one part of an object as a result of electric fields to another object.

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• Charges are not created or destroyed, only transferred. If an object gives up electrons, another object gains those electrons. This is the law of: Conservation of charge.

• When a negatively charged object and a positively charged object are brought together, electrons move until both objects have the same charge.

• The loss of static electricity as charges move from an object is called the static discharge “shock”

20-2 Circuit Measurements

• Current depends on the resistance of the material through which it travels.– The greater the resistance, the less current there

is for a given voltage.• Thickness of wire

• Length of wire

• What the wire is made of: copper, etc.

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• The potential energy per unit of electric charge is called electrical potential.

• The difference in electrical potential between two places is called the potential difference.

• Voltage is the unit of measurement of potential difference.

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• Voltage is measured with a device called a voltmeter. Current is measured with a device called an ammeter.

• Ohm’s law states that the resistance is equal to the voltage divided by the current:– Resistance = V/C or Ohm’s = Volts/Amps

20-2Insert Diagram

• When the battery is charged, and we connect the wires, voltage causes current to flow in the circuit.

• Adding a second batter would increase the voltage, therefore increasing brightness.

• If you added a 2nd light-bulb, more resistance would occur. Less current flows.

20-2

• R = Resistance

• I = Current

• V = Voltage

I = Voltage/Resistance

V = Current multiplied by Resistance

Electrical Safety

• If a person touches a downed power line, electricity can flow through the person in a short circuit. (a connection that allows current to take an unintended path).

• Insulation on wires protect you from getting shocked.

• Rubber, thick-soled shoes increases the resistance between you and the wire – reduces the current going through your body.

• A wire that carries more current than it is designed to carry will become hot – leading to a fire.– In order to prevent circuits from overheating, devices

called fuses and circuit breakers are added to circuits.

– A fuse contains a thin strip of metal that will melt if too much current flows through it. When the strip of metal melts, or blows, the circuit stops.

• Lightening rod is a metal rod mounted on the roof of a building in order to protect the building.

• A lightening rod is connected to a grounding wire. When lightening strikes the rod, charges flow through the rod, into the wire, and then into Earth – this protects the building.

• When a fuse burns out, it cannot be used again. To avoid this problem, we use circuit breakers, safety devices that use an electromagnet to shut of the circuit when the current gets too high.

The human body depends on electrical signals. For example, tiny electrical pulses control the beating of the heart. If your body receives an electric current from a source outside the body, the current interferes with the normal process with your body.

Current greater than 0.2 amp cause burns and stop the heart.

Chapter 20 Test

1. How do electric charges interact?2. How does static electricity differ from

electric current?3. What causes electric current to flow?4. How does increasing voltage affect

current?5. How does increasing resistance affect

current?