Electricity

Mr. Ahearn2009

Electricity

• Protons and Electrons have positive and negative charge

• Atom becomes + if it loses electrons, and – if it gains electrons

Electrons Move in Solids

e- can move from atom to atom by rubbing– Ex. Rubbing a balloon on hair

» Hair holds e- more loosely, so they go to the balloon, hair becomes +, balloon becomes –

» Causes static charge- imbalance of electric charge

Ions Move in Solution

• Movement of charge can be caused by movement of ions

• NaCl dissolved in water Na+ and Cl- move freely

• Play important role in human body (Nerve cell)

Electric Force• Attractive or repulsive force that all charged

objects exert on each other• “opposites attract and like repel” • depends on distance and amount of charge• Electric Fields- surrounds every electric

charge and exerts forces on other electric charges

Insulators and Conductors

• Insulator- e- can’t move freely• Ex. Plastic, wood, glass, rubber

• Conductor- e- can move easily• Ex. Copper, gold, aluminum• Metals are best because e- are loosely

attached

Induced Charge

• Rubbing feet across floor collects e- from carpet, spread over body as you go to touch metal (door knob), e- in your hand move to the metal, you feel an electric discharge

Induced Charge

Grounding

• Electrical discharge can cause damage and injury.

i. Lightening ii. Even small amt. can damage circuits• One way to avoid this is to direct that current

into the ground called grounding. Examples include: lightening rods, 3rd prong on a plug

Question?

1. Why when objects become charged it is electrons that are transferred from one object to another rather than protons?

2. Why are metals good conductors?

3. Why does an electric discharge occur?

http://virlab.virginia.edu/VL/VDG.htm

Virtual Lab

Electric Current

Vocabulary:Electric current VoltageCircuit Resistance

Flow of Charge

• Electric Current- flow of electric charge– Liquids ions– Solids electronsMeasured in amperes (A)

Model for Simple Circuit

• Flowing water is good model for electric current.

• Circuit- closed conducting loop that electric charge flows.

Electric Circuits• simplest electric circuit:

source of electrical energy (battery), and an electric conductor (wire), connected to the battery.

• As long as there is a closed path for electrons to follow, electrons can flow in a circuit.

Voltage

• voltage- of a battery is a measure of how much electrical potential energy each electron can gain.

• Measured in volts (V)• voltage increases, more electrical potential

energy is available to be transformed into other forms of energy.

How a Current Flows• When the ends of a wire are connected to

a battery, the battery produces an electric field in the wire.

• Electric field forces electrons to move (+) battery terminal.

• electron collides with other electric charges and continues (+) terminal

Batteries• battery supplies

energy to an electric circuit.

• alkaline battery two terminals are separated by a moist paste.

Why do batteries die?

Battery Life

• Battery contains limited amt. of chemical.• Reaction change chemicals into new

compounds.• When chemicals used up battery is dead

Resistance

• measure of how difficult it is for electrons to flow through a material.

• unit of resistance is the ohm (Ω).• Insulators generally have much higher

resistance than conductors

Buildings Use Copper Wires• Copper has low resistance and is one of

the best electric conductors.• Less heat is produced as electric current

flows in copper wires, compared other materials.

Resistance of Wires

• Depends on length, thickness, material.• Increases – longer, thinner

Resistance at work

Light bulb Filaments

• filament is made of wire so narrow that it has a high resistance.

• electric current flows in the filament, it becomes hot enough to emit light.

• made of tungsten metal (high melting pt)

Question1. Compare and contrast and electric discharge

with electric current?2. Describe how a battery causes electrons to

move in a circuit?3. Describe how the electric resistance of a wire

changes as the wire becomes longer? And thicker?

4. Explain why the electric wires in a house are made of copper?

5. In an electric circuit, where do the electrons come from that flow in the circuit?

Answers1. Both involve movement of e-. Discharge is rapid

and discontinuous. Current is continuous.2. A battery has a positive terminal that attracts e-

and a negative terminal that repels e-. These forces cause e- to flow.

3. Longer wire more resistance. Thicker wire less reistance.

4. Copper is a good conductor and has low resistance. Less heat produced when current flows in copper wire.

5. Electrons are in the atoms of the elements that make up the wire.

Electric Circuits

VocabularyOhm’s Law Series CircuitParallel Circuit Electric Power

Controlling the Current

Amount of current depends on:– voltage supplied by the battery – resistance of the conductor

Bucket height is like voltage higher up = more volts

Hose is like resistance longer and thinner = more resistance

Ohm’s Law

Voltage (v)= current (amps) x resistance (omh’s)

If voltage in a circuit increases current increases.

voltage in the circuit doesn’t change the current in the circuit decreases when the resistance is increased.

Ohm’s Law example

A lightbulb is plugged into a wall outlet. If the lightbulb has a resistance of 220Ω and the current is 0.5 A, what is the voltage of the outlet?

Current: I =0.5 AResistance: R= 220Ω

V = IR = (0.5 A) (220Ω) = 110V

Practice1. An electric iron plugged into a wall has a resistance of

24Ω. If the current in the iron is 5.0 A, what is the voltage provided by the wall socket?

2. What is the current in a flashlight bulb with a resistance of 30 Ω if the voltage of the batteries is 3.0 V?

3. What is the resistance of a bulb connected to a 110 V wall outlet if the current in the bulb is 1.0 A?

120 V

0.1 A

110 Ω

Series Circuits

• Series Circuit- has only one path for electric current to follow.

• electrical devices are connected along the same current path.

Series Circuit

Each new device that is added to the circuit decreases the current because each device has electrical resistance. The total resistance to the flow of electrons increases as each additional device is added to the circuit. By Ohm’s law, if the voltage doesn’t change, the current decreases as the resistance increases.

Parallel Circuit• A circuit that has more than 1

path for electric current to flow.

• If one path is broken, electrons continue to flow through the other paths.

• resistance in each branch can be different, depending on the devices in the branch.

Protecting Electric Circuits

• In a parallel circuit, current increases as more devices are added wire heats up.

• To keep the wire from becoming hot enough to cause a fire, the circuits in houses and other buildings have fuses or circuit breakers.

Protecting Electric Circuits

Electric Power

Rate at which electrical energy is converted into other forms of energy

• SI unit of power is the watt.

Power (watts) = Current (amps) x Voltage (volts)

Practice

A lightbulb is plugged into a 110 V wall outlet. How much electric power does the lightbulb use if the current in the bulb is 0.55 A?

Voltage: V = 110 VCurrent: I = 0.55 A

P = IV = (0.55 A) (110 V) = 60 W

Practice1. The batteries in a portable CD player provide 6.0 V. If

the current in the CD player is 0.5 A, how much power does the CD player use?

2. What is the current in a toaster uses 1,100 W of power when plugged into a 110 V wall outlet?

3. An electric clothes dryer uses 4,400 W of electric power. If the current in the dryer is 20.0 A, what is the voltage?

3.0 W

10 A

220 V

Cost of Electric Power

• sell it in units of kilowatt-hours, amount of electrical energy equal to using 1 kW of power continuously for 1 h.

Electrical Safety

• 1997- electric shocks killed an estimated 490 people (USA)

Electrical Safety• You experience an electric shock when an

electric current enters your body• In some ways your body is like a piece of

insulated wire.• The fluids inside your body are good conductors

of current. • The electrical resistance of dry skin is much

higher.• Skin insulates the body like the plastic insulation

around a copper wire.

Lightening Safety

• On average, more people are killed every year by lightning in the United States than by hurricanes or tornadoes.

• If you are outside and can see lightning or hear thunder, take shelter indoors immediately.

• If you cannot go indoors, you should take these precautions:– Avoid high places and open fields– Stay away from tall objects such as trees, flag poles,

or light towers.

Lightening Safety

• Avoid object that conduct current such as bodies of water, metal fences, picnic shelters, and bleachers

Study for your upcoming test!!