Chapter 21 Electricity

Objectives

• 21.1 Describe the effects of static electricity. • 21.1 Distinguish between conductors and

insulators • 21.1 Recognize the presence of charge in an

electroscope.

Objectives

• 21.2 Explain the occurrence of lightning in terms of induction and static discharge

• 21.2 Evaluate the positive and negative aspects of lightning induced forest fires

Objectives

• 21.3 Describe how static electricity is different from current electricity

• 21.3 Explain how a dry cell is a source of electricity

• 21.3 Conceptually and mathematically relate potential difference, resistance, and current.

Objectives

• 21.4 Sketch a series and a parallel circuit, and list applications of each type of circuit

• 21.4 Recognize the function of circuit breakers and fuses

• 21.5 Explain and calculate electrical power • 21.5 Calculate the amount of electrical energy

in kW/hrs

Terms

• Static Electricity: Net build up of charge– Charge is not created or destroyed, so the loss of

electrons for one is the gain for another• Electric Field: A force that attracts or repulses

charged objects– Objects not charge feel no force

• Insulator: Doesn’t allow electrons to move• Conductor: Allows passage of electrons

Atom (Review)

• Composed of protons, neutrons, and electrons. The Protons and Neutrons are stuck but the electrons are capable of moving

Rearranging electrons

Conductors/Insulators

• Conductors: Allow their electrons to move freely– Most metals

• Insulators: Electrons stuck in place– Plastic, Rubbers

Lightning

Positive charge exerts field away

• Thanks to Benjamin Franklin

Electric Fields

More Examples

• Doubled both charges

Potential Difference

• Also called the Volt– In the gravitational world: How high have you

been lifted from the earth?– In the electric world: How far are you away from

your opposite charge?

Voltage

Voltage

• Same q, differing v Same v, diff q

Voltage

Jupiter (lots of PE) Mercury (little PE)

AED

• Build up a large static charge, then release

Electric Current

• The flow of electrons• Only occurs when there is a potential

difference – One location must be at a lower potential than the

others– This is produced by batteries, photovoltaic cells

(solar panels), generators (spinning coils)• Measured in Amperes

The Ampere

• Abbreviated as Amp usually• 1 Amp = 1 C/second through a cross-sectional

area

Electric Circuit as related to Amps

Energy Changes

• Energy and Charge are conserved

Random Information• Effect of Various Electric Currents on the Body• Current in Amps Effect• 0.001 Can be felt• 0.005 Painful• 0.010 Involuntary Muscle

Contractions• 0.015 Loss of Muscle Control• 0.070 If through heart, serious

disruption, probably fatal if > 1s

Current

– While electrons have random motion, current is the net movement.

– The net movement of the electrons is the drift velocity. The drift velocity is actually very small.

– As the wire becomes thinner, the drift velocity becomes faster.

• Electrons in a current carrying wire move slower than a snail

Resistance

• Resistance of a circuit determines how much current will flow– Different materials have different resistances – Metals are low, Non-Metals are high– Insulators have lots of resistance, conductors next

to none• Resistance determines the amount of current– High resistance = low current because very few

electrons are able to be moved

Ohm’s

• Resistance is measured in Ohm’s• Resistors also cause a drop in voltage. They

resist the push that is being applied by the voltage. – Is resistance a bad thing? Not all the time. When

transporting energy across the country in big power lines = yes. When heating up your curling iron or light bulb = no.

Resistance

• Temperature as well influences the resistance. As temperature goes up, atom “jiggling” increases. Temp goes Up, Resistance goes up– The more the atom’s jiggle, the more they

interfere with the movement of electrons (the electrons crash into them and don’t move as fast).

• At very low temperatures, the atom jiggling is so little that some materials act as superconductors.

Resistance

• Ohm’s Law =

• Resistance is calculated by how much drop in voltage it causes

• Question• 2.0 Amps of current are flowing in a current.

A resistor causes a drop of 10.0 V. How many Ohm’s of resistance does the resistor have?

IVR

Water Pump Analogy

• If we have a water pump that exerts pressure (voltage) to push water around a "circuit" (current) through a restriction (resistance), we can model how the three variables interrelate.

Water Pump Analogy

• If the pressure stays the same and the resistance increases (making it more difficult for the water to flow), then the flow rate must decrease:

Water Pump Analogy

• If the flow rate were to stay the same while the resistance to flow decreased, the required pressure from the pump would necessarily decrease:

Capacitors

• Store up energy. As the electrons flow past a place, the capacitor acts as a holding dock of sorts for the electrons. – You can think of it as a dam, where water flows to

the dam, the dam stores up the water and releases it later.

– These are used for your windshield wipers. When enough energy is stored up, it causes the wiper to go .

Circuit Breakers/Fuses

• Designed to stop fires– Lots of current = lots of electrons bouncing around

= lots of friction = lots of heat• Typical setting is 30 amps– Two hairdryers on the same line usually enough to

trip the circuit

Power = Current times Voltage

• P = IV• Power measured in Watts (J/s)• Electric power is measured by the amount of

current flowing and the voltage difference– More current equals more power– More voltage (bigger potential difference) equals

more power

Kilo-Watt Hour

• The most important thing to take away from this chapter (as far as your every day life is concerned)

• It is what electricity is sold in• 1 kW/Hr = A device that runs on 1 kW and

runs for one hour– Equivalent to 3.6 million Joules of energy

Typical Ranges and Costs

• 60 W light bulb = 0.060 kW run for 1 hour = 0.060 kW/Hr of energy

• 3000 W stove = 3 kW run for 1 hour = 3 kW/hrs of energy

• Cost is approximately $0.05 to $0.20 per kW/hr (10 cents here in SEMO)

Cost

• A computer runs on 500 W. If the computer is run for 5 hours…

• A) How many kW/hr’s will be used?• B) If it costs 10 cents per kW/hr, how much

does it cost the consumer?

Why turn off the lights?

• A certain light fixture has five 60 W light bulbs. If the lights are left on in an unattended room for 8 hours a day for a month, how much does it cost (at 10 Cents per kW/hr) to light the room?

• A computer runs at 400 W. If it is left on at night every day for a year, how much will it cost you to leave it on at night (same cost rate)

Night Light

• Night light runs on 1 W. How much will it cost to run for an entire year (at rate as last)