Electricity

M. Rocha

Physics 2B

Electrostatics

Electricity at rest

Electrical Forces

There are two types of electrical charge:

Positive and Negative

Like charges repel, opposite charges attract

Origin of Charge

Charge comes from Electrons and Protons

Proton charge = e

Electron charge = -e

e is called the elementary charge

Chekpoint

What is the total charge of an object with 6 electrons and 5 protons?

Total charge = -6 e + 5 e = -1 e

Charge is quantized

The elementary charge e is the smallest unit of charge

No smaller units of charge have ever been observed, all charged objects have a charge that is a whole-number multiple of e, the smallest quantum of charge

quarks have never been observed alone, they are always in a combination so that their charge sums to e, -e or zero

Charging by Friction/Contact

Electrons move easily, so an object can become charged by rubbing electrons off an object’s surface

Chekpoint

When you rub a balloon against your hair the balloon gets electrically charged, this is because

Electrons are released from the fur and stick on the balloon

Human Electroscope

Van der Graaf Demo Video

Conservation of Charge

Charge cannot be created nor destroyed.

Objects become charged by transfer of charges

Conductors and Insulators

Conductors: Materials in which electrons flow freely, such as metals

Insulators: Materials in which electrons do not move easily, such as plastic and wood

Semiconductors: Materials that can act as conductors or insulators under different conditions, such as silicon

Lightning

Air is an insulator but at high voltages it can conduct electricity as lightning

Music Tesla Coil

Lightning

Air is an insulator but at high voltages it can conduct electricity as lightning

Coulomb's Law

Electric force acts at a distance just like Gravity

And also like Gravity the magnitude of the Electric Force is proportional to the product of charge and inversely proportional to the square of the distance

Coulomb's Law

Electric force acts at a distance just like Gravity

And also like Gravity the magnitude of the Electric Force is proportional to the product of charge and inversely proportional to the square of the distance

F_\mathrm{Electric} = k \frac{q_1 q_2}{d^2}

Coulomb's Law

F_\mathrm{Electric} = k \frac{q_1 q_2}{d^2}

The SI unit of charge is the coulomb (C)

A charge of 1 C is the charge of 6.24 × 10^18 e

e = 1.6 × 10^-19 C

k = 9.0 × 10^9 N·m^2/C^2

Chekpoint

What is the force between the following two charged particles if they are separated by 3 meter?

Particle 1 with q_1 = 1 x 10^-5 C , and

Particle 2 with q_2 = -1 x 10^-4 C

-1 N

F_\mathrm{Electric} = k \frac{q_1 q_2}{d^2}

k = 9.0 × 10^9 N·m^2/C^2

Electric Potential Energy

Potential energy associated with electric force, similar to potential energy associated with gravitational force

Electric Potential

Electric potential is potential energy per charge

Voltage is a measure of electric potential

1 \ \mathrm{volt} = 1 \ \frac{\mathrm{Joule}}{\mathrm{Coulomb}}

Electric Potential Energy = Voltage × Charge

Electric Potential

Electric potential is potential energy per charge

Voltage is a measure of electric potential

1 \ \mathrm{volt} = 1 \ \frac{\mathrm{Joule}}{\mathrm{Coulomb}}

Yes! A charged ballon can generate high electric potential (voltage), but it has small potential energy because of the small amount of charge, thus is safe

Electric Potential Energy = Voltage × Charge

Charging by Induction

If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface

Charging by Induction

If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface

Charge Polarization

Charge polarization occurs when the positions of the electrons and protons in an atom are skewed

Charge Polarization

Charge polarization occurs when the positions of the electrons and protons in an atom are skewed

Electric Field Lines

Electric field lines indicate the direction of electric force if a positive charge was placed in the electric field

Electric Current

Electricity in motion

Electric Current

Just like water flows whenever there is a altitude difference, electricity flows whenever there is a voltage difference

g \Delta h

\Delta V

Electric

Potential Difference

(voltage)

Gravitational Potential Difference

\frac{PE_\mathrm{grav}}{mass}

\frac{PE_\mathrm{electric}}{charge}

Checkpoint 1

Select the correct statements about Fig. 1 vs. Fig. 2

Fig. 1

100 m

12 V

100 m

12 V

Fig. 2

A) The electrical/gravitational potential in 2 is greater than in 1

B) The electrical/gravitational potential is the same in 1 and 2

C) The electrical/gravitational potential energy is greater in 2

Electric Current

Electric current is measured in amperes or amps (A)

An ampere is the flow of
1 coulomb of charge per second

1 coulomb = 6.24 x 10^18 e

= 6.24 billion billion electrons

Voltage Sources

A sustained current requires a suitable pumping device to provide a difference in electric potential (voltage)

Voltage Sources

A sustained current requires a suitable pumping device to provide a difference in electric potential (voltage)

Chemical Batteries

Mechanical Generators

Photovoltaics

Chemical Batteries

Batteries separate positive and negative charges by using a chemical reaction

Chemical potential energy is converted into electrical energy

Chemical Batteries

Batteries separate positive and negative charges by using a chemical reaction

Chemical potential energy is converted into electrical energy

The Lemon Battery

Mechanical Generators

Mechanical kinetic energy is converted into electrical energy

Photovoltaics

Radiation energy is converted into electrical energy

The photoelectric effect

Electrical Resistance

For a given pressure, more water passes through a large pipe than a small one

Similarly, for a given voltage, more electric current passes through a large-diameter wire than a small-diameter one

Less resistance

More resistance

Electrical Resistance

Voltage Source

A simple hydraulic circuit is analogous to an electric circuit

Ohm's Law

\mathrm{Current} = \frac{\mathrm{Voltage}}{\mathrm{Resistance}},

I = \frac{V}{R}

The Current flow is proportional to the Voltage and inversely proportional to the Resistance

1 \ ampere = 1 \ \frac{volt}{ohm}

Units:

A potential difference of 1 volt across a circuit with a resistance of 1 ohm will produce a current of 1 ampere

Ohm (symbol is Ω) is unit of resistance

Checkpoint 2

A typical light bulb has a resistance of about 100 ohms. What is the current flow across a light bulb when connected to an electric socket providing 120 volts?

I = \frac{V}{R} = \frac{120 \ volts}{100 \ ohms} = 1.2 \ amps

I = \frac{V}{R}

Checkpoint 3

If the resistance across your body is 1000 times greater than a typical light bulb, how much current would flow trough your body if you touch an electric socket? (current = 1.2 amps for the bulb)

\frac{I_\mathrm{you}}{I_\mathrm{bulb}} = \frac{V R_\mathrm{bulb}}{V R_\mathrm{you}} = \frac{1}{1000}

I = \frac{V}{R}

\Rightarrow I_\mathrm{you} = \frac{1}{1000} I_\mathrm{bulb} = .0012 \ amps

Electric Shock

Ohmic Heating

Flowing electrons strike atoms in a conductor, heating the material. For a given voltage the higher the resistance the higher the heating

Toaster

Oven

Light bulb

Fuses and Circuit Breakers

A fuse is designed to melt (due to ohmic heating) when current is too large

Circuit breaker does same job without needing replacement; flip the switch to reconnect

Direct vs. Alternating Currents

Direct current (DC) is current that flows in only one direction

Alternating current (AC) is current that flows back and forth with alternating direction

Electric Power

\mathrm{Power} = \mathrm{Voltage} \times \mathrm{Current} = V I

Units:

1 \ ampere \times 1 \ volt = 1 \ watt = 1 \frac{Joule}{s}

\mathrm{Power} = \frac{\mathrm{Energy}}{\mathrm{Time}}

P = V I = \mathrm{(\frac{Energy}{Charge}) (\frac{Charge}{Time})} = \mathrm{\frac{Energy}{Time}}

Checkpoint 4

A kilowatt is 1000 watts, and a kilowatt-hour is the amount of energy consumed in one hour at the rate of 1 kilowatt. If electric energy costs 20 cents per kilowatt-hour, what does it cost to operate a 100 watt light bulb for 10 hours?

100 watts x 10 hours (20 cents/kilowatt-hour)

= 1 kilowatt-hour (20 cents/kilowatt-hour)

= 20 cents

The End

Electric Current

The instrument to measure current is called an Ammeter

Due to charge conservation, same current into and out of light bulb

Capacitors

Capacitors are used to store electric potential energy

They produce a short lived current while discharging