Day 13 Current Electricity

LO: Current electricity involves continuously moving electrons

LO: Definition of “Current” and “Amps”

AGENDA:

Do Now

Notes

HWp 609(1-5) and p 615(1-6)

Read Holt p. 608-611

Do Now

Electrical Devices

Look around you. Chances are that there is an electrical device nearby - a cell phone, a computer, a projector - something that needs electrical energy to operate.

CD- Player

Typical Batteries and the Symbol Used to Represent Them in Electric

Circuits

Electromotive Force (emf)

The energy needed to run a CD player, for instance, comes from batteries.

Within a battery, a chemical reaction occurs that transfers electrons from one terminal (leaving it positively charged) to another terminal (leaving it negatively charged).

Because of the positive and negative charges on the battery terminals, an electric potential difference exists between them. The maximum potential difference is called the electromotive force* (emf) of the battery.

The electric potential difference is also known as the voltage, V.

The SI unit for voltage is the volt, after Alessandro Volta (1745-1827) who invented the electric battery. 1 volt = 1 J/C.

Emf’s or Voltages of Common Batteries

•Car battery = 12 V

•AAA, AA, C, D = 1.5 V

•9-volt battery = 9 V

•Lantern battery = 6 V

Electric Current

The electric current is the amount of charge per unit time that passes through a surface that is perpendicular to the motion of the charges.

The SI unit of electric current is the ampere (A), after the French mathematician André Ampére (1775-1836). 1 A = 1 C/s. Ampere is a large unit for current. In practice milliampere (mA) and microampere (μA) are used.

.t

QI

Direction of Current Flow

Electric current is a flow of electrons. In a circuit, electrons actually flow through the metal wires.

Conventional electric current is defined using the flow of positive charges.

It is customary to use a conventional current I in the opposite direction to the electron flow.

AC and DC

•If the charges move around a circuit in the same direction at all times, the current is said to be direct current (dc), which is the kind produced by batteries. •In contrast, the current is said to be alternating current (ac) when the charges move first one way and then the opposite way, changing direction from moment to moment. Outlets give us ac voltage.

Electrical Resistance

When electric current flows through a metal wire there exists a hindrance to the flow, known as electrical resistance.

This is because as the electrons move through they will collide with the atoms of the conductor.

The SI unit of resistance is the ohm (Ω), after Georg Simon Ohm (1787-1854), a German physicist, who discovered Ohm’s law, which will be discussed in the next section.

A resistor is a material that provides a specified resistance in an electric circuit.

Ohm’s Law

Ohm’s LawGeorg Simon Ohm (1787-1854), a German physicist, discovered Ohm’s law in 1826.

This is an experimental law, valid for both alternating current (ac) and direct current (dc) circuits.

When you pass an electric current (I) through a resistance (R) there will be a potential difference or voltage (V) created across the resistance.

Ohm’s law gives a relationship between the voltage (V), current (I), and resistance (R) as follows:

V = I R

Units

Quantity Symbol Unit Name

Unit Abbreviation

Current I Ampere A

Voltage V Volt V

Resistance R ohm Ω

Flashlight

Resistance,R and Resistivity,ρ

The resistance of a conductor is directly proportional to the length since the current needs to pass through all the atoms in the length.

The resistance is inversely proportional to the cross-sectional area since there is more room for the current to pass through.

The above observations can be combined and the resistance, R of the conductor is written as follows,

.A

LR

Resistivity of Materials

Resistivity is an inherent property of a material, inherent in the same sense that density is an inherent property.

The Heating Element of an Electric Stove

Electrical Energy

•Our daily life depends on electrical energy.

•We use many electrical devices that transform electrical energy into other forms of energy.

• For example, a light bulb transforms electrical energy into light and heat.

•Electrical devices have various power requirements.

Electric Power,P.

time

EnergyP

Since the electrical energy is charge times voltage (QV), the above equation becomes,

.t

QVP

Since the current is charge flow per unit time (Q/t), the above equation becomes,

.VIVt

Q

t

QVP

Since V = IR, the above equation can also be written as,

.2

2

R

VRIIVP SI Unit of Power: watt(W)

Killowatt-hour (kWh)

The SI unit of power is watt, after James Watt (1736-1819), who developed steam engines.

Utility companies use the unit kilowatt-hour to measure the electrical energy used by customers. One kilowatt-hour, kWh is the energy consumed for one hour at a power rate of 1 kW.

.sec s

J

ond

jouleWwatt

20.5 Alternating Current

V = V 0 sin 2 f t

Alternating Voltage from the outlet

Effective voltage ≈ 115 V, called the RMS value.

Electrons in a Circuit

• With no voltage on a wire, free electrons move rapidly, but in random directions.

• The drift speed is much smaller than the average speed between collisions

• When a circuit is completed, the electric field travels with a speed of about 2/3 the speed of light, causing electrons throughout the wire to move almost instananeously.

Electrons in a Circuit

• In a wire, excess charge moves to the outside of the wire.

Electrons in a Circuit

• In a wire, excess charge moves to the outside of the wire.

• Near the positive terminal of a battery, there is a small deficit of electrons on the wire’s surface.

Electrons in a Circuit

• In a wire, excess charge moves to the outside of the wire.

• Near the positive terminal of a battery, there is a small deficit of electrons on the wire’s surface.

• Near the negative terminal, there is a small excess.

Electrons in a Circuit

• This creates an electric field within the wire.

E

Electrons in a Circuit

• This creates an electric field within the wire.

• The electric field pushes electrons throughout the volume of the wire toward the more positive direction

E

Electrons in a Circuit

• This creates an electric field within the wire.

• The electric field pushes electrons throughout the volume of the wire toward the more positive direction

• This creates a current from higher to lower voltage

E

I

Resistance

Resistance in a Circuit

• The faster electrons flow in the wire, the greater is the current.

Resistance in a Circuit

• It takes energy from the battery to push electrons onto the negative end of the wire and to pull electrons from the positive end of the wire.

Resistance in a Circuit

• It takes energy from the battery to push electrons onto the negative end of the wire and to pull electrons from the positive end of the wire.

• Electrons in the wire lose this energy in colliding with atoms within the wire.

Resistance in a Circuit

• If the voltage across the wire is greater, the electrons move faster.

The more the wire opposes the flow of current, the greater is the resistance of the wire.

Resistance

• Resistance is defined as the ratio of the voltage across a conductor to the current flowing through it.

I

VR

Units of Resistance

• Units of resistance are ohms (Ω)– 1 Ω = 1 V / A

Georg Simon Ohm

• 1787 – 1854• Formulated the

concept of resistance

• Discovered the proportionality between current and voltages

Ohm’s Law

• Often, resistance remains constant over a wide range of applied voltages or currents

• This statement has become known as Ohm’s Law:

V = I R

• If resistance is constant in a material, it is said to be ohmic.

Ohm’s Law

• For an ohmic material, a graph of I vs. V is a straight line.

• The slope of the line is 1/R.

Ohm’s Law

• Non-ohmic materials are those whose resistance changes with voltage

Resistivity

Resistivity

• the resistance of an ohmic conductor is proportional to its length, L, and inversely proportional to its cross-sectional area, A

• ρ is the constant of proportionality and is called the resistivity of the material

LR

A

Temperature Variation of Resistivity

• For most metals, resistivity increases with increasing temperature– With a higher temperature, the metal’s atoms

vibrate with increasing amplitude– The electrons lose more energy in collisions

to the faster-moving atoms

Temperature Variation of Resistivity

• For most materials, resistivity increases approximately linearly with temperature over a limited temperature range, so

– where A and B are constants.BTA