Chapter 34

Electric Current

34.1 Flow of Charge

• Potential difference – difference in potential (voltage) across the ends of a conductor

• Conductor – a substance through which electricity can flow

Flow of Charge

• Water analogy: Water flows from high pressure (higher position) to low pressure (lower position) until water pressure equalizes

• To keep water flowing, keep pumping water back to high pressure region (higher level)

34.2 Electric Current

• Electric current – the flow of electric charge

–Measured in AMPERES

–= 1 coulomb per second

Electric Current

• We use the symbol I for current.

• I ~ V• Potential Difference can

cause flow of charge

34.3 Voltage Sources

• Voltage source – an “electrical pump” that can provide a sustained potential difference

• Can be dry cells, wet cells or generators

34.3 Voltage Sources

• Electromotive force – (EMF) the potential difference (voltage) that provides the “electrical pressure” to move electrons between the terminals in a circuit

Voltage Sources

• Voltage is impressed across a circuit, but charge flows through the circuit

• Water Analogy (again): Water flows from high pressure to low

pressure, but only water flows, not the pressure.

34.4 Electrical Resistance

• Electrical resistance (R) – the resistance of a material to the flow of an electric current through it–Measured in OHMS

• Named after Georg Simon Ohm

Electrical Resistance

• Depends on:• conductivity of the wire material

• thickness of the wire

• length of the wire

• I ~ 1/R

34.5 Ohm’s Law

voltage

• Current = resistance

volt

Units: ampere = ohm

34.5 Ohm’s Law

• Another way to say this law:

•V = voltage

•I = current

•R = resistance

V = I R

34.6 Ohm’s Law and Electric Shock

What causes electric shock in the human body?

Current flowing through the body

0.07 A can disrupt heart rhythm and kill

34.6 Ohm’s Law and Electric Shock

What effects current?

voltage and resistancevoltage and resistance

The voltage of the current’s source and the resistance of your skin

34.6 Ohm’s Law and Electric Shock

The resistance of your skin can range from 100 ohms to about 500,000 ohms100 ohms if you’re soaked in salt water500,000 ohms if your skin is very dry

34.6 Ohm’s Law and Electric Shock

Why is it dangerous to work with electrical equipment near water?

34.6 Ohm’s Law and Electric Shock

In order for a shock to occur, there must be a difference in electrical potential between one body part and another.

34.6 Ohm’s Law and Electric Shock

This is why a bird can stand on a live electrical wire and not get electrocuted.

(Notice both feet are touching the wire at ALL times!!)

34.7 Direct Current and Alternating Current

• Direct current – (DC) a flow of charge (electric current) that is always in one direction–EX: battery-operated

34.7 Direct Current and Alternating Current

• Alternating current (AC) – electric current that rapidly reverses direction –EX: electricity in our homes

• Rates:•60 hertz (in North America)•50 hertz (most everywhere else)

DC vs. AC

time

c

urr

ent

DC

time

c

urr

ent

AC

34.8 Converting AC to DC

• Diode – a tiny electronic device that acts as a one-way valve to allow electron flow only in one direction

34.9 Electron Speeds in Circuits

• Electric Fields propagate through a conductor at near the speed of light.

• However, electrons are another matter:

in direct current they drift through the circuit very slowly (3 hours for a meter of wire)

in alternating current there is no net movement at all – back and forth only

34.10 Source of Electrons• Electric power sources provide ENERGY,

not electrons!• Energy is carried by the electric field,

causing the electrons within an appliance to vibrate back and forth (AC) or drift in one direction (DC).

• Your don’t get electric shock from electrons entering your body, but from the electric field making your own electrons move.

34.11 Electric Power• Electric power =

current · voltage

ampere · volt =C/s · J/C =J/s =watt

Chapter 34 Vocabulary

• Alternating current

• Ampere

• Diode

• Direct current

• Electrical resistance

• Electric current

• Electric power

• Electromotive forces

• Ohm

• Ohm’s law

• Potential difference

• Voltage source