Electric Current

Chp. 20PreAP Physics

qI

t

SI Units: C/s = Ampere (A)Named after French Mathematician

-- Andre Ampere

ELECTRIC CURRENT 

The electric current (I) is rate at which electric charges (q) pass through a conductor each second.

How do you get charges to flow?

• It needs a push (a force) provided by an Electrical Potential Difference– A.K.A. Voltage Difference

• The charge flows from high potential energy to low potential energy.

• If a voltage difference is joined together with a conductor the charge will always move until the force acting on it is reduced to a minimum or until the voltage becomes the same.

Why can a bird sit on the power line?

• If power lines have 10000 V how much voltage does the bird have when it is on the wire?– 10000V

• Why doesn’t the bird get electrocuted?– No Voltage (potential)

Difference

What if the high voltage wire is near the ground?

• A chicken steps on a high voltage wire with one foot on wire and one foot on the ground. What happens to the Chicken?– Electrocuted

• WHY???– Voltage Difference between the ground and the

wire.

Use Conventional Current

• The direction is the same as the direction in which positive charges would move

• Positive terminal toward the negative terminal

• Historically it was believed that positive charges moved through metal wires.– Electricians still use it today

even though its NOT how charge really flows.

In more recent centuries we have learned that in a wire, electrons are the only charged particles moving in an electrical current.

ELECTROMOTIVE FORCE ()

A source of electromotive force (emf) is a device that converts, chemical, mechanical, or other forms of energy into the electric energy necessary to maintain a continuous flow of electric charge.

The source of the emf in this case is chemical energy.

emf continued• A.K.A. Maximum

Voltage (potential difference)

• Example: Car Battery– Positive terminal has a

max. voltage of 12 V higher then the negative terminal

– So emf = 12 V

2 Types of Current

• Direct Current (DC)– Charges move in an electric circuit in one

direction– Ex. Batteries

• Alternating Current (AC)– Charges change direction several times a

second• In USA 60 Hz

– Ex. Generators at power companies, wall outlets

War of the CurrentsLate 1800’s

• Battle over which current system would be used at the 1893 World’s Fair in Chicago and ultimately the USA

• Became a Brutal Rivalry– Edison went to great lengths to demonstrate that AC current

was dangerous• Electrocuted animals on stage and film• 1st criminal killed by electric chair hooked up to an AC motor

– Tesla insisted that his AC current was safe and more efficient then DC.

Thomas Edison Supporter of

DC current

Nikola Tesla Inventor of AC current

Who Won?• Tesla & Westinghouse won

the bid for the World Fair in 1893– Claimed they could light the

fair for half of what Edison was going to charge

– However, Edison refused to sell them any light bulbs so they had to create their own

• Ultimately AC became the wave of the future– More efficient– Cost effective– Fewer wires– Less bulky– Could be sent great

distances

Resistance• Which offers more resistance for water

to flow? – Narrow or wide pipe?– Long or short pipe?

• Could electric current flow be affected by resistance similar to water flow?– YES – Relationship between current, voltage and

resistance was discovered by German Physicist, George Simon Ohm

OHM’S LAW "For a given resistor at a particular temperature, the current is directly proportional to the applied voltage."

R

VI

R = Resistance (V/A) = ohm (Ω)V = Voltage (V)I = Current (A)

OR V IR

20.1 The voltage between the terminals of an electric heater is 80 V when there is a current of 6 A in the heater. What is the current if the voltage is increased to 120 V?

V1 = 80 V

I = 6 AV2 = 120 V

RV

I

80

6= 13.3 Ω

IV

R

120

133.= 9 A

Four devices are commonly used in the laboratory to study Ohm’s law: the battery, the voltmeter, the ammeter and a resistance or load.

Ammeter measures current through the battery, the filament, and itself. Current must pass through it so must be in the circuit. Low Resistance.

Voltmeter measures voltage drop across a battery, filament or resistor. The current does not travel through a voltmeter, has high resistance.

The following symbols are used in electric circuits:

ELECTRIC POWER AND HEAT LOSS•Change in Energy per unit Time •How fast electrical work is done.•Measured in Watts (W)•The rate at which heat is dissipated in an electric circuit is referred to as the power loss.

P = I2 RP

V

R

2P = V I

W qVP

t t

20.3 A current of 6A flows through a resistance of 300 Ω for 1 hour. a. What is the power loss?

I = 6 AR = 300 Ωt = 1 hour

P = I2R = (6A)2(300) = 10,800 W

b. How much heat is generated in loss?

W = Pt = 10800W (3600s) = 3.89x107 J

WP

tRemember Work =

Energy and heat is a form of energy.