Unit 3 Day 1: Voltage, Current, Resistance & Ohm’s Law

• Batteries

• Electric Current

• Conventional Current

• Resistance

• Resistors

• Energy Dissipated in a Resistor

Batteries

• Discovered by Luigi Galvani

(1780) And improved upon by

Alessandro Volta

• A battery produces electricity by transforming chemical energy into electrical energy, using various metal electrodes immersed in an electrolytic solution

Batteries• Exterior to the battery, the electrodes are connected to

terminals

• 2 or more batteries connected together in series ( + to – terminals) adds the voltage

• Batteries are the source of potential difference ΔV =VA-B & measured in SI units of volts (V)

Electric Current

• Electric current is the flow of electric charge in an electric circuit

• An electric circuit consists of a source of ΔV (battery), connecting wires, and a load (lamp)

Electric Current

• When a circuit forms a closed loop, conventional current flows out of the (+) terminal of the battery, into the load, and back into the (-) terminal of the battery

Electric Current

• Where ΔQ is the amount of charge that passes through a conductor at any location, during some interval of time.

• SI: Amperes (A) 1A = 1 Coulomb / sec

• Charge carriers are the electrons which flow out of the (-) terminal of the battery and into the (+) terminal

t

QI

Electric Current

• Conventional current flows in the opposite direction of electron flow !

Electric Current

+ E-Field -

-

• Current flows because the potential difference produced by the battery sets up an electric field parallel to the wires. Free electrons in the wire are attracted to the (+) end of the electric field

• There will be a continuous flow of electrons as long as ΔV is available

Resistance• In 1850 Georg Simon Ohm determined that in a

circuit, .

• The current is the circuit is limited or impeded by the amount of voltage applied to the circuit.

• This impediment to current flow is due to electrons colliding with metal atoms in the wire. This impediment to current flow is called resistance.

VI

Resistance

where R is the resistance, SI: Ohms (Ω)

1 Ω = 1 volt / ampere

• This is called Ohm’s Law

RIVorR

VIor

I

VR

Resistors• All electronic devices which represent a load in

an electronic circuit can be modeled as a resistor

• A resistor is a device which contains a defined amount of resistance and is used to control current in an electronic circuit

Resistors

Load Resistances

I

+

+

VL

-

• In an electronic circuit, current flows into the load resistor establishing a potential difference across it in the polarity shown

Energy Dissipated into a Load Resistor

+ - E-Field

I e

A B

• The energy dissipated into a load resistor is the energy delivered by the battery per unit time:

VQVqUU

VqVVqUUU

ie

BABAe

VQU