Current Electricity: The Voltaic Cell Battery (electricity) - Wikipedia, the free encyclopedia

Current Electricity

• Cation – positively charged ion• Anion – negatively charged ion

Conventional Current – a 19th century convention, still in use, that treats any electrical current as a flow of positive charge from a region of positive potential to one of negative potential. The real motion, however, in the case of electrons flowing through a conductor, is in the opposite direction, from negative to positive!

**when electrons move, they create positive charges that appear to move in the opposite direction from the positive plate to the negative plate.**

• Current – flow of electrons (or charged particles)

Why does current flow?

• Because of differences in electric potential (voltage). Recall that charges move to lower the energy of the system.

Conventional current Electron Flow

- + - +- + - +- + - +

- + - +

Between plates, the flow of charge wouldeventually cease…How do we keep charges flowing continuously?~ a source of energy to maintain a POTENTIAL

DIFFERENCEExample: generator, chemical reaction (voltaic

cell), solar energy (photovoltaic cell)cell- voltaic or galvanic (dry cells) – converts chemical energy into electrical energysolar cell – photovoltaic – converts solar energy into electrical energybattery – several cells connected together

Where does current flow? Why do we use it?

Electric Circuit – closed loop through which current passes.

consists of:

1. a charge pump or battery which increases the PE of the charges.

2. devices that reduce PE of the charges. (resistors, lamps, wires, etc.)

RATE OF FLOW OF CHARGE

(AND ENERGY TRANSFER)

The energy carried in an electric circuit (current)

depends on the charge transferred (q) and

potential difference (V).

E = q V or W = q V

Current – the rate of flow of charge

I = q / t units: C/s, Amperes, Amps or A

Ampere – a flow of 1 coulomb per second

CONSERVATION OF CHARGE & ENERGY IN A CIRCUIT

1) Charges cannot be created or destroyed, just separated, so, in a circuit, the total amount of charges does not change.

2) Energy is also conserved, E = q V. If q is conserved and E is conserved, the net change in potential difference must be zero.

Resistance and Ohm’s Law

• Resistance – the property that determines how much current will flow.

symbol: R units: Ohms (Ω)

R = V/I V= potential difference, I= current

Ohm(Ω) – a resistance that permits a current of 1 ampere to flow when a potential difference of 1 Volt is applied across the resistor.

Resistor – a device designed to have a specific resistance.

• often used to control current in entire circuits or parts of circuits

• Resistance does not depend on the size or direction of the voltage across it (for most conductors)

Ohm’s Law

• Georg Simon Ohm (1739-1789)

• Most conductors (resistors) obey this law.

The resistor (conductor) has constant

resistance that is independent of the

potential difference (ΔV).

V = I R

Devices that do not obey this law:Transistor radios, Diode, Light Bulbs

Continuous Resistance

• Sometimes it is desirable that current is constant.

• Other times we desire a smooth, continuous variation in resistance (or current)

Examples:light dimmers, electric fans, mixers, etc.

Variable Resistor (Rheostat or Potentiometer)- consists of a coil of resistance wire and a sliding

connecting point. Move the contact point to a different position to achieve a different resistance.

Circuits & Electrical Power

How to control the transfer of energy

(current) in a circuit:

1. VARY VOLTAGE(while R remains constant)

V = I R V = I R

2. VARY RESISTANCE(while V remains constant)

R = V R = V

I I

Using Electrical Energy

Recall: rate of energy transfer I = q/t

Power- product of potential difference and current

P = W/t and W = q V P = qV/t

P = V I• Power is the amount of energy per unit time

converted from another form of energy

When Ohm’s Law applies…

P= V I and V = I R

P = I2 R P =(I R) I

Or P = V2/ R P = V (V/R)

Rewrite the power equation for work or energy:

W = P t and P = I2 R

So W = I2 R t

Losses of Electrical Energy

• When a wire heats up, resistance increases so when wires stretch over great distance, they experience “Joule Heating” (W= I2 R t) or “IR Loss” – loss of energy in the form of thermal energy

• To reduce this effect, I or R have to be reduced. According to P=VI, when P is constant, large V minimizes I

• It is important not to transfer large amounts of current over long wires…instead, very high voltage is used

Resistance In A Wire

R = ρ L = ρ L

A π r2

ρ = ( rho) resistivity

A = cross-sectional area

L = length

The resistance in a wire is directly proportional to

its resistivity and length, and is inversely

proportional to the cross-sectional area

Series Circuits

Series circuit- a circuit in which there is only one path for current to flow.

• All current passes through each resistor

• The current is the same, so in each resistor I=IT

IT

Series Circuits

What happens to the potential energy as current passes through each resistor?

• It decreases• Voltage in a resistor = voltage drop• All of the drops add up to the total voltage

V1 + V2 + V3 = VT

Kirchoff’s Law – conservation of energy:• Algebraic sum of the potential drops and the

applied voltage is zero. 6V + (-3V) + (-3V) = 0

Series Circuits

Individual voltage drops: V = IR

V1 = I1 R1 V2 = I2 R2 V3 = I3 R

TOTAL VOLTAGE:

V1 + V2 + V3 = VT

I1 R1 + I2 R2 + I3 R3 = IT RT

TOTAL RESISTANCE(EQUIVALENT RESISTANCE)

R1 + R2 + R3 = RT

Series Circuits

Current is the same throughout…

IT = VT

RT

IT = I1 = I2 = I3

Parallel Circuits

Parallel circuit - a circuit in which there are several, separate current paths.

• Current flows through each loop but each loop is independent of the other.

Total Current - the sum of each individual path.• Each path draws its own current

IT = I1 + I2 + I3

Parallel Circuits

Potential difference – the voltage supplied still comes from the battery, so V is constant for each loop.

VT = V1 = V2 = V3

V = IR so…VT = I1 R1= I2 R2 = I3 R3

Parallel Circuits

Resistance varies in each loop, so current must also (if V = constant)

I = V/Rand…IT = I1 + I2 + I3

So…VT = V + V + V

RT R1 R2 R3

V factors out…1/RT = 1/R1 + 1/R2 + 1/R3 Equivalent Resistance

Parallel Circuits

Equivalent Resistance (Effective or Total)

In a parallel circuit, the total resistance is

smaller than the smallest resistor!!

Or… more resistors in a parallel circuit

means more current drawn (IT = I1 + I2 + I3)

And…the more current, the less effective resistance.

Parallel Circuits

Kirchoff’s Law – Conservation of Charge

• the algebraic sum of the current entering ant current junction is zero.

• 9A + (-3A) + (-6A) = 0

(see diagram!)