Electric Currents

AP Physics

Chapter 18

Electric Currents

18.1 The Electric Battery

The Electric Battery

Alessandro Volta – produces the

first battery made of zinc,

silver, and cloth soaked

in salt solution

Electric Cell – two plates or rods

(electrodes) of dissimilar

metal placed a solution (like

dilute acid)

Battery – several cells connected

together 18.1

The Electric Battery

The potential difference (voltage) between the terminals of a cell depend on

what the electrodes are

made of.

Remember half-cell reactions

from chemistry

When two cells are placed

end to end, or in series, their

voltages add up

18.1

Electric Currents

18.2 Electric Current

Electric Current

Circuit – continuous conducting path between terminals of a battery (or other source of EMF)

Electric Current – flow of charge (electrons)

I – current (amperes)

Q – charge (coulomb)

T – time 18.2t

QI

Electric Current

Ampere (for Andre’ Ampere)

Usually called an amp

Open Circuit – break in the

circuit, no current flow

18.2

sCA 11

Electric Current

Short Circuit – when the load is bypassed

Current increase

Ground – allows for a

continuous path for charge

flow

18.2

Electric Current

For historical reasons, current is defined as being in the direction that positive charge flows

18.2

Electric Currents

18.3 Ohm’s Law

Ohm’s Law

George Simon Ohm

The actual values depend on the resistance of the conductor

Called Ohm’s LawR – resistance measured in Ohms ()

18.3

I V

IR V

Ohm’s Law

Only true for Ohmic materials

Vacuum Tubes, Transistors, Diodes are

nonohmic

18.3

Ohm’s Law

A graph of current vs. potential difference

The metallic conductor is ohmic

The diode and filament are not

18.3

Ohm’s Law

Resistor – anything that uses

electric energy

Resistor – device used to control

current

The symbol for a resistor is

18.3

Ohm’s Law

The resistance value of a resistor is indicated by the colored bands on the resistor

18.3

Ohm’s Law

Misconceptions

1. Cells (batteries) do not put out a constant current. They maintain a constant potential difference.

2. Current passes through a wire and depends on the resistance of the wire. Voltage is across the ends of the wire.

3. Current is not a vector, it is always parallel to the conductor. The direction is from + to -.

18.3

Ohm’s Law

Misconceptions

4. Current or charge do not increase or decrease. The amount of charge in one end of the wire comes out of the other end.

18.3

Electric Currents

18.4 Resistivity

Resistivity

Resistance is found to be directly proportional to its length and inversely proportional to its cross sectional area.

r is called the resistivity (m)

Longer extension cords must

be thicker to keep

resistance low

A

LR

18.4

Resistivity

Some common resistivity values

18.4

Material Resistivity

(Wm)

Temperature Coefficient

(Co-1)

Silver 1.59x10-8 0.0061

Copper 1.68x10-8 0.0068

Gold 2.44x10-8 0.0034

Aluminum 2.65x10-8 0.00429

Tungsten 5.6x10-8 0.0045

Platinum 10.6x10-8 0.00651

Nichrome 100x10-8 0.0009

Resistivity

Best Conductor is Silver, but Copper is close and much cheaper

Tungsten is used in filaments

Nichrome

Apparently an Anime character

18.4

Electric Currents

18.5 Electric Power

Electric Power

The rate of energy flow for an electric circuit

That is more commonly written as

Combining with Ohm’s Law it can also be written

18.5

Vt

q

t

qV

t

WP

IVP

RIP 2R

VP

2

Electric Power

The power company charges by the kilowatt-hour (kWh)

Just a cool picture

18.5

JsWkWh 000,600,3)3600)(1000(1

Electric Currents

18.6 Power in Household Circuits

Power in Household Circuits

Household circuits – wires will heat up as current increases

In a 20A household circuit

In a 15A household circuit

Circuits are typically designed to run at 80% of the rated power output

Different circuits have different gauge wires (diameter)

18.6

WVAIVP 2400)120)(20(

WVAIVP 1800)120)(15(

Power in Household Circuits

Circuit Breakers and Fuses

Break the circuit

18.6