Cu

rrent E

lectricity

11.1 Electric Current

11.1 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 – timet

QI

11.1 Electric Current

Ampere (for Andre’ Ampere)

Usually called an amp

Open Circuit – break in the

circuit, no current flow

sCA 11

11.1 Electric Current

Short Circuit – when the load is bypassed

Current increase

Ground – allows for a

continuous path for charge

flow

11.1 Electric Current

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

11.3 Resistance and Ohm’s Law

11.3 Resistance and Ohm’s Law

George Simon Ohm

The actual values depend on the resistance of the conductor

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

I V

IR V

11.3 Resistance and Ohm’s Law

Resistor – anything that uses

electric energy

Resistor – device used to control

current

The symbol for a resistor is

11.3 Resistance and Ohm’s Law

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

11.3 Resistance and 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 -.

11.3 Resistance and 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.

11.5 Superconductors

11.5 Superconductors

An element or compound that conducts electricity without resistance

Become insulators above a critical temperature

Uses

MagLev Trains

11.6 Electrical Energy and Power

11.6 Electrical Energy and 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

Vt

q

t

qV

t

WP

IVP

RIP 2R

VP

2

11.6 Electrical Energy and Power

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

Just a cool picture

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

11.6 Electrical Energy and Power

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)

WVAIVP 2400)120)(20(

WVAIVP 1800)120)(15(

11.6 Electrical Energy and Power

Circuit Breakers and Fuses

Break the circuit

11.7 Sources of EMF

11.7 Sources of EMF

EMF – electromotive force – the potential difference between the terminals of a source when no current flows to an external circuit ()

11.7 Sources of EMF

A battery will have an internal resistance (r)

So there is a potential drop due to the current that travels through the cell

So the actual potential across the terminals of a cell will be

This is called the terminal

voltage

IrVc

IrEV

11.8 Resistors in Series

11.7 Sources of EMF

When resistors are place in a single pathway

They are said to be in

series

A schematic would look

like this

11.7 Sources of EMF

The current in a series circuit is the same throughout the circuit

The potential across the source of EMF is equal to the sum of the potential drops across the resistors

nT IIII ....21

nT VVVV ....21

11.7 Sources of EMF

Since potential can be defined as

We can rewrite the equation for potential as

IRV

nT VVVV ....21 nneqT RIRIRIRI ....2211 neq RRRR ....21

11.9 Resistors in Parallel

11.9 Resistors in Parallel

When resistors are place

in a multiple pathways

They are said to be in parallel

A schematic would look like this

11.9 Resistors in Parallel

The potential difference in a parallel circuit is the same throughout the circuit

The current through the source of EMF is equal to the sum of the current through the resistors

nT VVVV ....21

nT IIII ....21

11.9 Resistors in Parallel

Since current can be defined as

We can rewrite the equation for potential as R

VI

nT IIII ....21 n

n

eq

T

R

V

R

V

R

V

R

V....

2

2

1

1 neq RRRR

1....

111

21

11.9 Resistors in Parallel

Circuits that contain both series and parallel components need to be solved in pieces

This circuit contains20 resistors in series25 resistors and load series to each

other and parallel to the 40 resistor