Circuits

March 30th, 2009

Electric Circuits

A circuit is simply a closed loop through which charges can continuously move.

The word circuit is from the same base as the word circle, which describes how the electrons move

Requirements of a Circuit There must be an energy supply capable doing

work on charge to move it from a low potential energy to a high potential energy location. This establishes a potential difference (voltage) across the two ends of the external circuit.

There must be a closed conducting loop in the circuit which stretches from the high potential, positive terminal to the low potential, negative terminal.

An analogy to help you…A circuit is like a rollercoaster

It is a complete loop There is a place where energy is put in to

increase potential energy (a battery in a circuit and the chain pulling up the hill in the rollercoaster)

The rollercoaster naturally flows downhill just like charge in a circuit naturally flows from high potential to low potential

The diagram at the right shows a light bulb connected to a 12-V car battery. The + and - terminals are shown.

As a + charge moves through the battery from D to A, it ________ (gains, loses) electric potential.

As a + charge moves through the externalcircuit from A to D, ________ (gains, loses)electric potential.

Use >, <, and = signs to compare the electric potential (V) at the four points of the circuit.

VA ___ VB ____ VC ____ VD

Decide if each statement is true or false:1. When an electrochemical cell no longer works,

it is out of charge and must be recharged before it can be used again.

2. An electrochemical cell can be a source of charge in a circuit. The charge which flows through the circuit originates in the cell.

3. Charge becomes used up as it flows through a circuit. The amount of charge which exits a light bulb is less than the amount which enters the light bulb.

4. The local electrical utility company supplies millions and millions of electrons to our homes everyday.

All statements were FALSE!! When a battery is “dead” it is out of energy

required to push a charge from one terminal to another This energy can be restored in some batteries

with a reversible chemical reaction…so it is more like reenergizing than recharging your batteries

Batteries are the source of energy in a circuit, charges carry the energy

Charge is not used up in a circuit, energy is Electrons are already in your home, the

power company supplies the energy to these electrons

Circuits

A circuit is a set of electrical components connected so they provide one or more complete paths for the movement of charges

A load is any component or group of components that dissipates energy

A simple circuit consists of a source of potential difference (electrical energy) and a load

Drawing Circuits with Schematic Diagrams Graphic representation of an electric circuit Uses standardized symbols representing circuit components

COPY DOWN SYMBOLS FROM OVERHEAD

Light Bulbs

A light bulb contains a complete conducting path

See Overhead (regular and decorative lights)

A light bulb is considered a load because it dissipates electrical energy (by converting it to light energy)

EMFElectromotive Force

The energy per unit charge supplied by a source of electric current

Any device that increases the potential difference and energy of charges circulating in a circuit

Current flow showsthe direction of +charges EMF shows the flowof electrons

Types of Circuits Closed circuit

Path from one terminal to the other is complete. A potential difference exists and electrons move from one terminal to the other.

Open Circuit No complete path. No charge flow and no

current Short Circuits

Without a load the circuit contains little resistance to the movement of charges.

This causes high current which can damage circuit components.

Types of Circuits Series Circuit

A circuit or portion of a circuit that provides a single conducting path without junctions

Parallel Circuit A circuit or portion of a circuit in which two or more

components are connected across a common point or junction, providing multiple conducting paths for the current

Resistors in Series

Resistors in series all have the same current

The total (or equivalent) resistance can by found by finding the sum of all resistors

REQ= R1 + R2 + ….

Practice

A 9.0 V battery is connected in series to four resistors with values of 2.0, 4.0, 5.0, and 7.0. Find the equivalents resistance for the circuit and the current in the circuit.

Hint: Draw a diagram!

Answer: R= 18 and I=0.50A

Resistors in Parallel

Resistors in parallel have the same potential difference (voltage) across them.

The sum of the currents in parallel resistors equals the total current

Equivalent resistance is the sum of the reciprocals

1/REQ= 1/R1 + 1/R2 + ….

Practice

A 9.0 V battery is connected to four resistors in parallel with values of 2.0, 4.0, 5.0, and 7.0. Find the equivalents resistance for the circuit and the current in the circuit.

Hint: Draw a diagram!

Answer: R=0.917 and I =9.8 A