Grade 8 electricity

ElectricityOhm’s Law and Circuits

Electricity

• A form of energy from movement of electrons

• comes from the Greek word “elektron”

Amber

Electricity

e-, p+, n0

• It is associated with electric charge, a property of subatomic particles.

• Charges can be stationary or moving.

Electricity

Unlike charges, attract

Like charges,

repel

Static Electricity• The temporary

building up of charge on an object.

• Some atoms hold electrons more tightly than others.

Static ElectricityCharging by

conduction happens when a charged object touches

another object and electrons are transferred.

Static Electricity

Charging by induction happens when a charged object is held close to another object and that

causes charges to be rearranged.

Static Electricity

Charging by friction happens when objects rub

together and electrons are transferred.

Insulators and Conductors

Some materials allow charges to be transferred (conductor) while others do

not (insulator).

Insulators and Conductors

Conductors : Gold, Silver, Copper, Iron, Lead, Salt Water. Insulators : Plastics, Glass, Dry Air, Wood.

Electric Current

e-

e-

e-

e-

e- e-

e- e-

•flow of charges•The electric charge in a current is carried by minute particles called electrons.

Electric Current

Electric Current

For electricity to flow, you

need a closed continuous

path, called a circuit.

Current•The measure of how many electrons per second

•Also called amperage.

•Measured in amperes.

Current

Current

AC is an electric current that regularly reverses direction.

Current

DC is an electrical current that flows in only one direction and has a fairly constant average value.

Remember: Electric Potential Energy- Two Unlike Charges

Higher Potential Energy

Lower Potential Energy

+

-

In addition, for electrons to flow there must be a potential difference between two places.

Voltage

• the push that causes electrons to flow.• Electrical pressure.• Charges flow from high voltage to low voltage

or from High PE to Low PE.• Measured in Volts (V).

Electrochemical Cell• Electrochemical cells

can cause voltage that makes electrons flow.

• There are two kinds of electrochemical cells---wet cells such as a car battery and dry cells like flashlight batteries.

Electrochemical Cell

The cell stores chemical energy and converts it to electrical energy when a circuit is connected.

Electrochemical Cell

When two or more cells are connected together we call this a battery.

Electrochemical Cell

The cells chemical energy is used up pushing a current round a circuit. Current is DC.

Electrochemical Cell

Alessandro Volta’s (1800’s) cell

Electrochemical Cell

Wet Cell = Car Battery

Electrochemical Cell

Dry Cell = Household “battery”

Electrochemical Cell

Dry Cell = Household “battery”

Electrochemical Cell

What occurs in a wire when the circuit switch is closed?

Resistance• The tendency for a

material to oppose the flow of electrons.

• Different materials have different amounts of resistance to the flow of electrons.

• The unit of resistance is ohm (Ω).

Resistance• Resistance in wires

produces a loss of energy (usually in the form of heat), so materials with no resistance produce no energy loss when currents pass through them.

Which material has higher resistance?

Thin wires provide more resistance than do thick wires

Which material has higher resistance?

Insulators have high resistance.

Which material has higher resistance?

• Resistance also depends on temperature, usually increasing as the temperature increases resistance.

Circuits

To make the circuit, these components are connected together with metal connecting wires.

cell

lamp

switch

wires

Circuits

When the switch is turned on (closed), the lamp lights up. This is because there is a continuous path of metal for the

electric current to flow around.

cell

lamp

switch

wires

Circuits

When the switch is turned on (closed), the lamp lights up. This is because there is a continuous path of metal for the

electric current to flow around.

cell

lamp

switch

wires

Circuits

Scientists usually draw electric circuits using symbols.

Cell lamp switch wire

Circuits

Types of Circuits

Series Parallel

Series Circuits• The components are

connected end-to-end, one after the other.

• They make a simple loop for the current to flow round.

• If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out.

Parallel Circuits• The components are

connected side by side.

• The current has a choice of routes.

• If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.

circuit diagramsIn circuit diagrams components are represented by the following symbols;

cell battery

switch

lamp

motorammeter

voltmeter

buzzer

resistor

variable resistor

Measuring Current (Series)

Current is the same at all

points in the circuit.

I = constant

Measuring Current (Parallel)

Current is shared

between the components.

I = I1 +I2 +I3…

Measuring Voltage (Series)

Voltage is shared

between the components.

V = V1 +V2 +V3…

Measuring Voltage (Parallel)

Voltage is the same at all

points in the circuit.

V = constant

Measuring Resistance (Series)

•Resistance is shared between the components.

R = R1 +R2 +R3…Total resistance will go up because all of the current must go through each resistor.

Example 1

• If the battery’s voltage is 12V and the voltage across R1 is 5 V, and across R2 is 4V, find the voltage across R3 .

• Answer: 3V

Measuring Resistance (Parallel)

•Total Resistance is computed by the following equation.

1/R= 1/R1 +1/R2 + 1/R3

Parallel Circuits

I = I1 + I2 + I3

V =V1=V2=V3

Using Ohm’s Law:V/Req= V/R1 +V/R2 + V/R3

Equivalent Resistance:1/Req= 1/R1 +1/R2 + 1/R3

When are parallel circuits used?

Examp l

e

• Find the Req , I’s.

• How does Req compare with each R?

Ans: 0.55Ω, I= 22A, (12A, 6A, 4A)

=1Ω=3Ω

=2Ω12V

Toll Road—Circuit Analogy

Batteries in Series and Parallel: