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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: