Section 2.1Controlling the Flow of Electrical

Current

Insulators: • Electrons are tightly bound to the nucleus

allowing little movement. Eg. woodConductors:• Electrons are free to move to the positive

terminal when a voltage source is applied.eg. metal

• The more charged particles (or ions) present in a solution, the better it conducts.

• Pure water will not conduct electricity. Tap water will.

Neon Signs

• Neon signs are a circuit where electrons must flow through a gas.

• Usually the gas would be an insulator, but when current is applied the electrons are excited and free themselves from the atoms.

• This creates a mixture of charged particles which conducts electricity.

• Different gases emit different colors of light.

Semiconductors

• At high temps they act like conductors.• At low temps they act like insulators.• Examples: Germanium and Silicon

Superconductors• Perfect conductors (no resistance)• Some metals act as superconductors at

extremely low temperatures.

Video: How Superconducting Levitation Works

Resistance: A measure of how difficult it is for electrons to flow. Measured in ohms (Ώ)

• High resistance creates heat or light.

• Resistors: allow current to pass through but provides resistance, limiting the current.

• Read P. 301. How does a polygraph work?Put your answer into your notebook.

Switches and Variable ResistorsA switch is a device that allows the flow of electrons or stops the flow. When the switch is open, there is no flow, because there is a gap in the conductor. When the switch is closed, the switch becomes the 'gap replacement' and allows the flow of electrons to continue. To change the electron flow gradually, a variable resistor, or rheostat is used (a dimmer switch, volume control knob).

Dimmer Switch

• Do p299 Act D-3 Investigating Conductivity

• Do 2.1 Check and Reflect P. 303 # 1-3,5-9

2.2. Modeling and Measuring Electricity

Modeling VoltageA waterfall is used, as a model, to demonstrate voltage. Water flows when there is a change in the gravitational potential energy (elevation). Electricity will not flow unless there is a change in electrical potential (voltage).

Modeling Resistance and CurrentFlow of water in pipes is used, as a model, to demonstrate resistance. The size of pipe determines the volume of water allowed through it. The amount of resistance, in a circuit, determines the size of the current.

Ohm's Law

Georg Simon Ohm, a mathematician, proved a link between voltage (V), current (I) and resistance (R). The unit of resistance was named after him, the ohm.

Ohm's Law states that as long as temperature stays the same:

• the resistance of a conductor stays constant, and

• the current is directly proportional to the voltage applied

Ohm's Law

V = I x R, I = V/R, R = V/I

V = voltage in volts (V) I = current in amperes (A) R = resistance in ohms (Ω)

Do 1-3 p. 307

If the temperature of a resistor changes, the resistance changes as well (resistance is usually low when the resistor is cool, and as the temperature increases, so does resistance).

Types of Resistors

Different resistors are used for different applications, especially in electronics. There are many styles, sizes and shapes. The two most common are the wire-wound and carbon-composition types.

• Wire wound: The longer and thinner the wire the higher the resistance.

• Carbon-composite: By varying the composition and diameter different resistances are available.

Using Test MetersVoltmeters measure voltage difference (voltage drop).

Ammeters measure current (rate of flow) in amperes.

Small currents are measured using galvanometers.Multimeters can measure voltage, current and resistance in a circuit.

2.3 Analyzing and Building Electrical Circuits

• Engineers and designers of electrical circuits use symbols to identify components and connections. A drawing made with these symbols is called a schematic or schematic diagram.

Parts of a Circuit

1. A source provides energy and a supply of electrons for the circuit.

2. A conductor provides a path for current.

3. A switching mechanism controls current flow, turning it on and off, or directing it into different parts of the circuit.

4. A load converts electrical energy into another form of energy.

Circuits

• A circuit which is open is said to be off

• A circuit which is closed is said to be on (the load is working) because the current is able to flow freely

• Open (off)

• closed (On)

Circuit Analysis Example - BulldozerThe toy bulldozer has 2 loads, a motor and a bulb. 2 1.5V cells act as the energy source. A switching mechanism connects to 4 wires. Label these on the diagrams.

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What are "series" and "parallel" circuits?

• There are two basic ways in which to connect more than two circuit components: series and parallel.

*Check it out yourself at: http://www.autoshop101.com/trainmodules/elec_circuits/circ101.html

• In a series circuit there is only one pathway for the current to flow. If there is a break in a circuit the whole circuit cannot function. Adding components to a series circuit increases the total resistance in the circuit and decreases the current.eg. lights get dimmer as you add more.

• In parallel circuit there are multiple pathways for the current to flow. Components in the circuit can have their own pathway to the current source.

• A break in one section of the circuit will not affect components in another section of a circuit. Adding resistors in parallel decreases the total resistance of the circuit.

Applications of Series and Parallel Circuits

House Wiring - uses parallel circuitsMicrocircuits are made up of transistors and resistors

built on an extremely small scale. Microcircuits (Integrated Circuits) - transistors are used

with three layers of specially treated silicon, with the middle layer receiving a small voltage, allowing it to control the voltage in the outer layers

This allows the transistors to act as switches. • `