PEP Conceptual Physics Class Notes€¦ · •Resistance and Ohm’s Law •Electrical Resistance •Measuring Resistance •Ohm’s Law •Resistance of Common Objects •Conductors

PEP Conceptual PhysicsClass Notes

Unit 5 – ElectricityChapter 12 – Electric Circuits

Section 12.1• Electric Circuits

• Electricity

• Electric Circuits

• Circuit Diagrams

• Electrical Symbols

• Open and Closed Circuits

Electricity• Electricity – charged particles that carry energy

from one place to another

• Electric Current – the flow of electric charge

• Analogy to a water system• Electric current is like water current – it has energy

• Electric wires are like water pipes – they direct and restrict the flow of current

• Resistors are like even more restrictive water pipes

• Switches are like water valves – they shut off flow

• Batteries are like water pumps – they provide potential energy to create currents

• Motors are like water wheels – they turn and do work

Electric Circuits• Electric Circuit – a complete path through which

electricity travels• Electricity is powerful – a small motor can do the work of

several horses and even more people

• Electricity is dangerous – enough electric current flowing through a body can damage nerves or stop the heart

• Electric wires are unlike water pipes if they break• Water continues to flow, but electricity stops immediately

• Natural electric circuits• Human nervous system transmitting signals

• Electric eel stunning its prey

• Lightning strike from cloud to ground

Electric Circuits• Look at the toaster on page 295

• Electricity enters the toaster through the plug that plugs into the wall outlet

• The switch activates by pushing down the lever

• The heater element is a resistor that dissipates the electric power as heat to toast the bread

• Look at Figure 12.2 on page 295 (colors incorrect)• “Hot” (black) goes to the right (narrow) slot

• “Neutral” (white) goes to the left (wide) slot

• “Ground” (green/bare) goes to the round bottom hole

Circuit Diagrams

• Circuit Diagram – a drawing that uses symbols to represent each part of an electrical circuit• Look at the bulb, switch, and battery circuit and its

adjacent diagram on page 296

• Electrical Symbol – a symbol used for the parts of an electrical circuit in a circuit diagram• The symbols are industry standard so that all trained

electricians can read the diagram correctly

• Look at Figure 12.3 on page 296

• Look at symbols on the next slide

• Resistor – a device that controls or uses the energy carried by an electric current

Electrical Symbols• Wire

• Battery (cell)

• Light Bulb

• Resistor

• Switch (open)

+ -

Open and Closed Circuits• Closed Circuit – an electrical circuit with no breaks

in which current (or charge) flows

• Open Circuit – an electrical circuit with a break in which current (or charge) cannot flows

• Switch – a device used to open and close a circuit• Other things, such as burned-out light bulbs or tripped

circuit breakers, can also cause open circuits

• Types of power supplies• Electric generators supplying electricity to your home

• Batteries (a single “battery” is properly called a cell)

• Solar cells

Section 12.2• Current and Voltage

• Current and Voltage

• Batteries

• Electrical Instruments

• Measuring Voltage

• Measuring Current

Current and Voltage• Current – the flow of electric charge

• Ampere – the unit of electric current• Named for Andre-Marie Ampere (France, 1775-1836)

• Its short name is amps

• Its abbreviation is A

• Voltage – a measure of electrical potential energy

• Volt – the unit of voltage• Named for Alessandro Volta (Italy, 1745-1827)

• Its abbreviation is V

Current and Voltage• Current and voltage are different

• Current is charge that flows and does work

• Voltage provides energy for the charge to flow

• Voltage is the power per amp that flows• A 1.5 V battery carries 1.5 W of power per amp

• A 120 V house outlet carries 120 W of power per amp

• Electric current flows from a battery’s positive terminal and returns to its negative terminal• Current leaving a battery carries energy

• Current passing through a resistor gives up energy

• Look at Figure 12.7 on page 298

Batteries• Battery – a device that transforms chemical energy

into electrical energy and moves the current in a circuit• A common non-rechargeable dry cell battery is 1.5 V

• A common rechargeable dry cell battery is 1.2 V

• A common automobile (wet cell) battery is 12 V

• Batteries are like pumps – they restore energy• Current leaving a battery carries renewed energy

• Current passing through a resistor gives up energy

Batteries• For a common flashlight battery

• The positive terminal is a raised bump on one end

• The negative terminal is flat on the other end

• For button type batteries (like a CR2025)• Pay attention to the + and – marks on the sides

• A 1.5-volt battery allows a 1-ampere current to carry 1.5 joules of energy every second, which equals to 1.5 watts

Electrical Instruments• Voltmeter – an instrument that measures voltage

• Ammeter – an instrument that measures current

• Ohmmeter – an instrument that measures resistance

• Multimeter – an instrument that measures voltage, current, and resistance• The function and the range is selectable

• Meters usually have two probes, one red (positive) probe and one black (negative) probe

Measuring Voltage• Set the meter to ‘volts’

• Place the probes across the device in the circuit

• The meter will display the voltage difference between the two points

• If you put the probes as the same point in the circuit, the voltage will read zero

Source: Science Buddies

Measuring Current• Set the meter to ‘amps’

• Place the probes so the current passes through the meter• First, disconnect the

circuit where you want to measure

• Then, insert the meterto reconnect the circuit

• The meter will display the current flowing between the two points

Source: Science Buddies

Section 12.3• Resistance and Ohm’s Law

• Electrical Resistance

• Measuring Resistance

• Ohm’s Law

• Resistance of Common Objects

• Conductors and Insulators

• Resistors

Electrical Resistance• Resistance – a measure of an object’s opposition to

the flow of electric current

• Water analogy• Look at Figure 12.12 on page 302

• Bottle on left has a small neck opening so resistance ishigh and water flow is low

• Bottle on right has a large neck opening so resistance is low and water flow is high

• Greater circuit resistance means smaller current

• Resistances can add together• Look at light bulbs at the bottom of page 302

Measuring Resistance• Disconnect power so

you don’t destroy the meter!

• Set the meter to ‘ohms’

• Place the probes across the device in the circuit

• The meter will display the resistance of the device

Source: Science Buddies

Ohm’s Law• Ohm’s Law – states that current is directly related to

voltage and inversely related to resistance

• 𝐼 is the current in amps (A)

• 𝑉 is the voltage in volts (V)

• 𝑅 is the resistance in ohms (Ω)

𝐼 =𝑉

𝑅𝑅 =

𝑉

𝐼𝑉 = 𝐼 × 𝑅

Resistance of Common Objects• Every electrical device is designed with a resistance

that causes just the right amount of current for the voltage it uses

• Human skin• About 100,000 Ω when dry, much lower when wet

• You can feel current of about 0.0005 A and get shocked above 0.0012 A

• Resistance varies with temperature and age

Device Voltage Current Resistance

Flashlight bulb 1.5 V 0.5 A 3 Ω

100-W house bulb 120 V 0.83 A 145 Ω

Conductors and Insulators• Conductor – material with low electrical resistance

• Conducts electric current well – used for wires

• Copper, aluminum, and most other metals

• Insulator – material with high electrical resistance• Conducts electric current poorly

• Glass, rubber, and most plastics

• Semiconductor – material with electrical resistance between conductor and insulator• Conducts current depending on various conditions

• Used for computer chips and communication circuits

• Silicon, germanium, and certain III-V compounds

Resistors• Resistor – a device constructed to have a specified

resistance value required by an electrical circuit

• Fixed resistors• Single value, usually small, color-coded bands

• Look at Figure 12.17 on page 307

• Red, green, orange, gold – 25 × 103 = 25,000 Ω ± 5%

• We’ll use the color chart in Lab 12/13

• Variable resistors (potentiometers)• Adjustable within a range of values

• Used as light dimmers and volume controls