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