DE Resistance in Electrical Systems 1 Resistance Resistance

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DE: Resistance in Electrical Systems 1

DE: Resistance in Electrical Systems 1

Resistance: Resistance is defined as the opposition to motion. In electrical systems, resistance opposes

Resistance: Resistance is defined as the opposition to motion. In electrical systems, resistance opposes flow of charge. 2

Why is there Resistance in Electrical Systems? Electrical resistance occurs because the electrons moving

Why is there Resistance in Electrical Systems? Electrical resistance occurs because the electrons moving in the wire bump, or collide with the atoms in the wire (from the substance that the wire is made out of). 3

A by-product of the collisions of atoms (resistance) is heat. 4

A by-product of the collisions of atoms (resistance) is heat. 4

Is Electrical Resistance Good or Bad? Examples of useful electrical resistance: 1. Dimmer switches,

Is Electrical Resistance Good or Bad? Examples of useful electrical resistance: 1. Dimmer switches, volume controls. 2. Electric stoves, heaters, lights, fuses. 5

Examples of non-useful electrical resistance: 1. Heat build up, motor windings. 2. Voltage drop

Examples of non-useful electrical resistance: 1. Heat build up, motor windings. 2. Voltage drop - losses in wires. Short circuits, extension cords. 6

Formula for Electrical Resistance: voltage Electrical Resistance = -------current or, RE V = -----I

Formula for Electrical Resistance: voltage Electrical Resistance = -------current or, RE V = -----I 7

Electrical resistance is measured in units called ohms. The symbol for electrical resistance is

Electrical resistance is measured in units called ohms. The symbol for electrical resistance is the Greek letter “omega” . Voltage is measured in volts. Current is measured in amps. 8

Rearranging the Formula to Solve for Current or Voltage: To find voltage (V) or

Rearranging the Formula to Solve for Current or Voltage: To find voltage (V) or current (I), the formula needs to be rearranged. Voltage = Current x Resistance or, V = I x R 9

voltage drop Current = -----------resistance or, ΔV I = ----RE 10

voltage drop Current = -----------resistance or, ΔV I = ----RE 10

The relationship between voltage, current, resistance, & power is known as Ohm’s Law. Georg

The relationship between voltage, current, resistance, & power is known as Ohm’s Law. Georg Simon Ohm 11

The relationship between voltage, current, resistance, & power is known as Ohm’s Law. 12

The relationship between voltage, current, resistance, & power is known as Ohm’s Law. 12

Sample Problems – Use Ohm’s Law to Solve: 1. A light bulb operates on

Sample Problems – Use Ohm’s Law to Solve: 1. A light bulb operates on a 110 volt circuit. The bulb draws a current of. 91 amps. What is the resistance of the light bulb? Answer: (include unit or symbol): 120. 8 ohms ( ) 13

Sample Problems – Use Ohm’s Law to Solve: 2. A toaster operates on 120

Sample Problems – Use Ohm’s Law to Solve: 2. A toaster operates on 120 volts, and has a resistance of 20 ohms. What is the current? Answer: 6 amps 14

Sample Problems – Use Ohm’s Law to Solve: 3. A 220 volt stove draws

Sample Problems – Use Ohm’s Law to Solve: 3. A 220 volt stove draws a current of 40 amps. What is the resistance of the stove? Answer: 5. 5 ohms ( ) 15

Wire Length & Diameter: What Effect Do They Have On Resistance? A longer wire

Wire Length & Diameter: What Effect Do They Have On Resistance? A longer wire will have more resistance than a shorter wire (extension cords). A thinner wire will have more resistance than a thicker wire (speaker wire). 16

Resistors Devices used to resist the flow of current are called resistors. Resistors are

Resistors Devices used to resist the flow of current are called resistors. Resistors are used in all types of electrical and electronic circuits to limit & control current. Resistors also control how much energy an electrical device uses. 17

Two Main Types of Resistors: 1. Fixed resistor: Has a specific (set) value of

Two Main Types of Resistors: 1. Fixed resistor: Has a specific (set) value of resistance. They are usually made of carbon, metal-film, or wire wound. A color code designates the resistance value. 18

Two Main Types of Resistors: 2. Variable resistor: Has a movable element that allows

Two Main Types of Resistors: 2. Variable resistor: Has a movable element that allows for the adjustment of resistance. Volume controls and dimmer switches are examples of variable resistors. 19

Ohm’s Law & Series Circuits: In a series circuit, the electrical loads are connected

Ohm’s Law & Series Circuits: In a series circuit, the electrical loads are connected to each other to form a single path for current to flow. If that path is disrupted or if a load fails, burns out or is disconnected, the other loads are disrupted (shut off). 20

Series Circuit Schematic In series circuits, there are “rules” for how resistance, voltage &

Series Circuit Schematic In series circuits, there are “rules” for how resistance, voltage & current are determined. 21

Rules for Series Circuits: 1. Resistance in a series circuit is found by adding

Rules for Series Circuits: 1. Resistance in a series circuit is found by adding individual resistance values together. Total resistance = Resistance 1 + Resistance 2 + Resistance 3, etc… (RT = R 1 + R 2 + R 3, etc…) 22

Rules for Series Circuits: 2. Current in a series circuit is the same throughout

Rules for Series Circuits: 2. Current in a series circuit is the same throughout the circuit and through each resistor. Total current = Current 1 = Current 2 = Current 3, etc… ( IT = I 1 = I 2 = I 3, etc…) 23

Rules for Series Circuits: 3. Voltage in a series circuit splits up, or divides

Rules for Series Circuits: 3. Voltage in a series circuit splits up, or divides across the individual resistors or loads. Total voltage = Voltage 1 + Voltage 2 + Voltage 3, etc… (VT = V 1 + V 2 + V 3, etc…) 24

Ohm’s Law & Parallel Circuits In a parallel circuit, each electrical load has a

Ohm’s Law & Parallel Circuits In a parallel circuit, each electrical load has a separate connection, or path, to the voltage source. If that path is disrupted or if a load fails, burns out or is disconnected, the other loads stay on. 25

Parallel Circuit Schematic In parallel circuits, there are “rules” for how resistance, voltage &

Parallel Circuit Schematic In parallel circuits, there are “rules” for how resistance, voltage & current are determined. 26

Rules for Parallel Circuits: 1. Resistance in a parallel circuit is less than the

Rules for Parallel Circuits: 1. Resistance in a parallel circuit is less than the smallest, single individual resistance value. To find resistance in a parallel circuit, the inverse of each resistor value must be added. (RT = 1/R 1 + 1/R 2 + 1/R 3, etc…) 27

Rules for Parallel Circuits: 2. Current in a parallel circuit splits up, or divides

Rules for Parallel Circuits: 2. Current in a parallel circuit splits up, or divides across the individual resistors or loads. Total current = Current 1 + Current 2 + Current 3, etc… (IT = I 1 + I 2 + I 3, etc…) 28

Rules for Parallel Circuits: 3. Voltage in a parallel circuit is the same throughout

Rules for Parallel Circuits: 3. Voltage in a parallel circuit is the same throughout the circuit and across each resistor. Total voltage = Voltage 1 = Voltage 2 = Voltage 3, etc… ( VT = V 1 = V 2 = V 3, etc…) 29

Adding too many items in parallel lowers the resistance too much. This allows too

Adding too many items in parallel lowers the resistance too much. This allows too much current to flow!! 30

Classifying the Electrical Properties of Materials: 1. Conductors: materials with low resistivity values (such

Classifying the Electrical Properties of Materials: 1. Conductors: materials with low resistivity values (such as copper, gold, silver, & solder). 31

Classifying the Electrical Properties of Materials: 2. Insulators: materials with high resistivity values (such

Classifying the Electrical Properties of Materials: 2. Insulators: materials with high resistivity values (such as glass, ceramics, and plastics). 32

Classifying the Electrical Properties of Materials: 3. Semi-conductors: materials with medium resistivity values (such

Classifying the Electrical Properties of Materials: 3. Semi-conductors: materials with medium resistivity values (such as silicon and germanium which are used to make transistors and integrated circuits (IC’s). 33

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