3 Resistance

Handout: Understanding Resistance: Key Concepts

What is Resistance?

Resistance (Ω) is the opposition to the flow of electric current. It converts electrical energy into other forms, most commonly heat.

How Resistance Affects Circuits:

1. Current Limiting: A resistor limits the amount of current that can flow in a circuit (according to Ohm's Law: I=V/R).

2. Voltage Drop: When current flows through a resistor, there is a voltage drop across it. This is the amount of electrical "pressure" consumed by the resistor.

3. Power Dissipation: Resistors convert electrical energy into heat. The amount of power dissipated is calculated by P=I2R or P=V2/R. Resistors have a power rating (in Watts) that they can safely dissipate.

Factors Affecting a Material's Resistance:

1. Material Type (Resistivity ρ):

   • Different materials have different abilities to conduct electricity.

   • Good conductors (e.g., copper, silver) have low resistivity.

   • Insulators (e.g., rubber, plastic) have very high resistivity.

2. Length (L):

   • Resistance is directly proportional to length. A longer wire has more resistance.

   • Analogy: A longer hallway has more obstacles and takes more effort to walk through.

3. Cross-sectional Area (A):

   • Resistance is inversely proportional to cross-sectional area. A thicker wire has less resistance.

   • Analogy: A wider hallway allows more people to pass easily.

4. Temperature (T):

   • Positive Temperature Coefficient (PTC): For most metals (conductors), resistance increases as temperature increases.

   • Negative Temperature Coefficient (NTC): For some materials (like carbon, semiconductors, thermistors), resistance decreases as temperature increases.

Types of Resistors:

A. Fixed Resistors (Constant Value):

• Carbon Composition: Older, less precise, used where exact value not critical.

• Metal Film/Metal Oxide Film: Common, good precision, stable with temperature.

• Wirewound: Used for high power applications (heating, power dissipation).

• Surface Mount Device (SMD/SMT): Tiny chips for printed circuit boards (PCBs).

B. Variable Resistors (Value Can Be Changed):

• Potentiometer: Three terminals. Used as a voltage divider to provide a variable output voltage.

• Rheostat: Two terminals (or potentiometer used with two). Used in series to vary current to a load.

C. Special Resistors (Resistance Changes with Condition):

• Thermistor: Resistance changes with temperature (NTC or PTC). Used as temperature sensors.

• Photoresistor (LDR): Resistance changes with light intensity. Used in light sensors.

• Varistor (VDR/MOV): Resistance changes drastically with voltage. Used for surge protection.

Common Resistor Applications:

• Current Limiting: Protecting LEDs, limiting control circuit currents.

• Voltage Division: Creating specific voltage levels for sensors or control signals.

• Heating Elements: Converting electrical energy to heat (e.g., electric heaters).

• Sensing: Using thermistors, LDRs, etc., to detect physical changes.

• Loads: Providing a necessary electrical load for a circuit or power supply.

Measuring Resistance with a DMM:

• CRITICAL SAFETY RULE: Always ensure the circuit or component is DE-ENERGIZED (power OFF) before measuring resistance.

• Set DMM dial to Ohms (Ω).

• Connect red lead to V$\Omega$mA jack, black lead to COM jack.

• Place probes across the component to be measured.

• Note: For accurate readings, it's often best to measure resistors out of circuit to avoid parallel paths that can affect the reading.

Handout: Resistance Review Questions

Resistance Review Questions

Answer the following questions to check your understanding of today's topics.

1. In an electrical circuit, how does resistance affect the flow of current?

2. If a resistor has a large amount of current flowing through it, what form of energy is typically dissipated by the resistor?

3. List the four main factors that determine the resistance of a material.

4. If you have two wires made of the same material and same thickness, but one is twice as long as the other, which wire will have more resistance?

5. Why does a thicker wire generally have less resistance than a thinner wire of the same material and length?

6. Explain the difference between a material with a Positive Temperature Coefficient (PTC) and one with a Negative Temperature Coefficient (NTC) in terms of how their resistance changes with temperature.

7. What is the primary difference between a fixed resistor and a variable resistor?

8. Which type of fixed resistor is commonly used for high power applications due to its ability to dissipate heat?

9. You need a resistor to create a variable voltage signal from a fixed voltage source. Which type of variable resistor would you use?

10. Name one type of "special resistor" whose resistance changes based on an external physical condition (other than just turning a knob).

11. Describe a common application of a resistor for "current limiting."

12. What is the most important safety rule to remember when measuring resistance with a Digital Multimeter (DMM)?

Quiz: Resistance (15 Questions)

Name: ____________________________________ Date: ________________________________

Instructions: Choose the best answer for each question or fill in the blank.

1. Resistance in an electrical circuit is defined as the:
   a) Force that pushes electrons
   b) Rate of flow of electrons
   c) Opposition to the flow of electric current
   d) Energy consumed by the circuit
   
   

2. According to Ohm's Law (I=V/R), if the resistance in a circuit increases while the voltage remains constant, the current will:
   a) Increase
   b) Decrease
   c) Stay the same
   d) Become infinite
   
   

3. Electrical energy dissipated by a resistor is primarily converted into:
   a) Light
   b) Sound
   c) Motion
   d) Heat
   
   

4. Which of the following factors is inversely proportional to a material's resistance?
   a) Length
   b) Temperature
   c) Cross-sectional area
   d) Material type
   
   

5. A longer wire made of the same material and thickness will have:
   a) Less resistance
   b) More resistance
   c) The same resistance
   d) Zero resistance
   
   

6. A material with a Negative Temperature Coefficient (NTC) will experience what change in resistance as its temperature increases?
   a) Resistance increases
   b) Resistance decreases
   c) Resistance stays the same
   d) Resistance becomes zero
   
   

7. Which type of fixed resistor is commonly used for high-power applications, such as in heating elements or power supplies?
   a) Carbon composition
   b) Metal film
   c) Wirewound
   d) Surface mount
   
   

8. A three-terminal variable resistor used to obtain a variable output voltage from a fixed input voltage is a:
   a) Rheostat
   b) Thermistor
   c) Potentiometer
   d) Varistor
   
   

9. If a potentiometer is wired as a rheostat, it is typically connected in what way with the load?
   a) In parallel
   b) In series
   c) As a voltage divider
   d) Directly to ground
   
   

10. A special type of resistor whose resistance changes in response to changes in light intensity is called a:
    a) Thermistor
    b) Photoresistor (LDR)
    c) Varistor (VDR)
    d) Rheostat
    
    

11. What is the primary application of a resistor when placed in series with an LED (Light Emitting Diode)?
    a) To increase the LED's brightness
    b) To limit the current flowing through the LED
    c) To convert AC to DC for the LED
    d) To measure the voltage across the LED
    
    

12. To measure the resistance of a component using a Digital Multimeter (DMM), the circuit containing the component must always be:
    a) Connected to AC power
    b) Connected to DC power
    c) De-energized (power OFF)
    d) Continuously supplied with current
    
    

13. When measuring resistance, the DMM's black test lead should always be connected to the:
    a) A (Amps) jack
    b) V$\Omega$mA jack
    c) COM (Common) jack
    d) Frequency jack
    
    

14. What unit indicates the maximum power a resistor can safely dissipate without burning out?
    a) Ohms
    b) Volts
    c) Amperes
    d) Watts
    
    

15. If you observe a metal wire getting hot when current flows through it, this is a direct result of the wire's:
    a) Inductance
    b) Capacitance
    c) Resistance
    d) Voltage