Electicity PDF

Resisting the Movement of Charge

• Resistance is a property that hinders the movement of electric charge, converting electrical energy into other forms of energy, such as light and heat.
• Tungsten wire in a light bulb has significantly higher resistance than copper wire.
• Voltage, current, and resistance are closely related in an electric circuit.
• Resistance opposes the motion of electrons, while voltage drives the electrons.
• Good conductors (e.g., copper) have low resistance, allowing electric charge to move easily.
• Poor conductors have high resistance, opposing the movement of charge.
• Georg Ohm discovered that the ratio of voltage to current remains constant for a given wire (ohmic resistors).

Find Out: Resistance Roadblock

• High resistance is useful in circuits for specific electronic components.
• Increasing or decreasing resistance affects the energy of electrons in the circuit.
• Experiment involves using Nichrome™ wire to observe the effects of varying resistance on bulb brightness and current.

Calculating Resistance

• The ohm (Ω) is the standard unit for resistance.
• Resistance can be measured with an ohmmeter or a multimeter.
• Georg Ohm found that resistance can be calculated from current and voltage measurements.
• Ohm's Law: The mathematical relationship between resistance (R), voltage (V), and current (I).
  • $R = \frac{V}{I}$ (Resistance = Voltage / Current)
  • Units of resistance are volts per ampere (V/A), which are equivalent to ohms (Ω).
  • $V = IR$ (Voltage = Current × Resistance)
  • $I = \frac{V}{R}$ (Current = Voltage / Resistance)
• Examples of resistance:
  • 100-watt light bulb: ~144 Ω (at 120 V)
  • Stereo speakers: 2 Ω to 8 Ω
  • Headphones/earbuds: 32 Ω
  • Dry human skin: ~500,000 Ω (500 kΩ or 0.5 MΩ)

Model Problem

• Problem: Find the resistance of an electric heater with a current of 12.5 A when connected to a 120V outlet.
• Solution:
  • Given: $I = 12.5 A$, $V = 120V$
  • Using Ohm's Law: $R = \frac{V}{I} = \frac{120V}{12.5A} = 9.60 Ω$

Practice Problems

1. Resistance of a light bulb with a 12 V battery and 2.4 A current:
   • $R = \frac{V}{I} = \frac{12V}{2.4A} = 5 Ω$
2. Current through a toaster with 145 Ω resistance connected to a 120 V source:
   • $I = \frac{V}{R} = \frac{120V}{145Ω} ≈ 0.828 A$
3. Potential difference across a 1500 Ω resistor with a current of 0.075 A:
   • $V = IR = (0.075A)(1500Ω) = 112.5 V$
4. Smallest resistance for a 15 A extension cord connected to a 120 V supply:
   • $R = \frac{V}{I} = \frac{120V}{15A} = 8 Ω$

Resistors

• Resistors are electronic components used to control current or voltage in circuits.
• Made of heat-conducting ceramic core coated with materials like carbon, metal oxides, or resistance wire.
• Modern chip resistors are small and mounted directly on circuit boards.

Variable Resistors

• Resistance changes based on temperature, light, voltage, or other variables.
• Nichrome™ wire acts as a variable resistor when length is adjusted.
• Used in light dimmers, motor speed controls, surge protectors, and temperature controls.

Common Types of Variable Resistors

• Rheostats: Control total resistance, used in high-current circuits (e.g., power distribution equipment).
• Thermistors: Sensitive to temperature changes, used in thermostats and heat sinks.
• Varistors: Voltage-dependent resistors, protect circuits from high-voltage surges.

Voltage, Current, and Resistance Investigation

• Objective: Examine the relationship between electric current, voltage, and resistance in a resistor.
• Safety Precautions: Avoid touching resistors during current flow; limit current to 0.5 A and voltage to 15 V.
• Procedure: Construct a circuit with a resistor, ammeter, voltmeter, and variable power supply.
  Adjust the power supply to vary voltage and record corresponding current readings.
  Repeat for a second resistor.

Analysis and Conclusion

1. Electric current increases as voltage increases.
2. For similar voltages, the resistor with higher resistance allows less current to pass through.
3. Calculate resistance for each trial using Ohm's Law ($R = V/I$).
4. Calculated resistance values for each resistor should be consistent but may vary slightly due to experimental error.
5. Compare calculated resistance to the marked value; differences may occur due to resistor tolerance.
6. Results should support Ohm's law.

Types of Circuits

• Series Circuits:
  • Have only one current path.
  • All moving charges travel through each component.
  • Components are connected end to end.
• Parallel Circuits:
  • Have multiple current paths.
  • Total current is divided among the branches.
  • Components are connected side by side.
• Circuits can be a combination of series and parallel connections.

Series and Parallel Circuits Investigation

• Objective: Explore the effects of adding bulbs to series and parallel circuits.
• Hypothesis: Predict the effect on bulb brightness and current as bulbs are added.

Part A: Series Circuit

1. Bulb brightness decreases as more bulbs are added.
2. Electric current decreases as more bulbs are added.
3. When one bulb is unscrewed, the circuit breaks, and all bulbs go out.

Part B: Parallel Circuit

1. Bulb brightness remains constant as more bulbs are added.
2. Electric current increases as more bulbs are added.
3. When one bulb is unscrewed, the other bulbs remain lit.

House Wiring

• Household circuits are parallel circuits.
• Voltage across each load is the same.
• Turning on one appliance does not affect others.
• High current can cause wires to overheat.
• Fuses or circuit breakers protect against electrical fires.

Is Resistance Futile? Challenge

• Design five circuits with specific characteristics:
  • One bulb can be turned off while the other stays lit.
  • Both bulbs can be turned off simultaneously.
  • Maximum current flow.
  • Minimum current flow.
  • One light can be dimmed without affecting the other.
• Use provided materials: bulbs, milliammeter, D-cells, switches, Nichrome™ wire, copper wire, resistors.

Evaluation

1. Demonstrate circuit operation and explain circuit types.
2. Revise circuit diagrams as needed; explain changes.
3. Label circuits as series or parallel.
4. Energy transformations:
   • Nichrome™ wire: Electrical to thermal.
   • Light bulbs: Electrical to light and thermal.
   • Resistors: Electrical to thermal.
5. Practical applications for variable resistance circuits.
6. Dimming bulbs: "Lost" energy is converted to heat; could cause problems for variable resistor designers.

Factors Affecting Resistance of Wire

• Length: Resistance increases with length.
• Cross-sectional Area: Resistance decreases with area (thicker wires have lower resistance).
• Temperature: Resistance increases with temperature.
• Material: Different materials have different resistance properties.

Topic 3 Review

1. Electrical resistance hinders the flow of electric charge.
2. Ohm is equivalent to volts per ampere (V/A).
3. Ohm’s Law: $V = IR$, where V is voltage, I is current, and R is resistance.
4. Series circuits have one current path; parallel circuits have multiple current paths.
5. Factors affecting resistance: length, cross-sectional area, temperature, material.
6. Parallel circuits produce the highest current through three bulbs because the voltage across each bulb is the same, and the total current is divided among the branches.
7. Voltage across a water heater element with 32 Ω resistance and 6.8 A current: $V = IR = (6.8A)(32Ω) = 217.6 V$
8. Angela is correct: as resistance increases, current decreases (Ohm's Law).