Electrical Principles: Circuits, Static & Quantities

Understanding Electrical Principles and Circuits

Static Electricity Fundamentals

When an atom becomes an ion, it becomes charged. When two insulating materials are rubbed together (e.g., wool and a plastic rod), electrons move from one material to another (as protons cannot move). This process results in one material becoming negatively charged and the other positively charged. Once charged, these materials can attract uncharged objects and objects with an opposite charge.

Understanding Electrical Sparking

A significant charge builds up on one object due to electrons being rubbed off by another. If a large enough charge accumulates, the voltage becomes sufficiently high to ionize the air molecules. This allows electrons to jump to earth, causing a visible spark.

I-V Characteristics: Required Practical

This practical investigates the relationship between current (I) and voltage (V) for different components.

1. Set up a circuit using wires, a power source, an ammeter, and a voltmeter around component X.
2. Place a resistor as component X. Measure five positive values of voltage, then swap the power supply connections and measure five negative values of voltage.
3. Repeat the process with a filament lamp and a diode as component X.
4. Plot a graph showing the relationship between current (I) and voltage (V) for all three components.

Results:

• Resistor: Current is directly proportional to voltage when temperature is constant. This demonstrates Ohm's Law.
• Filament Lamp: The graph shows two curved lines in opposite directions, indicating that resistance increases with temperature.
• Diode: There is negligible current flow until approximately 0.6V (forward bias voltage). After this point, the current shows a steep linear increase. Current through a diode flows only one way; it has a very high resistance in the reverse direction.

Key Electrical Quantities and Conductors

• Charge (Q): Measured in Coulombs (C). One Coulomb is equivalent to 6.24 x 10¹⁸ electrons. Charge can only flow if there is a source of potential difference.
• Resistance (R): Measured in Ohms (Ω). It quantifies how difficult it is for current to flow through a material.
• Current (I): Measured in Amperes (A). This is the rate of flow of charge.
• Power (P): Measured in Watts (W). This represents the rate of energy transfer.

Resistance of a Wire: Required Practical

This practical investigates how the resistance of a wire changes with its length.

1. Set up a circuit using wires, a power source, an ammeter, and a voltmeter around a variable resistor.
2. Attach a length of resistance wire to a meter ruler and secure it with tape. Attach one crocodile clip to the end of the wire.
3. Attach a second crocodile clip at the 10 cm mark on the ruler. Record the current and voltage through this section of the wire.
4. Repeat by moving the second crocodile clip in 10 cm increments along the wire, recording the current and voltage for each length.
5. Calculate the resistance at each point using the equation V=IR (rearranged as R=V/I).
6. Plot a graph showing the relationship between the resistance and the length of the wire.

Results:

As the length of the wire increases, so does its resistance. This demonstrates a direct proportionality between wire length and resistance.

Series and Parallel Circuits Explained

• Series Circuits:
  • Current is the same everywhere throughout the circuit.
  • The total voltage across components adds up to the voltage of the power source (battery).
• Parallel Circuits:
  • The total current from the power source is the sum of the currents in each branch.
  • The voltage across each branch is the same as the voltage of the power source (battery).

Essential Electrical Equations

• Voltage = Current × Resistance (V = IR) - Ohm's Law
• Power = Energy Transferred / Time (P = E/t)
• Power = Work Done / Time (P = W/t)
• Efficiency (%) = (Useful Power Output / Total Power Input) × 100
• Charge Flow = Current × Time (Q = It)
• Power = Voltage × Current (P = VI)
• Power = Current² × Resistance (P = I²R)