Electricity and Circuits

Introduction

  • Electrical Circuit

    • Definition: A closed loop that contains something for the electrons to flow through.

  • Current

    • Definition: The measure of the flow of electrons

    • Denoted by: I

    • Measured in: Amps (A)

  • Potential Difference

    • Definition: The force driving the flow of electricity

    • Measured in: Volts (V)

  • Resistance

    • Definition: Everything that slows down the flow of electricity

    • Measured in: Ohms (Ω)

  • Electrical Charges and Fields

    • Components of an Atom:

    • Protons (+)

    • Neutrons (0)

    • Electrons (-)

    • Electrons have a NEGATIVE CHARGE.

    • When an atom loses an electron, it has a POSITIVE CHARGE.

    • These two charges attract each other, creating an electric field.

    • Positive charges attract negative charges.

    • The closer the charges are, the stronger the field.

Electrostatics

  • Static Electricity

    • Generated through FRICTION:
      1) When insulating materials are rubbed together, electrons are transferred from one object to another.
      2) This results in one object having a positive charge and the other a negative charge.
      3) The direction of charge transfer depends on the materials involved.

  • SPARKS

    • Occur when:
      1) An electric charge builds on an object, and the potential difference (P.D.) between the object and the ground increases.
      2) If the P.D. becomes large enough, electrons can jump across the object to the Earth or to any earthed conductor.

  • IONIZATION

    • Sparks are caused by:
      1) A high potential difference created by a strong electric field.
      2) This causes the removal of electrons from the air, allowing the air to conduct electricity, thus forming a current.

Current and Charge

  • Current

    • Definition: The flow of electrical charge (electrons).

    • Charge will only flow around a circuit if there is a potential difference.

    • Direction of current flow: From positive to negative.

  • Charge

    • Definition: A measure of the total electrical current passing in a circuit over a certain period of time.

    • Measured in: Coulombs (Q)

    • Equation:

    • Q=It

      • Where:

      • Q = charge in Coulombs

      • I = current in Amps

      • t = time in seconds

  • Example of charge calculation:

    • For a battery charger passing a current of 2A over 2.5 hours:

    • Calculate total charge:

      • Q = 2.0 imes (2.5 imes 60 imes 60)

      • Result: Charge = 18000 C

Potential Difference and Resistance

  • Potential Difference

    • Definition: The driving force that pushes the current around a circuit, also known as Voltage.

    • Measured in: Volts (V)

    • Equation:

    • V=IR

      • Where:

      • V = potential difference in Volts

      • I = current in Amps

      • R = resistance in Ohms

  • Power

    • Definition: The rate at which energy is transferred or converted.

    • Equation:

    • t{Power}=\frac{energy}{time}

    • Power in electrical terms:

      • ext{Power} = I^2 imes R

Resistance

  • Definition: Anything that slows down the flow of electricity.

  • Measurement: Ohms (Ω)

  • Resistance relationships:

    • The greater the resistance, the smaller the current that flows.

    • Equation:

    • R=\frac{V}{I}

  • Factors Affecting Resistance:

    • Whether components are connected in series or parallel.

    • The length of the wire.

Graphs

  • Ohmic Conductors

    • Current and voltage are DIRECTLY PROPORTIONAL (assuming temperature remains constant).

  • Non-Ohmic Conductors:

    • Filament lamps: As current flows, the filament heats up, increasing resistance and thus reducing current flow at higher voltages.

    • This results in a non-linear relationship on a graph, becoming less steep as the current increases.

Practical Investigations

  • Setting Up a Test Circuit
    1) Set up a test circuit.
    2) Vary the variable resistor to change current and potential difference.
    3) Take readings from an ammeter and a voltmeter.
    4) Swap wires connected to the battery for complete readings.
    5) Plot a graph of current against voltage.

  • Resistance Increase with Temperature

    • Why?

    • Because positive ions vibrate, increasing collisions and thus impeding electron flow through the metal.

Component Characteristics

  • Key Components of Circuits:

    • Cell: Provides electric power.

    • Switch: Controls the flow of electricity.

    • Filament bulb: Emits light when current flows through in the forward direction.

    • LED: Similar to filament but more efficient.

    • Variable resistor: Modifies the amount of resistance in the circuit.

    • Ammeter: Measures electric current.

    • LDR (Light Dependent Resistor): Resistance depends on light intensity.

    • Generator: Produces electrical energy.

    • Speaker: Converts electric signals into sound.

Series Circuits

  • Description: Circuit components connected in a single loop.

    • If one component fails, the circuit is broken (except voltmeters which are connected in parallel).

  • Potential Difference

    • Total potential difference across components is shared.

    • Equation:

    • V{total} = V1 + V2 + … + Vn

  • Current

    • Current remains the same throughout a series circuit.

    • Equation for total resistance:

    • R{total} = R1 + R2 + … + Rn

    • Higher resistance components receive a larger share of the voltage.

    • Example of cell potential differences adding up:

    • If there are two cells with potential differences of 3V each, the total supply becomes 6V.

Investigating Resistance with Series Circuits

  • Method:
    1) Use at least 4 identical resistors.
    2) Build a circuit using one resistor.
    3) Measure current using an ammeter and calculate resistance.
    4) Add another resistor, measure the current, and calculate.
    5) Repeat and plot graphs for the number of resistors against current.

Parallel Circuits

  • Description: Circuits with multiple junctions; components are separately connected.

    • If one fails, it does not greatly affect others.

    • Common in household wiring for safety and redundancy.

  • Potential Difference

    • Same voltage (P.D.) is maintained across all components:

    • V{total} = V1 = V2 = … = Vn

  • Current

    • Total current divides across parallel branches.

    • Total current equation:

    • I{total} = I1 + I2 + … + In

    • More components generally lead to lower total resistance:

    • Equation:

      • R{total} = rac{ 1 }{ R1 } + rac{ 1 }{ R2 } + … + rac{ 1 }{ Rn }

Investigating Resistance with Parallel Circuits

  • Method:
    1) Use the same equipment to set up a parallel circuit.
    2) Measure total current and calculate resistance.
    3) Add another resistor in parallel, measure total current and potential difference of the battery.
    4) Repeat the above steps multiple times.
    5) Draw a graph of the number of resistors against total current.