Electricity & Circuits - Grade 8 Notes

Parallel Circuits

• In a parallel circuit, the electric current is divided into branches.
• Electrical components (e.g., bulbs) are connected to each branch.
• Electric current passes through all branches, meaning there is more than one path for the current to flow.
• Current Flow: In a parallel circuit, current splits at junctions and flows through multiple paths before recombining.
• The total current in a parallel circuit equals the sum of the currents in each branch.
• Voltage: The voltages across each of the branches of a parallel circuit are equal to the voltage of the supply.

Voltage

• In series circuits, voltage is divided among the components. For example, a 9V battery might result in 6V across one component and 3V across another ($9V = 6V + 3V$).
• In parallel circuits, voltage remains the same across all the lamps/components.

Types of Circuits

• Series Circuit:
  • Components are connected along a single path.
  • The same current flows through all components.
  • Different voltage drops can occur across different components.
  • Cannot be switched off/on separately.
  • If cells are increased in the same series circuit it will increase the current in the circuit and increase the voltage across each component.
• Parallel Circuit:
  • Components are connected side by side.
  • Current divides among the branches.
  • The same voltage is present across all branches.
  • Can be switched off/on separately.

Current

• The current through the cell is more than the current through each lamp in parallel.

Series Circuits

• The electrical components are joined one after another to form a single loop.
• Electric current passes through a single path.

Variable Resistor

• A variable resistor allows adjustment of the amount of resistance easily from low to high by moving a slider.
• Unit for resistance is ohm ($\Omega$).

Cell and Battery Function

• Cells and batteries create a potential difference (voltage).
• Current flows from high potential (+) to low potential (-).
• The S.I. unit for potential difference is volt (V).

Measuring Voltage

• Voltage measures the electrical energy that the cell can give to the electrons.
• Voltage is measured using a device called a voltmeter.
• The unit of voltage is volt (V).
• Voltmeters should be connected in parallel.

Ohm's Law

• Ohm's law states that the current $I$ through a given conductor is directly proportional to the potential difference $V$ between its end points.
• Formula: $V = I \times R$, where:
  • $V$ = Voltage
  • $I$ = Current
  • $R$ = Resistance
• Rearranged formulas:
  • $I = V / R$
  • $R = V / I$

Adding Cells to a Parallel Circuit

• Adding cells to a parallel circuit increases the supply voltage, which:
  • Increases the voltage across each branch.
  • Increases the current through the cell.
  • Increases the current through each branch.

Resistance

• Resistance is a measure of how easy or difficult it is for electrons to move through a material.
• Conductors have very low resistance, while insulators have very high resistance.
• A resistor is an electrical component that resists or hinders the flow of electric charges when it is connected in a circuit.
• Analogy: Traffic moving from three lanes to one lane. Cars move fast in three lanes, but when they have to merge into one lane, their speed slows down.
• Resistors are used to control the amount of current flowing in a circuit.

Resistance in Series and Parallel

• Series: High resistance, low current
• Parallel: Less resistance, high current

Parallel Circuits Calculations

• $V<em>{total} = V</em>1 = V<em>2 = V</em>3 = …$ (Voltage is the same across all parallel branches)
• $I<em>{total} = I</em>1 + I<em>2 + I</em>3 + …$
• $1/R<em>{total} = 1/R</em>1 + 1/R<em>2 + 1/R</em>3 + …$
• Another way to calculate total parallel resistance for two resistors:
  • $R<em>{tot} = (R</em>1 \times R<em>2) / (R</em>1 + R_2)$

Series Circuits Calculations

• $I<em>{total} = I</em>1 = I<em>2 = I</em>3 = ….$ (Current is the same throughout a series circuit)
• $V<em>{total} = V</em>1 + V<em>2 + V</em>3 + …$
• $R<em>{total} = R</em>1 + R<em>2 + R</em>3 + ….$