grade 8 Electric Circuit

Electric Circuit & Ohm's Law

An electric circuit is a closed path where voltage drives current and resistance opposes the flow. Ohm's law V = IR links the three quantities: higher V means larger I, and higher R means smaller I. In series the current is the same and voltage divides; in parallel the voltage is the same and current divides. Change voltage and resistance and the current shifts; series and parallel sit side by side.

What is a Circuit?

🔋 Understand Circuits as Water Flow

①Voltage (V) = water-tank height difference → pushes current

②Current (I) = water flow rate → charge flowing through the wire

③Resistance (R) = narrow pipe → opposes current

Electric-Circuit Diagram

Voltage V6V

Resistance R3Ω

💡 Observe

①Higher voltage → larger current (taller tank → faster water)

②Higher resistance → smaller current (narrower pipe → less flow)

③Current = voltage ÷ resistance

Ohm's Law — V = IR graph

Ohm's Law

V = I × R

Voltage (V) = current (A) × resistance (Ω)

Solving for Current

I = VR

Current (A) = voltage (V) ÷ resistance (Ω)

Series vs Parallel

Series Connection

R_total = R₁ + R₂ + R₃ + ...

Same current, voltage is divided

Parallel Connection

1R_total = 1R₁ + 1R₂ + ...

Same voltage, current is divided

💡 Series vs Parallel Analogy

①Series: pass narrow streets in turn → resistance accumulates

②Parallel: many roads simultaneously → total resistance decreases

③Home outlets are parallel — each device gets the same 220 V

Summary

Ohm's Law (Core)

V = IR, I = V/R, R = V/I

Knowing two of V, I, R lets you find the third

🎯 Exam Key Points

①Ohm's Law: V = IR ties V, I, R together

②Series: same current, divided voltage, summed resistance

③Parallel: same voltage, divided current, reciprocal sum of resistance

④Ammeter: connected in series (low internal resistance)

⑤Voltmeter: connected in parallel (high internal resistance)

Examples and Unit Test

Example 1

If a 3 Ω resistor and a 5 Ω resistor are connected in series, what is the total (combined) resistance?

In a series connection, the total resistance is the sum of the resistances (R_total = R₁ + R₂).

3 Ω + 5 Ω = 8 Ω.

▸ 8 Ω

In a series connection, the combined resistance is the sum of the resistances.

Example 2

When bulbs are connected in series versus in parallel, how do the voltage across each bulb and the current through it differ?

In a series connection the current is the same everywhere, and the voltage is shared among the bulbs.

In a parallel connection the same voltage is applied across each bulb, and the current splits among the branches.

▸ Series = same current, shared voltage / parallel = same voltage, split current

Series keeps the current constant; parallel keeps the voltage constant.

Unit-test style

Which statement about series and parallel circuit connections is correct?

①In a series connection, the total resistance is the sum of the resistances

②In a series connection, the current is different through each bulb

③In a parallel connection, the voltage is different across each bulb

④In a series connection, if one bulb breaks the others stay lit

⑤Ohm law is V = I ÷ R

▸ ① In a series connection, the total resistance is the sum of the resistances

In a series connection the resistances add up, so the total resistance = R₁ + R₂ + ...

In series the current is the same everywhere (②), in parallel the voltage is the same (③), in series if one point breaks all go off (④), and Ohm law is V = IR (⑤).

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Static Electricity

Ohm's Law

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