grade 8 Ohm's Law

Ohm's Law

Ohm's law is the basic link among voltage, current, and resistance, written V = IR. In a water-pipe picture, pressure is voltage, flow is current, and a narrow pipe is resistance. Raising V raises I; raising R lowers I; and the slope of a V-I graph is the resistance. Change voltage and resistance and the current is found; ammeter and voltmeter connections sit beside it.

Voltage, Current & Resistance

🚿 Ohm's Law Through Water Pipes

①Pressure (V) — higher pressure pushes water faster

②Flow (I) — the actual amount flowing

③Narrow pipe (R) — narrower pipe lets less water through

V-I Graph View

Voltage V6V

Resistance R3Ω

💡 Current Now: 2.0 A

①Slope of the line = Resistance (R)

②Larger R → steeper slope → more voltage needed for the same current

③Raise V → current rises; raise R → current falls

Ammeter & Voltmeter

Connecting an Ammeter

Ammeter → series in circuit (internal resistance ≈ 0)

Connect inline with the section being measured for accurate current

Connecting a Voltmeter

Voltmeter → parallel across the section (internal resistance ≈ ∞)

Connect across the two ends to measure voltage accurately

Ohm's Law Formulas

Ohm's Law

V = I × R

Voltage (V) = Current (A) × Resistance (Ω)

Finding Current

I = VR

Current (A) = Voltage (V) ÷ Resistance (Ω)

Finding Resistance

R = VI

Resistance (Ω) = Voltage (V) ÷ Current (A)

Summary

Ohm's Law (Core Formula)

V = IR

Voltage = Current × Resistance — knowing any two gives the third

🎯 Exam Points

①V = IR: V↑ → I↑ (R fixed), R↑ → I↓ (V fixed)

②V-I graph slope = resistance R — steeper means larger R

③Ammeter: series connection, very low internal resistance

④Voltmeter: parallel connection, very high internal resistance

⑤Ohmic conductor: current rises proportionally with voltage (straight line)

Examples and Unit Test

Example 1

If 12 V is applied to a bulb with a resistance of 4 Ω, what current flows? (Ohm law)

By Ohm law, the current I = V ÷ R (current = voltage ÷ resistance).

I = 12 V ÷ 4 Ω = 3 A.

▸ 3 A

By Ohm law V = IR, the current is I = V ÷ R.

Example 2

In a circuit, how is an ammeter (which measures current) connected, and how is a voltmeter (which measures voltage) connected?

An ammeter must be connected in series (in one line) with the part being measured for an accurate reading.

A voltmeter must be connected in parallel (side by side) across the section being measured for an accurate reading.

▸ Ammeter = series / voltmeter = parallel

An ammeter is connected in series; a voltmeter in parallel.

Unit-test style

Which statement about Ohm law is correct?

①At constant voltage, a larger resistance gives a larger current

②Ohm law is expressed as V = IR

③An ammeter is connected in parallel in a circuit

④At the same resistance, raising the voltage decreases the current

⑤On a V-I graph, the slope represents the voltage

▸ ② Ohm law is expressed as V = IR

Ohm law is voltage = current × resistance, that is V = IR.

A larger resistance gives a smaller current (①), an ammeter is connected in series (③), raising the voltage increases the current (④), and the slope of a V-I graph represents the resistance (⑤).

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