Physics Lecture 5 Ohms Law 2

Ohm’s Law (formula)

V = I × R ; I = V ÷ R ; R = V ÷ I

Three factors in Ohm’s Law

Voltage (V), Current (I), Resistance (R)

What does Ohm’s Law state?

The voltage across a resistor is directly proportional to current through it and inversely proportional to its resistance.

Georg Ohm

Physicist who discovered the relationship between voltage, current, and resistance.

Circuit requirements

Source of potential difference (battery), conductor, and a load (resistor). Circuit must be closed for current to flow.

Current definition

Flow of electric charge (electrons move from negative to positive; conventional current is defined positive to negative).

Potential difference (definition)

The energy difference that pushes electrons; created by a source like a battery or generator.

Polarity in circuits

Electrons flow from negative to positive; conventional current flows from positive to negative.

Example problem: V=220 V, R=8.63 Ω → Find I

I = V ÷ R = 220 ÷ 8.63 ≈ 25.49 A

Electrical power (definition)

Amount of energy per unit time produced or consumed by a device. Unit: Watt (W). Formula: P = V × I

Work (definition)

Energy converted when current moves through a circuit. Unit: Joule (J). Formula: W = V × I × t = P × t

Example: Power with I=4 A, V=60 V

P = 4 × 60 = 240 W

Example: Work with I=4 A, V=60 V, t=6 min (360 s)

W = 4 × 60 × 360 = 86,400 J = 86.4 kJ

Power loss example: I=75 A, V=110 V

P = 75 × 110 = 8.25 kW (~8.4 kW)

Power with current & resistance example: I=25 A, R=8 Ω

P = I² × R = 25² × 8 = 5000 W = 5 kW

Series circuit definition

Components are end to end; current passes consecutively through each part.

Series circuit rules

Current is constant; total voltage = sum of voltages; total resistance = sum of resistances; total power = sum of powers.

Series circuit mnemonic

CIC = current constant, VAT = voltages add, RAT = resistances add, PAT = power adds.

Series circuit failure

If one component fails, the entire circuit goes out.

Series circuit example: R1=3kΩ, R2=10kΩ, R3=5kΩ

Total resistance = 18kΩ. Total current (9 V ÷ 18kΩ) = 0.0005 A = 500 µA.

Series circuit voltage distribution

R1: 1.5 V ; R2: 5 V ; R3: 2.5 V ; Total = 9 V

Parallel circuit definition

Components connected in branches; current is divided among branches.

Parallel circuit rules

Voltage is constant; total current = sum of branch currents; total resistance = reciprocal of sum of reciprocals.

Parallel circuit properties

If one branch fails, others continue. Adding resistors decreases total resistance and increases total current.

Parallel circuit resistance formula

1/Rt = 1/R1 + 1/R2 + … ; total resistance is always less than any single resistor.

Effect of adding more resistors (parallel)

Total resistance decreases; total amperage increases; may trip breaker or blow fuse.

Meters in circuits

Used to measure current, voltage, and resistance.

Ammeter (function)

Measures current; must be connected in series; reads same anywhere in a series circuit.

Voltmeter (function)

Measures potential difference (voltage); connected in parallel across a component.

Ohmmeter (function)

Measures resistance; usually handheld; connected in parallel with component.

Work problem practice

Example: Voltage in circuit with I=53 A and R=7 Ω: V= I × R = 371 V. Power = 53 × 371 = 19.66 kW. Power loss = I² × R = 19.6 kW.

Summary of Ohm’s Law usage

Use V=IR for circuit relationships, P=VI for power, W=VIt for work; apply series and parallel rules for combined circuits.