Electrical Quantities

Quiz 1:
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Chapter 16: Electrical Quantities – Theory and Formulas

16.1 Electric Charge

Atoms contain:

• Protons – Positive charge
• Neutrons – No charge
• Electrons – Negative charge

An atom is neutral when it has equal numbers of protons and electrons.

Charging by Friction

Electrons transfer from one material to another. The material that loses electrons becomes positively charged, and the one that gains electrons becomes negatively charged.

Interaction of Charges

• Like charges repel
• Unlike charges attract

Conductors and Insulators

• Conductors: Allow electrons to move freely (e.g., metals)
• Insulators: Hold electrons tightly (e.g., plastic, glass)

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16.2 Electric Field

An electric field is a region where a charge experiences a force.

Field Line Directions

• Positive charge: Field lines point away
• Negative charge: Field lines point toward

Field Strength

Stronger where field lines are closer together.

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16.3 Electric Current

Electric current is the flow of electric charge.

Formula

I = Q / t
Where:
I = Current (A)
Q = Charge (C)
t = Time (s)

Conventional vs. Electron Flow

• Conventional current: From positive to negative
• Electron flow: From negative to positive

Measuring Current

• Use an ammeter in series

Types of Current

• DC (Direct Current): Flows in one direction
• AC (Alternating Current): Changes direction periodically

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16.4 Electromotive Force (e.m.f.) and Potential Difference (p.d.)

Electromotive Force (e.m.f.)

Energy provided by a source per unit charge.

E = W / Q
Where:
E = e.m.f. (V)
W = Work done (J)
Q = Charge (C)

Potential Difference (p.d.)

Energy used by a component per unit charge.

V = W / Q
Where:
V = p.d. (V)
W = Work done (J)
Q = Charge (C)

Measuring Voltage

• Use a voltmeter in parallel

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16.5 Resistance

Resistance measures how much a component opposes current flow.

Formula

R = V / I
Where:
R = Resistance (Ω)
V = Voltage (V)
I = Current (A)

Ohm's Law

For ohmic conductors: V ∝ I (at constant temperature)

Factors Affecting Resistance

• Length (l): R ∝ l
• Cross-sectional area (A): R ∝ 1/A
• Material and temperature

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16.6 Electrical Energy and Power

Power

P = W / t = V × I
Where:
P = Power (W)
W = Energy (J)
t = Time (s)
V = Voltage (V)
I = Current (A)

Energy

E = P × t = V × I × t

Energy Units

• Joule (J): Small unit
• Kilowatt-hour (kWh): Used in electricity bills

Cost Calculation

Cost = Energy (kWh) × Price per kWh

Note: Always ensure meters and units are correctly matched in calculations.

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