Simple Magnetism And Magnetic FieldsCopy

Cheat Sheet: Electricity – Charge, Current, e.m.f., p.d., and Resistance (O Level / IGCSE Physics)

4.2.1 Electrical Charge

1. Charge Basics

• Two types: positive (+) and negative (−)
• Unit: Coulomb (C)

2. Charge Interaction

• Unlike charges attract
• Like charges repel

3. Electrostatic Charging by Friction

• Experiment: Rub a balloon on hair or plastic rod with cloth
• Results in electrostatic attraction/repulsion (e.g. balloon sticks to wall)

4. Only Electrons Move

• Negative charge (electrons) transfer from one object to another
• Object gains electrons → becomes negatively charged
• Loses electrons → becomes positively charged

5. Electric Field

• Region where a charge experiences a force

6. Electric Field Direction

• Field lines show the direction of force on a positive test charge

7. Electric Field Patterns

• (a) Point charge: radial lines outward (positive), inward (negative)
• (b) Charged sphere: same as point charge
• (c) Between two plates: parallel, straight lines from + to −

8. Conductors and Insulators

• Conductors: allow current to flow (e.g. metals like copper, aluminum)
• Insulators: block current flow (e.g. rubber, plastic, wood)

9. Conductor vs Insulator Experiment

• Insert material into a simple circuit (battery + bulb)
• If bulb lights → conductor
• If not → insulator

10. Electron Model Explanation

• Conductors: have free electrons that move easily
• Insulators: electrons are bound tightly, don’t move freely

4.2.2 Electrical Current

1. Definition

• Current (I) = charge flow per unit time
• Equation:
  I = Q / t
  where:
  I = current (A), Q = charge (C), t = time (s)

2. Conduction in Metals

• Free electrons move through metal and carry charge

3. Current Units

• Measured in amperes (A)
• 1 A = 1 Coulomb per second (C/s)

4. Direct vs Alternating Current

• d.c.: flows in one direction (batteries)
• a.c.: changes direction regularly (mains electricity)

5. Conventional Current vs Electron Flow

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

6. Using Ammeters

• Connected in series
• Use correct range setting for digital or analogue meters

4.2.3 Electromotive Force (e.m.f.) & Potential Difference (p.d.)

1. e.m.f. Definition

• Work done by source to move 1 C of charge in a complete circuit
• Equation:
  E = W / Q
  E = e.m.f. (V), W = work done (J), Q = charge (C)

2. p.d. Definition

• Work done on a component per unit charge
• Equation:
  V = W / Q

3. Units

• Both e.m.f. and p.d. measured in volts (V)
• 1 V = 1 J/C

4. Using Voltmeters

• Connected in parallel
• Use appropriate range setting

5. e.m.f. in Series

• Total e.m.f. = sum of individual sources
  (if aligned in same direction)

6. e.m.f. in Parallel

• For identical sources in parallel, total e.m.f. = e.m.f. of one source

4.2.4 Resistance

1. Definition & Equation

• Resistance (R) = opposition to current
• Equation:
  R = V / I
  R = resistance (Ω), V = p.d. (V), I = current (A)

2. Experiment to Find Resistance

• Set up circuit with voltmeter in parallel and ammeter in series
• Measure V and I
• Apply R = V / I

3. Resistance vs Wire Properties

• Resistance ∝ length of wire
• Resistance ∝ 1 / area (thicker wire = less resistance)

4. Ohm’s Law

• At constant temperature, V ∝ I
• Graph: straight line through origin
  (Applies to ohmic conductors like resistors)

5. I–V Graphs

6. Effect of Temperature

• As temperature increases, resistance of metal (e.g. filament) increases
• Due to more lattice vibrations → more collisions with electrons