Grade 10 physics Electricity and Magnetism – Current Electricity Notes

Strand:   3.0 Electricity and Magnetism  Sub-Strand:   3.2 Current Electricity  Specific Learning Outcomes:   By the end of the sub-strand, the learner should be able to: a) explain the terminologies used in current electricity, b) verify the relationships  𝑉 = 𝐼 𝑅 , 𝐸 = 𝐼 ( 𝑅 + 𝑟 )  as used in current electricity, c) determine the resistance and resistivity of various conductors using different methods, d) determine effective resistance of various resistor networks, e) determine the relationship of potential difference and current to power as used in heating effect of an electric current, f) appreciate the applications of current electricity in day-to-day life, g) Terminologies: current, potential difference, electromotive force, internal resistance, h) Resistor classification: ohmic and non-ohmic, i) Factors affecting resistance of ohmic resistors: temperature, length, cross-sectional area, type, j) Methods of determining resistance: resistor colour codes, ammeter-voltmeter method, Wheatstone bridge, metre bridge, resistor networks.)

Key concepts — simple and clear

• Current (I) — flow of electric charge through a conductor, measured in amperes (A).
• Potential difference (V) — energy per unit charge between two points, in volts (V).
• Electromotive force (E) — the open-circuit voltage supplied by a source (e.g., a cell).
• Internal resistance (r) — small resistance inside a cell/battery which reduces terminal voltage under load.
• Resistance (R) — how much a component opposes current, measured in ohms (Ω).
• Resistivity (ρ) — material property: R = ρ · L / A (L = length, A = cross-sectional area).
• Ohmic vs Non‑ohmic — ohmic: V ∝ I (constant R, e.g., metal wire at moderate T). Non‑ohmic: R changes with V or current (e.g., filament lamp, diode).

Important formulas (to verify / use)

• Ohm's law: V = I·R
• Cell supplying current with internal resistance: E = I(R + r) (or terminal V = E − I r)
• Resistance of wire: R = ρ · L / A
• Series resistors: R_total = R1 + R2 + ...
• Parallel resistors (two): 1/R_total = 1/R1 + 1/R2 (or R_total = R1·R2 / (R1+R2) )
• Electric power (heating effect): P = V·I = I²·R = V² / R

Visuals — simple circuit sketches

Measuring resistance — practical methods (simple explanation)

• Resistor colour codes — read coloured bands to get nominal resistance and tolerance (useful for fixed resistors in kits and school lab components).
• Ammeter–Voltmeter method
  Place the ammeter in series with the resistor to measure I, and the voltmeter in parallel to measure V across the resistor. Then R = V / I.
  Note: Use appropriate meter ranges. Minimise meter error by using high-resistance voltmeter and low-resistance ammeter when possible.
• Wheatstone bridge — accurate lab method: balance condition gives unknown resistor from known arms; nice for precise school measurements.
• Metre bridge — simpler version of Wheatstone using a long uniform wire and sliding contact: used in many school practicals to find resistivity or unknown R.

Determining resistivity (ρ) — idea for a school experiment

1. Measure length L of a uniform wire sample and its diameter d (use micrometer or vernier). Compute area A = π(d/2)².
2. Use metre‑bridge or ammeter‑voltmeter to find R of the sample.
3. Use R = ρ·L/A to calculate ρ. Compare with tabulated values (copper, aluminium, nichrome).

Heating effect of current — everyday relevance

Power dissipated as heat: P = I²R = V·I = V² / R. This explains how kettles, irons, electric stoves and small heaters work. Relate to energy cost: Energy (J) = P (W) × time (s). In Kenya, understanding power helps students compare appliances (e.g., 1000 W kettle vs 200 W immersion heater).

• Example: A 230 V heater of resistance R uses P = V² / R. If P = 1000 W, current ≈ 1000/230 ≈ 4.35 A.
• Practical note: high current leads to more heating in wires — importance of correct wire gauge and fuses.

Factors affecting resistance (easy points)

• Temperature — for most metals, resistance increases with temperature (important for bulbs and cables).
• Length (L) — longer wire → more resistance (R ∝ L).
• Cross-sectional area (A) — thicker wire → less resistance (R ∝ 1/A).
• Material (ρ) — copper and aluminium lower ρ than nichrome (used for heating elements).

Applications & safety — relate to daily life in Kenya

• Domestic wiring: correct fuse rating and wire thickness prevent overheating and fires.
• Household appliances: understanding power rating helps manage electricity bills.
• Electronics kits and school projects: using resistors correctly to protect LEDs and components.
• Public safety: earthing, circuit breakers and safe handling of batteries and chargers.

Suggested classroom & practical activities (age 15, Kenyan schools)

1. Build simple series & parallel circuits on a breadboard; measure V and I, and verify R_total calculations.
2. Use the ammeter‑voltmeter method to measure an unknown resistor; compare with colour-code value.
3. Metre‑bridge practical: find unknown resistance and compute resistivity of a wire sample; discuss measurement errors.
4. Measure current drawn by small appliances (phone charger, lamp) and calculate power; discuss energy use and cost.
5. Group project: investigate how temperature affects resistance of a filament lamp or a metal wire (qualitative observation of brightness/reading changes).
6. Safety exercise: identify correct fuse ratings for common household appliances and explain why.

Short worked example

Tips for exam revision

• Memorise key formulas and know when to use each form of power expression.
• Practice series/parallel reductions with mixed networks (start by simplifying simple parallel or series sections first).
• Be able to sketch correct ammeter/voltmeter connections — wrong connection gives incorrect readings.
• For practical questions, always state sources of error (meter resistance, contact resistance, heating).