Grade 10 electricity Electrical Machines – Inductors and Inductance Notes

Specific learning outcomes

• a) Explain the concept of energy storage in inductors.
• b) Construct inductors for use in electrical circuits.
• c) Analyse characteristics of an inductor in DC circuits.
• d) Devise DC circuits that incorporate inductors in an appliance.
• e) Appreciate application of inductors in day-to-day life.
• f) Identify categories: energy storage, inductor construction, inductor characteristics, DC circuit applications, and practical uses.

1. What is an inductor?

An inductor is a coil of wire that opposes changes in electric current through it. When current flows, a magnetic field forms around the coil and energy is stored in that magnetic field. The property that measures how well a coil stores magnetic energy is called inductance, symbol L (unit: henry, H).

2. Energy storage in inductors (Outcome a)

When current increases, the magnetic field grows and energy is stored. When current decreases, the field collapses and the inductor releases energy back into the circuit. Example: E = 1/2 L I². A larger L or larger current means more stored energy.

3. Constructing inductors (Outcome b)

Basic materials: insulated copper wire, a former (plastic tube or nail), and optionally an iron core. Steps for a simple coil:

1. Wrap the insulated copper wire evenly around the former (10–100 turns depending on desired L).
2. Leave 2–4 cm of free wire at each end for connections.
3. If an iron core (e.g., a soft iron nail) is inserted, inductance increases.
4. Insulate and fix the windings to prevent short-circuits.

How number of turns and core affect inductance: inductance L grows roughly with N² (N = number of turns) and with the magnetic permeability of the core (iron > air). Increasing coil area also increases L.

Safety note: do not connect large inductors to high voltage; avoid heating the coil; use teacher supervision.

4. Inductors in DC circuits — characteristics and analysis (Outcome c)

Important points for DC:

• When the switch is first closed, an inductor opposes the change in current. Current rises gradually (not instant).
• At steady-state with a constant DC, an ideal inductor behaves like a short circuit (only small resistance remains), so DC current passes freely.
• When the switch is opened, the inductor can produce a large voltage spike (back EMF) because it resists sudden fall in current.

Transient formulas (simple)

Time constant τ = L / R (seconds). This tells how fast current changes in an RL DC circuit with resistance R.

Current when switch closes (starting at 0): I(t) = I_final × (1 − e^(−t/τ)), where I_final = V / R.

Current when switch opens (decay): I(t) = I0 × e^(−t/τ).

5. Using inductors in DC appliance circuits (Outcome d)

Examples and simple designs:

• Choke (smoothing in DC power supplies): an inductor smooths ripples after rectification (use a coil in series with the load).
• Relay or solenoid coil: a DC coil creates magnetic force to move a switch or plunger (used in doorbells and automatic switches).
• Motor starter: inductors are part of starter circuits to limit surge current to motors.
• Spark suppression: placing a diode (flyback diode) across a coil in DC circuits prevents high-voltage spikes.

Simple appliance circuit idea for class project: build a battery-powered relay driver. Use a coil (relay), transistor switch, and diode to protect against back EMF. This shows how inductors control devices in everyday equipment.

6. Everyday applications (Outcome e)

Where students see inductors around them:

• Electric motors in fans, grinders and ceiling fans (motor coils are inductive).
• Doorbells and car starter solenoids (coils produce magnetic motion).
• Power supplies and phone chargers (chokes and inductors filter noise).
• Radio and TV — tuning circuits use inductors with capacitors to select stations.

Relate to Kenyan context: washing machine motors, boda-boda motorcycle ignition coils, and local radio receivers all use inductive components.

7. Categories and identification (Outcome f)

When identifying or classifying inductors, use these categories:

• Energy storage: coils designed to store magnetic energy (e.g., large inductors in power supplies).
• Construction: air-core vs iron-core, solenoids, toroidal coils.
• Characteristics: inductance value (L), DC resistance, saturation current, time constant in circuits.
• DC circuit applications: chokes, flyback inductors, relay coils.
• Practical uses: motors, transformers (related device), noise filters, tuners.

8. Suggested learning experiences & classroom activities

1. Construct a small coil: wind 50–100 turns of thin insulated copper wire on a plastic tube or empty pen. Test with a battery and a small bulb or LED (use resistor) to observe response when you switch on/off.
2. Observe transient: connect coil and resistor to a battery through a switch and measure current over time with a simple ammeter or multimeter logging — notice slow rise to steady value.
3. Compare air-core and iron-core coils: insert an iron nail and observe difference in strength (e.g., attraction of small iron filings) or measure inductance if possible.
4. Build a simple choke filter for a rectified DC LED lamp and observe reduced flicker.
5. Demonstrate back EMF: connect a small relay coil to a battery through a switch. When opening the switch, show why a diode (flyback diode) is needed to protect electronics.
6. Field visit/homework: identify inductive devices at home (fan, radio, charger) and list how the inductor is used.

Teacher tips: Use low voltages (1.5–9 V) and limit current. Supervise when using iron cores or larger batteries. Use insulated wire and ensure safe connections.

9. Simple assessment questions

1. State the formula for energy stored in an inductor and explain each symbol.
2. Why does current not change instantly in an inductor when a DC supply is switched on?
3. Describe how to make a simple inductor using materials found in school workshops.
4. Sketch a simple DC circuit using a battery, resistor and inductor and write the expression for current as a function of time after the switch is closed.
5. Name three everyday devices that use inductors and explain one way an inductor helps that device.

10. Summary checklist for learners

• Can explain energy stored by an inductor (E = ½ L I²).
• Can build a simple coil and know how turns & core change L.
• Can explain DC transient behaviour and time constant τ = L/R.
• Can design a simple DC circuit using an inductor and protect it with a diode.
• Can list practical uses in everyday equipment.