Voltage, Resistance, & Capacitance: Basic Electricity – JSS1 Basic Technology Lesson Note (Third Term)

Basic electricity deals with voltage, resistance, inductance and capacitance, which are the main quantities that describe how electricity behaves in a circuit.

Behavioural objectives

1. Define voltage and resistance and state their units and measuring instruments.
2. Define inductor, inductance and induced current.
3. Define capacitor and capacitance and explain how a capacitor works.
4. Name the types of capacitor.
5. Describe the parts and uses of a transformer.
6. Use the transformer formula to find a secondary voltage.

What is voltage?

Voltage is the work done in moving a unit of electric charge between two points in an electric field.

• Other name: potential difference (p.d.).
• Unit: the volt (V).
• Instrument: a voltmeter, connected across the component being measured.
• Everyday example: a dry cell gives about 1.5 V, while mains supply in Nigeria is about 220 V to 240 V.

What is resistance?

Resistance is the opposition that a circuit, component or substance presents to the flow of electricity.

• Symbol: R.
• Unit: the ohm (Ω).
• Instrument: an ohmmeter.
• Resistor: a component made to oppose the flow of electricity in a circuit.

What affects the resistance of a wire?

• Length: a longer wire has more resistance.
• Thickness: a thicker wire has less resistance.
• Material: copper resists less than iron, so copper is used for cables.
• Temperature: the resistance of a metal wire rises when it gets hotter.

What is an inductor?

An inductor is a circuit element, usually a compact coil, that produces an induced voltage in response to a changing current.

• Action: it opposes any change in the current in a circuit.
• Inductance: the ability of a coil to generate an induced electromotive force (e.m.f.) when the current through it changes.
• Unit of inductance: the henry (H).

What is induced current?

Induced current is the current made to flow by an induced e.m.f. in a conductor.

• Induced e.m.f.: the e.m.f. set up in a conductor by relative motion between the conductor and a magnetic field.
• Simple example: current generated when a magnet moves in a coil.
• Larger induced e.m.f.: move the magnet faster, use a stronger magnet or use more turns of wire.

What is a capacitor?

A capacitor is a device for storing electrical charge.

• Capacitance: the ratio of the electric charge on the plates to the potential difference across them.
• Formula: C = Q / V, where Q is charge and V is voltage.
• Other meaning: the charge-storing ability of a capacitor.
• Unit: the farad (F).

How does a capacitor work?

A capacitor works by holding opposite charges on two parallel plates that are separated by an insulator.

1. Place two flat plates parallel to each other.
2. Connect a cell to the plates at points A and B.
3. One plate gains positive charge and the other gains negative charge.
4. After charging for some time, remove the cell.
5. The plates keep their charges, so the pair acts as a bank, or store, of electric charge.

What are the parts of a capacitor?

• Plates: the two conducting surfaces, usually called electrodes.
• Dielectric: the insulator in the gap between the plates.
• Dielectric materials: air, mica, paper or rubber, chosen by the capacitance required.
• Case: plates and dielectric are rolled up to a convenient size and enclosed in plastic, cardboard or metal.

What are the types of capacitor?

There are five types of capacitor named at this level: air, paper, mica, electrolytic and polyethene.

1. Air capacitor: the dielectric is air. Two aluminium plates are used, with one part fixed and the other turned to change the capacitance.
2. Paper capacitor: a thin strip of paraffin-waxed paper is rolled between coatings of thin aluminium foil.
3. Mica capacitor: the dielectric is a solid material, mica.
4. Electrolytic capacitor: it stores a large charge in a small size and has marked positive and negative terminals.
5. Polyethene capacitor: it uses a thin plastic film as the dielectric.

How else are capacitors classified?

What are the uses of capacitors?

• Vehicles: used in the electrical circuits of a motor vehicle.
• Electronics: used in many electronic circuits, such as radios.
• Power supplies: used to store charge and smooth the supply.

What is a transformer?

A transformer is a device that transfers electrical energy from one circuit to another by electromagnetic induction.

• Supply: it works only with alternating current (a.c.).
• Main use: to step a supplied voltage up or down.

What are the parts of a transformer?

• Primary winding (A): the coil connected to the input supply.
• Secondary winding (B): the coil that gives the output voltage.
• Soft iron core: carries the magnetic flux and helps to minimise flux leakage.

What are the types of transformer?

There are two types of transformer by use: step-up and step-down.

What is the transformer formula?

The ratio of primary voltage to secondary voltage equals the ratio of primary turns to secondary turns.

E_p / E_s = N_p / N_s

• E_p: voltage in the primary.
• E_s: voltage in the secondary.
• N_p: number of turns in the primary.
• N_s: number of turns in the secondary.

What are the uses of transformers?

• Step-down at home: a step-down transformer can change 220 V to 110 V for an appliance made for 110 V.
• Small transformers: step down voltage for very low-voltage household appliances.
• Substations: very large transformers step high voltage, for example from the Kainji dam, down to low voltage for residential houses.

Worked example

The primary voltage of a transformer is 10 volts and the primary winding has 20 turns. If the secondary winding has 100 turns, find the secondary voltage.

Solution

1. Write the given data: E_p = 10 V, N_p = 20 turns, N_s = 100 turns, E_s = ?
2. Write the formula: E_p / E_s = N_p / N_s
3. Make E_s the subject: E_s = (E_p × N_s) / N_p
4. Substitute: E_s = (10 × 100) / 20
5. Answer: E_s = 50 volts. This is a step-up transformer, since the voltage increased.

Evaluation

1. Define (a) inductor (b) inductance (c) induced current.
2. Define (a) capacitor (b) the capacitance of a capacitor (c) electrode.
3. Explain the principle of operation of a capacitor.
4. Name the three parts of a transformer.
5. A transformer has a primary voltage of 200 V, 400 primary turns and 100 secondary turns. Find the secondary voltage.

Assignment

1. (a) State the unit and the measuring instrument for voltage and for resistance. (b) List the five types of capacitor named in this note.