Basic Electrical Concept – SS1 Solar Photovoltaic Installation & Maintenance Lesson Note (Second Term)

Basic electrical concepts are the ideas of voltage, current, resistance and power that describe how electricity behaves in a circuit.

Behavioural objectives

1. Explain voltage, current and resistance using the water pipe comparison.
2. State the unit and symbol of voltage, current, resistance and power.
3. Write the formulas P = V × I and V = I × R.
4. Use the triangle trick to find V, I or R.
5. Solve simple problems using Ohm’s Law and the power formula.
6. Explain why thick wires are used for high current in solar installations.

What is an electric circuit?

An electric circuit is a closed path along which electric current flows.

• Source: supplies the voltage, for example a battery or a solar panel.
• Conductors: wires that carry the current, usually copper.
• Load: the device that uses the electricity, for example a bulb, TV or fan.
• Switch: opens or closes the path to stop or allow current.

How can a water pipe help us understand electricity?

A storage tank and a garden hose give a simple picture of how voltage, current and resistance work together.

To get water to flow out of a hose, you need three things:

1. Pressure: comes from the height of the tank.
2. The water itself: flows through the hose.
3. The size of the hose: wide or narrow.

In electricity, the same three things have scientific names:

• Voltage: the pressure that pushes the water.
• Current: the flow of water.
• Resistance: the narrowness of the pipe, which holds back the flow.

What are voltage, current, resistance and power?

Voltage, current, resistance and power are the four basic electrical quantities, each with its own unit and symbol.

1. What is voltage?

Voltage is the “push” that moves electric current through a circuit, measured in volts (V).

• Mains socket: Nigerian wall sockets supply about 220 volts.
• AA battery: gives 1.5 volts.
• The rule: the higher the voltage, the more push the electricity has.

2. What is current?

Current is the flow of electric charge through a wire, measured in amperes (A).

• More bulbs: switching on many bulbs asks for more flow, which means more current.
• The hazard: current is what actually gives you an electric shock.
• In solar: thick wires are used for high current so that the wires do not overheat and melt.

3. What is resistance?

Resistance is the opposition that slows down or stops the flow of electric current, measured in ohms (Ω).

• Thick wire: has low resistance, so electricity flows easily.
• Thin wire: has high resistance, so it is harder for electricity to pass.
• Long wire: has higher resistance than a short wire of the same kind.
• Heat: current flowing against resistance produces heat, as in an electric iron or a boiling ring.

4. What is power?

Power is the rate at which electrical work is done, measured in watts (W).

• 10 W bulb: uses little power.
• 2000 W air conditioner (AC): uses a lot of power.
• The formula: Power = Voltage × Current (P = V × I).
• Other forms: I = P ÷ V and V = P ÷ I.

What is the difference between direct current and alternating current?

Direct current (DC) flows in one direction only. Alternating current (AC) keeps changing direction.

What is Ohm’s Law?

Ohm’s Law states that the voltage across a conductor equals the current through it multiplied by its resistance (V = I × R).

• Discovered by: Georg Ohm, who found that voltage, current and resistance are always connected.
• Formula: V = I × R.
• Meaning: for the same resistance, more voltage gives more current.

How does the triangle trick work?

The triangle trick is a memory aid for rearranging V = I × R.

Draw a triangle with V at the top, and I and R at the bottom.

• To find V: cover V, you see I × R.
• To find I: cover I, you see V ÷ R.
• To find R: cover R, you see V ÷ I.

How do we solve problems with Ohm’s Law and the power formula?

Write the formula, substitute the values, then calculate.

1. Find voltage: I = 2 A, R = 6 Ω. V = I × R = 2 × 6 = 12 V.
2. Find current: V = 12 V, R = 4 Ω. I = V ÷ R = 12 ÷ 4 = 3 A.
3. Find resistance: V = 12 V, I = 3 A. R = V ÷ I = 12 ÷ 3 = 4 Ω.
4. Find power: V = 12 V, I = 5 A. P = V × I = 12 × 5 = 60 W.
5. Find current from power: a 120 W load on a 12 V battery. I = P ÷ V = 120 ÷ 12 = 10 A.

Why does Ohm’s Law matter in solar?

In a solar system, wire resistance wastes voltage and produces heat.

• Voltage loss: a 12 V battery connected with a very thin wire loses “pressure” along the wire, and the TV may not turn on.
• Low voltage means high current: a 12 V system needs more current than a higher-voltage system to deliver the same power (I = P ÷ V).
• Wire choice: thick wires have low resistance, so they carry high current safely.

Which instruments measure electrical quantities?

Voltage, current and resistance are measured with a voltmeter, an ammeter and an ohmmeter.

• Voltmeter: measures voltage and is connected across the component.
• Ammeter: measures current and is connected in line with the circuit.
• Ohmmeter: measures resistance.
• Multimeter: one instrument that can do all three jobs, used by solar technicians.

What are the electrical quantities at a glance?

Each quantity has a symbol, a unit and a unit symbol.

Evaluation

1. In the water pipe comparison, what do voltage, current and resistance stand for?
2. State the unit and unit symbol for voltage, current, resistance and power.
3. Write the formula for power and the formula for Ohm’s Law.
4. Use the triangle trick to write the formula for finding current. Then find the current when V = 12 V and R = 4 Ω.
5. Why do solar installations use thick wires for high current?

Assignment: class “Home Audit” activity

At home, look at the stickers on your appliances (chargers, TV, fans) and answer these questions.

1. Find one device that shows watts (W). Write down how many watts it shows.
2. Find your phone charger and look for the “Output”. It might say 5 V and 2 A. Using P = V × I, how many watts is your charger? (5 × 2 = ?)
3. Why do you think an electric heater has a thicker wire than a small desk lamp? (Hint: think about current, resistance and heat.)