Electrical Energy – JSS3 Basic Science Lesson Note (Second Term)

Electrical energy is the energy that appliances use when an electric current flows through them.

• Uses: homes, offices and industries use it to run blenders, televisions, irons and fridges.
• Covered in this note: circuits, their components, basic terms, house wiring and meter billing.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define electric current and electric circuit;
2. explain series and parallel connections and state the formula for total resistance in each;
3. name the components of a circuit and state the use of each;
4. state the relationship between V, I and R (Ohm’s law), and explain potential difference and e.m.f.;
5. explain why houses are wired in parallel;
6. work out the number of units consumed from meter readings.

What is electric current?

Electric current is the flow of electrons from one point to another in a circuit.

• Path: the electrons move through a metallic conductor called a wire.
• Unit: current is measured in amperes (A).
• Direction: conventional current is taken to flow from the positive terminal to the negative terminal.
• Electron flow: electrons move the opposite way, from the negative terminal to the positive terminal.

What is a simple electric circuit?

An electric circuit is the path through which an electric current flows from a cell or battery to an appliance, such as a lamp, and back to the cell.

• Closed circuit: the path is complete, so current flows and the lamp lights.
• Open circuit: the path is broken, for example by an open switch or a broken wire, so no current flows.

What are the ways of connecting a circuit?

There are two ways of connecting a circuit: series connection and parallel connection.

What is a series connection?

In a series connection, the circuit elements are joined end to end so that the same current flows through each element.

• Current: the same in every element.
• Potential difference: shared among the elements.
• Break in the circuit: if one element fails, the whole circuit stops working.
• Total resistance: R = R₁ + R₂ (add more terms for more resistors).

Example: two resistors of 4 Ω and 6 Ω in series give R = 4 + 6 = 10 Ω.

What is a parallel connection?

In a parallel connection, the high potential terminals of the elements are joined to one point and their other terminals are joined to another point.

• Potential difference: the same across every element.
• Current: different in each element, and it divides at the joining point.
• Break in the circuit: if one element fails, the others keep working.
• Total resistance: 1/R = 1/R₁ + 1/R₂.

Example: 4 Ω and 6 Ω in parallel give 1/R = 1/4 + 1/6 = 5/12, so R = 2.4 Ω.

What is the difference between series and parallel connections?

What are the components of a circuit?

The components of a circuit are the parts joined together to make a circuit work.

• Cell or battery: the source of the electrical energy.
• Resistor: a wire or other material used to reduce or resist the flow of current. Its unit is the ohm (Ω).
• Lamp or bulb: used in a circuit to indicate the flow of electricity.
• Switch: used in a circuit to put the electric current on or off.
• Ammeter: an instrument used to measure the amount of current flowing. Its unit is the ampere (A).
• Voltmeter: an instrument used to measure potential difference.
• Galvanometer: an instrument used to detect the flow of an electric current.
• Fuse: a piece of wire made of a tin-lead alloy with a low melting point. It cuts off the electric current when there is danger in the circuit.
• Circuit breaker: a modern device introduced into the house circuit to increase safety in the use of electricity.

What are the basic terms used in electricity?

The basic terms are current, resistance, potential difference and electromotive force.

• Current (I): the flow of electrons along a conductor.
• Resistance (R): the ratio of the potential difference (V) across the ends of a conductor to the current (I) flowing through it.
• Potential difference (p.d.): the difference in electrical potential between the terminals of a cell or battery in a closed circuit.
• Electromotive force (e.m.f.): the electrical driving force between the terminals of a cell or battery in an open circuit.

What is Ohm’s law?

Ohm’s law states that the current through a conductor is directly proportional to the potential difference across it, if the temperature stays the same.

• Resistance: R = V/I
• Potential difference: V = I × R
• Current: I = V/R

Example: a 12 V battery across a 4 Ω resistor gives I = 12/4 = 3 A.

What are the symbols and units?

What are the parts of a house circuit?

A house circuit is the wiring that carries electricity from the supply to the sockets, lights and appliances in a building.

• Sockets: used for connecting electrical appliances.
• Fuse box: holds the fuses that protect the circuits.
• Control switch: used for putting off the electricity supply during repair or installation.
• Circuit breaker: switches off the supply when the current is too high for safety.

Why are houses wired in parallel?

Houses are wired in parallel so that each appliance works on its own and gets the full supply voltage.

• Independent use: if one bulb in the house burns out, the others stay on.
• Series circuit: all the bulbs go off as soon as one of them burns out.
• Separate switches: each appliance can be switched on or off without affecting the others.

How are electric meter reading and billing done?

An electric meter is a device that measures the amount of electric energy consumed by a residence or an industry through its electric gadgets.

1. Read the previous meter reading from the last bill.
2. Read the present meter reading.
3. Units consumed = present reading minus previous reading.
4. Total amount = units consumed × cost per unit.

Worked example: 48167 – 47896 = 271 units. Then 271 × ₦12.91 = ₦3,498.61.

Evaluation

1. Define electric current.
2. Write the formula for the total resistance of two resistors connected (a) in series and (b) in parallel.
3. State the use of a fuse and of a switch in a circuit.
4. Give one reason why houses are wired in parallel.
5. The previous meter reading is 47896 and the present reading is 48167. How many units were consumed?

Assignment

1. The flow of current is usually in the opposite direction to the flow of:
   
   A. neutrons B. protons C. electrons D. atoms
2. Electric circuits are connected in _____ ways:
   
   A. four B. two C. three D. five
3. A voltmeter is an instrument for measuring:
   
   A. current B. resistance C. capacitance D. potential difference
4. What is the name of the electrical element in the house circuit that is used for connecting electrical appliances?
   
   A. sockets B. fuse box C. control switch D. circuit breaker
5. What is the name of the electrical element in the house circuit that is used for putting off the electricity supply whenever there is a repair or installation?
   
   A. sockets B. fuse box C. control switch D. circuit breaker
6. The amount of current flowing through an electric circuit is measured by:
   
   A. ammeter B. ohmmeter C. voltmeter D. fuse
7. The unit of measuring current is:
   
   A. ohm B. voltage C. coulomb D. ampere
8. • (a) Mention four components of the electric circuit.
   • (b) Explain the term electric circuit.
   • (c) Mention two ways of connecting the electric circuit.
9. What is the full meaning of e.m.f.?