Electricity - Class 10 Science Notes (SEE Nepal)

Electricity is one of the most important chapters of Class 10 Science for SEE Nepal students. This chapter helps students understand the basic concepts of electric current, voltage, resistance, electrical energy, and their practical applications in daily life. Electricity is also one of the most frequent sources of numerical questions in the SEE Science examination, so every formula in this chapter must be practised until it becomes automatic.

Introduction to Electricity#

Electricity is a form of energy produced due to the movement or presence of electric charges. It is widely used in homes, industries, communication systems, and electronic devices. From the light bulb in your room to the computer you are reading this on, everything works because electric charges flow through conductors in a controlled way.

Electric Current#

Electric current is defined as the rate of flow of electric charge through a conductor.

Formula#

$$I = \frac{Q}{t}$$

Where:

• I = Electric current
• Q = Quantity of electric charge
• t = Time taken

The SI unit of electric current is Ampere (A). One ampere means one coulomb of charge passing through a cross-section of the conductor in one second.

Measuring Instrument#

Electric current is measured using an ammeter. An ammeter is connected in series with the electrical circuit.

---

Potential Difference (Voltage)#

Potential difference is the amount of work done to move a unit charge from one point to another in an electric field.

Formula#

$$V = \frac{W}{Q}$$

Where:

• V = Potential difference
• W = Work done
• Q = Electric charge

The SI unit of potential difference is Volt (V). The potential difference between two points is one volt when one joule of work is done to move one coulomb of charge.

Measuring Instrument#

Potential difference is measured using a voltmeter. A voltmeter is connected in parallel across the component.

---

Resistance#

Resistance is the property of a conductor that opposes the flow of electric current.

The resistance of a conductor depends on:

• Length of the conductor
• Area of cross-section
• Type of material
• Temperature

Formula#

$$R = \frac{V}{I}$$

Where:

• R = Resistance
• V = Voltage
• I = Current

The SI unit of resistance is Ohm (Ω). A conductor has a resistance of one ohm if one volt produces one ampere of current through it.

---

Ohm's Law#

Ohm's law states that:

At constant temperature, the current flowing through a conductor is directly proportional to the potential difference applied across it.

Mathematically:

$$V \propto I$$

or,

$$V = IR$$

Where:

• V = Potential difference
• I = Current
• R = Resistance

Ohm's law is the most important law of this chapter and is asked in almost every SEE Science paper — both as a statement question and as a numerical.

---

Resistors in Series and Parallel#

Series Combination#

When resistors are connected one after another so that the same current flows through each of them, they are said to be in series. The total (equivalent) resistance is the sum of all resistances:

$$R = R_1 + R_2 + R_3 + ...$$

The equivalent resistance of a series combination is always greater than the largest individual resistance.

Parallel Combination#

When resistors are connected between the same two points so that each one gets the full supply voltage, they are said to be in parallel. The equivalent resistance is given by:

$$\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + ...$$

The equivalent resistance of a parallel combination is always less than the smallest individual resistance. This is why household appliances are connected in parallel — each appliance receives the full voltage and works independently.

---

Electrical Energy#

Electrical energy is the amount of electrical work done by an electrical device.

Formula#

$$E = Pt$$

Where:

• E = Electrical energy
• P = Power
• t = Time

The commercial unit of electrical energy is kilowatt-hour (kWh).

Important Conversion#

1 unit of electricity = 1 kWh

One kilowatt-hour is the energy consumed by a device of power 1 kW operated for 1 hour. Electricity bills in Nepal are calculated in units (kWh), which is why you should always convert watts into kilowatts before multiplying by time.

---

Electrical Power#

Electrical power is the rate at which electrical energy is consumed or converted.

Formula#

$$P = VI$$

Where:

• P = Power
• V = Voltage
• I = Current

The SI unit of power is Watt (W). Since V = IR, power can also be written as:

$$P = I^2R \quad \text{or} \quad P = \frac{V^2}{R}$$

---

Heating Effect of Electric Current (Joule's Law)#

When current flows through a conductor, the conductor gets heated. This is called the heating effect of electric current. According to Joule's law, the heat produced in a conductor is:

$$H = I^2Rt$$

That is, the heat produced is directly proportional to the square of the current, the resistance of the conductor, and the time for which the current flows. This law is the working principle of electric heaters, electric kettles, irons, and most importantly, the fuse wire used for safety in every household circuit.

---

Solved Numerical Problems (SEE Pattern)#

Problem 1: A bulb of 100 W works on 220 V for 5 hours daily. Calculate the energy consumed in kWh and the cost at Rs 7.30 per unit.

Solution: P = 100 W = 0.1 kW, t = 5 h.

E = Pt = 0.1 × 5 = 0.5 kWh

Cost = 0.5 × 7.30 = Rs 3.65

Problem 2: A current of 2 A flows through a conductor of resistance 10 Ω. Calculate the voltage across it.

Solution: V = IR = 2 × 10 = 20 V

Problem 3: An electric heater of 2000 W is operated at 220 V. Find the current flowing through it.

Solution: I = P / V = 2000 / 220 = 9.09 A

Problem 4: Two resistors of 4 Ω and 6 Ω are connected (a) in series and (b) in parallel. Find the equivalent resistance in each case.

Solution (a): R = 4 + 6 = 10 Ω

Solution (b): 1/R = 1/4 + 1/6 = (3 + 2)/12 = 5/12, so R = 2.4 Ω

---

Household Electricity and Safety#

Fuse#

A fuse is a safety device that works on the heating effect of current. It is a thin wire of low melting point (usually tin-lead alloy) connected in series with the live wire. When the current exceeds the safe limit, the fuse wire heats up, melts and breaks the circuit, protecting appliances and preventing fire. A fuse of 5 A is used for lighting circuits and 15 A for heavy appliances like geysers and air conditioners.

MCB (Miniature Circuit Breaker)#

An MCB is a modern replacement for the fuse. It automatically switches off the circuit when the current exceeds the rated value, and unlike a fuse, it does not need to be replaced — it can simply be switched back on after the fault is removed.

Earthing#

Earthing connects the metallic body of appliances to the earth through a wire. If a live wire accidentally touches the metal body, the current flows to the earth instead of through the user, protecting people from electric shock.

---

Important SEE Exam Points#

• Electric current is measured in Ampere.
• Voltage is measured in Volt.
• Resistance is measured in Ohm.
• Ammeter is connected in series.
• Voltmeter is connected in parallel.
• Ohm's law is valid when temperature remains constant.
• Commercial unit of electricity is kilowatt-hour.
• Resistance increases with increase in length of conductor.
• Fuse works on the heating effect of electric current.
• 1 unit of electricity = 1 kWh.

---

Frequently Asked Questions#

1. What is electric current?#

Electric current is the rate of flow of electric charge through a conductor.

2. State Ohm's Law.#

Ohm's law states that current flowing through a conductor is directly proportional to the potential difference across its ends when temperature remains constant.

3. What is the SI unit of resistance?#

The SI unit of resistance is Ohm (Ω).

4. Which instrument measures electric current?#

An ammeter measures electric current.

5. What is Joule's law of heating?#

Joule's law states that the heat produced in a conductor is directly proportional to the square of the current, the resistance of the conductor, and the time for which the current flows (H = I²Rt).

6. Why is a voltmeter connected in parallel?#

A voltmeter is connected in parallel because it measures the potential difference across a component, and its resistance is very high so that only a negligible current flows through it.

7. What happens to the resistance of a wire when its length is doubled?#

Resistance is directly proportional to length, so the resistance becomes double.

8. Calculate the energy consumed by a 100 W bulb used for 5 hours.#

E = Pt = 0.1 kW × 5 h = 0.5 kWh, which is 0.5 unit of electricity.

9. What is the function of a fuse in a household circuit?#

A fuse protects the circuit from excessive current by melting and breaking the circuit when the current exceeds the safe limit.

---

Summary#

Electricity is an important chapter for SEE Class 10 Science. Understanding electric current, voltage, resistance, Ohm's law, series and parallel circuits, heating effect, power, and electrical energy helps students solve numerical problems and prepare effectively for the SEE examination. Practise at least the solved problems above, and rewrite every formula three times before the exam — numericals from this chapter almost always appear in the SEE paper.