Electricity
Understanding electric current, circuits, resistance, and the heating effect of electric current
Introduction to Electricity
Electricity has an important place in modern society. It is a controllable and convenient form of energy for a variety of uses in homes, schools, hospitals, industries and so on. In this chapter, we will study what constitutes electricity, how it flows in an electric circuit, and the factors that control or regulate the current through an electric circuit.
Key Concepts
- Electric current and circuits
- Electric potential and potential difference
- Ohm's law and resistance
- Factors affecting resistance of a conductor
- System of resistors (series and parallel)
- Heating effect of electric current
- Electric power and energy
We will also discuss the heating effect of electric current and its applications in various electrical devices.
Learning Outcomes
After studying this chapter, students will be able to:
- Define electric current and its unit
- Understand potential difference and its measurement
- State and apply Ohm's law
- Explain factors affecting resistance of a conductor
- Calculate equivalent resistance in series and parallel combinations
- Understand Joule's law of heating and its applications
- Calculate electric power and energy consumption
- Solve numerical problems related to electricity
Period-Wise Teaching Plan
This chapter is designed to be covered over 6 periods, each lasting 45 minutes. Below is the detailed period-wise plan:
Topics: Electric current, charge, circuit components, conventional current vs electron flow.
Activities: Demonstration of simple electric circuit, discussion on current flow.
Topics: Electric potential, potential difference, volt, Ohm's law, resistance.
Activities: Experiment to verify Ohm's law, measurement using voltmeter.
Topics: Resistance, resistivity, factors affecting resistance, applications.
Activities: Experiment with wires of different materials, lengths, and thicknesses.
Topics: Series combination, parallel combination, equivalent resistance.
Activities: Experimental verification of series and parallel resistance formulas.
Topics: Joule's law, applications of heating effect, electric fuse.
Activities: Demonstration of heating in wire, discussion on electrical safety.
Topics: Electric power, electrical energy, commercial unit of energy, revision.
Activities: Calculation of power consumption, energy bills, chapter quiz.
Teaching Methodology
The teaching approach for this chapter should be a blend of:
- Interactive lectures with visual aids and circuit diagrams
- Hands-on experiments and demonstrations
- Problem-solving sessions for numerical problems
- Group discussions on applications of electricity
- Real-life examples and case studies
- Regular assessment through quizzes and worksheets
Electric Current and Circuit
Electric current is the flow of electric charge. In metallic conductors, this flow is due to the movement of electrons.
Electric Current: The rate of flow of electric charge. Mathematically, I = Q/t, where I is current, Q is charge, and t is time.
I = Q / t
Where:
I = Electric current (in amperes, A)
Q = Electric charge (in coulombs, C)
t = Time (in seconds, s)
Unit of Current
The SI unit of electric current is ampere (A). One ampere is defined as one coulomb of charge flowing per second.
Conventional Current vs Electron Flow
- Conventional current: Direction of flow of positive charges (from positive to negative terminal)
- Electron flow: Direction of flow of electrons (from negative to positive terminal)
- In circuit analysis, we use conventional current direction
Electric Circuit
An electric circuit is a closed path through which electric current can flow. It consists of:
- Source: Battery or cell that provides energy
- Conductors: Wires that provide path for current
- Load: Device that uses electrical energy (bulb, resistor, etc.)
- Switch: Device to open or close the circuit
- Measuring instruments: Ammeter, voltmeter
Construct a simple circuit using a cell, connecting wires, and a bulb. Show how the bulb lights up when the circuit is closed and goes off when the circuit is open.
A current of 0.5 A is drawn by a filament of an electric bulb for 10 minutes. Find the amount of electric charge that flows through the circuit.
Electric Potential and Circuit Diagrams
Electric potential difference is the work done to move a unit charge from one point to another in an electric field.
Potential Difference: The work done per unit charge in moving a charge between two points. Mathematically, V = W/Q, where V is potential difference, W is work done, and Q is charge.
V = W / Q
Where:
V = Potential difference (in volts, V)
W = Work done (in joules, J)
Q = Charge (in coulombs, C)
Unit of Potential Difference
The SI unit of potential difference is volt (V). One volt is defined as one joule of work done per coulomb of charge.
Circuit Components and Symbols
Standard symbols are used to represent various components in circuit diagrams:
| Component | Symbol | Function |
|---|---|---|
| Cell | | | | Provides electrical energy |
| Battery | | | | | | Combination of cells |
| Switch (open) | ┴ ┬ | Breaks the circuit |
| Switch (closed) | ├ ├ | Completes the circuit |
| Resistor | ─══─ | Opposes current flow |
| Variable resistor | ─═╦─ | Adjustable resistance |
| Ammeter | ─A─ | Measures current |
| Voltmeter | ─V─ | Measures potential difference |
| Bulb | ╳ | Converts electrical to light energy |
Draw circuit diagrams for various configurations using standard symbols. Practice converting pictorial circuits to schematic diagrams.
How much work is done in moving a charge of 2 C across two points having a potential difference of 12 V?
Ohm's Law
Ohm's law establishes the relationship between potential difference across a conductor and the current flowing through it.
Ohm's Law: The potential difference across the ends of a conductor is directly proportional to the current flowing through it, provided its physical conditions remain unchanged.
V ∝ I or V = IR
Where:
V = Potential difference (in volts, V)
I = Current (in amperes, A)
R = Resistance (in ohms, Ω)
Experimental Verification
Ohm's law can be verified experimentally using a circuit with a nichrome wire, ammeter, voltmeter, and variable voltage source.
- Set up the circuit with nichrome wire, ammeter in series, voltmeter in parallel
- Vary the voltage using different numbers of cells
- Record current (I) and potential difference (V) for each setting
- Plot a graph of V vs I
- The straight line through origin confirms Ohm's law
- Slope of the graph gives resistance (R = V/I)
Applications of Ohm's Law
Ohm's law is used to:
- Calculate current when voltage and resistance are known
- Calculate voltage when current and resistance are known
- Calculate resistance when voltage and current are known
- Design and analyze electrical circuits
Perform the experiment to verify Ohm's law using nichrome wire. Record readings and plot V-I graph. Calculate resistance from the slope.
How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 Ω?
Resistance and Resistivity
Resistance is the property of a conductor that opposes the flow of electric current through it.
Resistance: The ratio of potential difference across a conductor to the current flowing through it. Mathematically, R = V/I.
R = V / I
Where:
R = Resistance (in ohms, Ω)
V = Potential difference (in volts, V)
I = Current (in amperes, A)
Factors Affecting Resistance
The resistance of a conductor depends on:
- Length (l): R ∝ l (longer wire has more resistance)
- Cross-sectional area (A): R ∝ 1/A (thicker wire has less resistance)
- Material: Different materials have different resistivities
- Temperature: For most conductors, resistance increases with temperature
R = ρl / A
Where:
R = Resistance (in ohms, Ω)
ρ = Resistivity (in ohm-meter, Ωm)
l = Length of conductor (in meters, m)
A = Cross-sectional area (in square meters, m²)
Resistivity
Resistivity (ρ) is a characteristic property of the material. It is defined as the resistance of a conductor of unit length and unit cross-sectional area.
| Material | Resistivity (Ωm) | Classification |
|---|---|---|
| Silver | 1.60 × 10-8 | Conductor |
| Copper | 1.62 × 10-8 | Conductor |
| Aluminum | 2.63 × 10-8 | Conductor |
| Tungsten | 5.20 × 10-8 | Conductor |
| Nichrome | 100 × 10-8 | Alloy |
| Glass | 1010 - 1014 | Insulator |
| Rubber | 1013 - 1016 | Insulator |
Investigate how resistance changes with length, thickness, and material of wire by measuring current for same voltage.
Resistance of a metal wire of length 1 m is 26 Ω at 20°C. If the diameter of the wire is 0.3 mm, what will be the resistivity of the metal? Identify the material using the resistivity table.
Heating Effect of Electric Current
When electric current flows through a conductor, heat is produced due to the resistance offered by the conductor to the flow of current.
Joule's Law of Heating: The heat produced in a resistor is directly proportional to the square of the current, the resistance, and the time for which the current flows.
H = I²Rt
Where:
H = Heat produced (in joules, J)
I = Current (in amperes, A)
R = Resistance (in ohms, Ω)
t = Time (in seconds, s)
Since V = IR, the formula can also be written as:
H = VIt
or
H = V²t / R
Practical Applications
The heating effect of electric current is utilized in many electrical appliances:
- Electric heater: Converts electrical energy to heat energy
- Electric iron: Uses heating element to generate heat for ironing clothes
- Electric kettle: Heats water using heating element
- Electric bulb: Filament gets heated to high temperature and emits light
- Fuse: Safety device that melts when excess current flows
Electric Power
Electric power is the rate at which electrical energy is consumed or converted into other forms of energy.
P = VI
Where:
P = Power (in watts, W)
V = Potential difference (in volts, V)
I = Current (in amperes, A)
Using Ohm's law (V = IR), power can also be expressed as:
P = I²R
or
P = V² / R
Electrical Energy
Electrical energy is the total amount of electrical work done or energy consumed.
E = Pt = VIt
Where:
E = Energy (in joules, J)
P = Power (in watts, W)
t = Time (in seconds, s)
The commercial unit of electrical energy is kilowatt-hour (kWh).
1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J
Calculate the power rating of various electrical appliances at home using their voltage and current ratings. Estimate their energy consumption.
An electric refrigerator rated 400 W operates 8 hour/day. What is the cost of the energy to operate it for 30 days at Rs 3.00 per kWh?
Teaching Resources
Key Terms
- Electric current: Flow of electric charge
- Potential difference: Work done to move unit charge between two points
- Ohm's law: V ∝ I (at constant temperature and physical conditions)
- Resistance: Opposition to flow of electric current
- Resistivity: Characteristic property of material (ρ = RA/l)
- Series combination: Resistors connected end to end
- Parallel combination: Resistors connected between same two points
- Joule's law: H = I²Rt
- Electric power: Rate of doing electric work (P = VI)
- Kilowatt-hour: Commercial unit of energy (1 kWh = 3.6 × 10⁶ J)
Assessment Questions
Chapter Review Questions
- What does an electric circuit mean?
- Define the unit of current.
- Calculate the number of electrons constituting one coulomb of charge.
- Name a device that helps to maintain a potential difference across a conductor.
- What is meant by saying that the potential difference between two points is 1 V?
- How much energy is given to each coulomb of charge passing through a 6 V battery?
- On what factors does the resistance of a conductor depend?
- Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source? Why?
- Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?
- Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?
- Use the data in the resistivity table to answer which among iron and mercury is a better conductor?
- Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 Ω resistor, an 8 Ω resistor, and a 12 Ω resistor, and a plug key, all connected in series.
- Judge the equivalent resistance when the following are connected in parallel – (a) 1 Ω and 10⁶ Ω, (b) 1 Ω and 10³ Ω, and 10⁶ Ω.
- What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?
- Why does the cord of an electric heater not glow while the heating element does?
- Compute the heat generated while transferring 96000 coulomb of charge in one hour through a potential difference of 50 V.
- An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 s.
- What determines the rate at which energy is delivered by a current?
- An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h.
Additional Resources
- Circuit simulation software
- Interactive Ohm's law applets
- Videos on electrical safety and fuse operation
- Resistance calculation worksheets
- Electrical appliance tear-down videos
- Printable circuit diagrams for practice
- Online quizzes and interactive learning tools
- Sample electrical bills for energy calculation exercises
