Electricity
Current, potential difference, Ohm's law, resistance combinations, heating effect and power.
Based on NCERT Class 10 Science, Chapter 11
11.1 Electric current and circuit
- Electric current is the rate of flow of charge through a conductor; its SI unit is the ampere (A).
- One ampere is one coulomb of charge flowing per second. Conventional current flows opposite to the flow of electrons.
- Current is measured with an ammeter connected in series in the circuit.
I = Q / t 1 A = 1 C/sWatch it happen — Charge, Current & Electrons
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One coulomb equals about 6.25 × 10¹⁸ electrons.
11.2 Electric potential and potential difference
- The potential difference between two points is the work done to move a unit charge from one point to the other.
- Its SI unit is the volt (V): 1 V = 1 J/C. A voltmeter is always connected in parallel across the two points.
- A cell or battery maintains this potential difference and keeps the current flowing.
V = W / Q11.4 Ohm's law
- At constant temperature the current through a metallic conductor is directly proportional to the potential difference across it.
- The constant of proportionality is the resistance R, measured in ohms (Ω). A V–I graph for an ohmic conductor is a straight line through the origin.
- A rheostat changes the resistance in a circuit and so controls the current.
V = I R R = V / IWatch it happen — Ohm's Law Explorer
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Same electrons pass through every resistor — current is common, voltage divides. Bigger current = faster dots.
At 6 V across 12 Ω, current is 0.5 A. Doubling resistance halves the current.
Q1.State Ohm's law. Draw/describe the V–I graph for a metallic conductor and state what its slope gives.
2 marks11.5 Factors affecting resistance
- Resistance increases with length, decreases with area of cross-section, and depends on the material and temperature.
- The resistivity ρ is a property of the material: metals like copper and aluminium have very low resistivity and are used for wires, while alloys such as nichrome have high resistivity and are used in heating elements.
R = ρ L / AWatch it happen — Resistivity of a Wire
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Nichrome has high resistivity, which is why it is used in heating elements.
Q1.On what factors does the resistance of a conductor depend? Write the relation.
3 marks11.6 Resistors in series and in parallel
- In series the same current flows through every resistor and the total resistance is the sum — the resistance always increases.
- In parallel the potential difference across each resistor is the same, currents divide, and the equivalent resistance is smaller than the smallest resistor.
- Household appliances are connected in parallel so each gets the full 220 V and can be switched independently.
Series: Rs = R₁ + R₂ + R₃ Parallel: 1/Rp = 1/R₁ + 1/R₂ + 1/R₃Watch it happen — Resistors in Series
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Same electrons pass through every resistor — current is common, voltage divides. Bigger current = faster dots.
In series the current is common and voltage divides in the ratio of resistances.
Q1.Two resistors of 6 Ω and 3 Ω are connected in parallel to a 6 V battery. Find the equivalent resistance and the total current.
3 marks11.7 Heating effect of electric current
- When current flows through a resistor, electrical energy is converted into heat — Joule's heating.
- This effect is used in electric irons, toasters, heaters, geysers and the filament of an incandescent bulb (tungsten, melting point above 3000 °C).
- A fuse is a safety device: a thin wire of low melting point melts and breaks the circuit when the current exceeds a safe value.
H = I² R tWatch it happen — Joule's Heating Effect
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Power stays constant (blue dashed) while heat piles up in a straight line — H = P × t. Raise the current and the blue line jumps by I², so the orange heat line climbs far steeper.
Electrons collide with the metal ions and lose energy as heat. Double the current and the wire gets 4× hotter — that's why a thin fuse wire melts first and breaks the circuit.
Heat depends on the square of current — doubling current gives 4× the heat.
Q1.(a) Derive the expression for heat produced in a resistor. (b) An electric iron of 1 kW works on 220 V for 2 hours daily — find the energy used in 30 days in kWh.
5 marks11.8 Electric power
- Power is the rate at which electrical energy is consumed; its SI unit is the watt (W).
- The commercial unit of electrical energy is the kilowatt hour (1 kWh = 3.6 × 10⁶ J), the 'unit' on your electricity bill.
P = V I = I² R = V² / R 1 kWh = 3.6 × 10⁶ JWatch it happen — Electricity Bill Calculator
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1 unit = 1 kWh = 3.6 × 10⁶ J of energy.
Q1.Why is the series arrangement not used for domestic circuits?
2 marks