Electricity ICSE Class 8 Physics: Household Energy, Safety and Static Electricity

What Electricity is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Physics: your school sets and marks the exam. Electricity sits in the Theme 8: Electricity theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

The kilowatt hour

Electrical energy used $=$ power × time. For household bills:
$\text{energy (kWh)}=\frac{\text{power (W)}\times \text{time (h)}}{1000}$.

1 kilowatt hour is the energy used by a 1 kW (1000 W) appliance working for 1 hour. It is called 1 unit on the bill.

In joules: $1\text{kWh}=1000\text{W}\times 3600\text{s}=3600000\text{J}=3.6\times 10^6\text{J}$.

For several identical appliances, multiply by the number of appliances; for a month, multiply by the number of days.

Worked example

Three fans of 60 W each run for 8 hours a day for 30 days. How many units of electricity do they use?

Try it first, then show the solutionSolution

1. Total power $=3\times 60=180$ W.
2. Energy $=\frac{180\times 8\times 30}{1000}$.
3. $=43.2$ kWh (units).

Answer: 43.2 units

Live, neutral and earth wires

Household supply reaches us through three wires:

• Live (phase) wire: at a high voltage (about 220–230 V in Indian homes). Current comes to the appliance along it. It is the dangerous wire.
• Neutral wire: at (nearly) zero voltage; it completes the circuit, carrying current back.
• Earth wire: connected to a metal plate buried deep in moist ground. It normally carries no current. It is joined to the metal body of appliances, giving a safe path to the ground if the live wire touches the body.

Colour codes: older wiring uses red (live), black (neutral) and green (earth); newer wiring uses brown (live), blue (neutral) and green-yellow (earth).

Fuses and switches are always placed in the live wire, so that switching off or a blown fuse cuts the appliance off from the high voltage.

Worked example

Why is a switch connected in the live wire and not in the neutral wire?

Try it first, then show the solutionSolution

1. If the switch were in the neutral wire, the appliance would still be connected to the live wire when 'off'.
2. Touching a faulty part could then give a shock. With the switch in the live wire, the appliance is cut off from the high voltage.

Answer: So that the appliance is disconnected from the high voltage when switched off.

Safety devices: fuse, circuit breaker, earthing

• Fuse: a short piece of thin wire of low melting point, placed in the live wire. If too much current flows (an overload or a short circuit), the wire gets hot, melts and breaks the circuit.
• Circuit breaker (MCB): a switch that trips (switches off) automatically when the current is too large. It can simply be switched on again after the fault is cleared, unlike a fuse, which must be replaced.
• Earthing: connecting the metal body of an appliance to the earth wire. If a fault makes the body live, the current flows safely to the ground (and usually blows the fuse) instead of through a person.

Appliances with metal bodies (iron, refrigerator, washing machine, heater) must be earthed and use three-pin plugs.

Worked example

The metal body of a washing machine becomes connected to the live wire because of a fault. What happens if it is (i) earthed (ii) not earthed?

Try it first, then show the solutionSolution

1. (i) Earthed: a large current flows through the earth wire to the ground; the fuse blows or the MCB trips, cutting off the supply.
2. (ii) Not earthed: the body stays at a high voltage, and anyone touching it gets a shock, because the current flows through their body to the ground.

Answer: (i) Supply cut off safely (ii) risk of electric shock

Static electricity and conservation of charge

Rubbing two different materials together transfers electrons from one to the other:

• the material that gains electrons becomes negatively charged;
• the one that loses electrons becomes positively charged, by an equal amount.

Examples: a glass rod rubbed with silk becomes positive (the silk negative); a plastic comb rubbed in dry hair becomes negative.

Like charges repel, unlike charges attract. A charged body also attracts small uncharged pieces of paper.

Conservation of charge: charge is neither created nor destroyed; it is only transferred. The total charge before and after rubbing is the same (zero).

Worked example

A glass rod is rubbed with a silk cloth and becomes positively charged. What charge does the silk get, and what does this show?

Try it first, then show the solutionSolution

1. Electrons move from the glass to the silk.
2. The silk gets an equal negative charge, so the total charge stays zero: charge is conserved, only transferred.

Answer: An equal negative charge; conservation of charge

Conduction, induction and the electroscope

• Charging by conduction: touching an uncharged conductor with a charged body; some charge flows across, and the conductor gets the same kind of charge.
• Charging by induction: bringing a charged body near (without touching) a conductor. The near side gets the opposite charge and the far side the same charge. If the far side is then earthed (touched) and the earth connection removed before the charged body is taken away, the conductor keeps the opposite charge.

Gold leaf electroscope: a metal rod with a metal disc (cap) at the top and two thin gold leaves at the bottom, inside a glass case. When charge reaches the leaves, they get the same charge and repel, so they diverge. It is used to detect charge and to find its type (with a body of known charge).

Draw-and-label checklist: metal cap, metal rod, insulating plug (cork), glass case, two gold leaves, earthed metal strip (optional).

Worked example

A negatively charged rod is brought near the cap of an uncharged electroscope without touching it. What happens to the leaves, and why?

Try it first, then show the solutionSolution

1. The negative rod repels electrons down the metal rod to the leaves; the cap becomes positive.
2. Both leaves become negatively charged, repel each other and diverge.
3. When the rod is removed, the electrons spread back and the leaves collapse.

Answer: The leaves diverge (by induction) and collapse again when the rod is removed.

Lightning, batteries and safety

Lightning is a huge electric discharge between charged clouds and the ground (or between clouds). A lightning conductor is a thick metal (copper) strip with pointed spikes at the top of a tall building, running down the wall to a metal plate buried in the ground. It gives the charge a safe, easy path to the earth, protecting the building.

Battery: a collection of cells connected in series (positive of one to negative of the next). The voltages add: four 1.5 V cells give $4\times 1.5=6$ V.

Dangers and precautions: electric shock, fires from overloading or short circuits. Never touch switches with wet hands; do not overload sockets; replace damaged wires; use the correct fuse; switch off the main supply before repairs; during a thunderstorm avoid open fields, tall trees and using wired appliances.

Worked example

How many 1.5 V cells must be joined in series to make a 12 V battery?

Try it first, then show the solutionSolution

1. In series the voltages add.
2. $12÷1.5=8$ cells.

Answer: 8 cells

Physics in real life: Electricity

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Kilowatt Hour and Electricity Bills
• Live, Neutral and Earth Wires, Fuse, MCB and Earthing
• Static Charge, Charging by Conduction and Induction, Gold Leaf Electroscope

Electricity formula sheet

Energy in kWh

$E(\text{kWh})=\frac{P(\text{W})\times t(\text{h})}{1000}$

1 kWh = 1 unit.

kWh in joules

$1\text{kWh}=3.6\times 10^6\text{J}$

1000 W × 3600 s.

Cost

$\text{cost}=\text{units}\times \text{rate per unit}$

Rate given in the question.

Cells in series

$V_{\text{total}}=V_1+V_2+\dots$

Four 1.5 V cells give 6 V.

Electricity: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Electricity

The question patterns to expect, and the method that works for each.

Tricks to remember Electricity

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Watts × hours ÷ 1000

  To get units, multiply watts by hours and divide by 1000.

  Use it when: Electricity bills.

• Live is lethal

  Switches and fuses go in the live wire because that is where the danger is.

  Use it when: Wiring questions.

• Earth is the escape route

  The earth wire gives stray current an easy way to the ground instead of through you.

  Use it when: Earthing.

• Conduction same, induction opposite

  Touching gives the same charge; induction (then earthing) leaves the opposite charge.

  Use it when: Charging methods.

Electricity: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Electricity

The slips examiners see most often, and what to write instead.

• ✗Mistake: Forgetting to divide by 1000

  Watt-hours must be changed to kilowatt hours.

  Wrong working

  100 W × 5 h × 30 = 15 000 units

  Right working

  $15000÷1000=15$ units

  ✓Do this: Units = W × h ÷ 1000.

  Usually costs: The answer.

• ✗Mistake: Using minutes as hours

  Time must be in hours.

  Wrong working

  30 min: $t=30$

  Right working

  $t=0.5$ h

  ✓Do this: Divide minutes by 60.

  Usually costs: The answer.

• ✗Mistake: Putting the fuse in the neutral wire

  A blown fuse in the neutral would leave the appliance connected to the live wire.

  ✓Do this: Fuse and switch always in the live wire.

  Usually costs: 1 mark.

• ✗Mistake: Saying rubbing creates charge

  Charge is only transferred, not created.

  ✓Do this: Write 'electrons are transferred'; total charge is conserved.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up conduction and induction

  Conduction needs contact and gives the same charge; induction needs no contact and gives the opposite charge.

  ✓Do this: Touch = same; near = opposite.

  Usually costs: 1 to 2 marks.

Practice questions on Electricity

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   A fuse is connected in the:
   (a) neutral wire
   (b) earth wire
   (c) live wire
   (d) any wire

   Hint

   The dangerous wire.

   Answer

   (c)

2. Q2 · 1 mark · Very short answerExam level

   How many joules are there in 1 kWh?

   Hint

   1000 × 3600.

   Answer

   $3.6\times 10^6$ J.

3. Q3 · 2 marks · Short answerExam level

   State the law of electric charges with one example.

   Hint

   Like and unlike.

   Answer

   Like charges repel; unlike charges attract. Two glass rods rubbed with silk repel each other; a glass rod (positive) attracts a plastic comb rubbed with hair (negative).

4. Q4 · 2 marks · Short answerHarder exam level

   A 60 W fan runs for 10 hours a day. How many units does it use in a day and in 30 days?

   Hint

   W × h ÷ 1000.

   Answer

   0.6 unit a day; 18 units in 30 days.

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): A fuse is always connected in the live wire.
   Reason (R): When the fuse melts, it disconnects the appliance from the high voltage of the live wire.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Why the live wire?

   Answer

   (c)

6. Q6 · 2 marks · Short answerHarder exam level

   What is the function of the earth wire?

   Hint

   Safety path.

   Answer

   It connects the metal body of an appliance to the ground, so that if the body becomes live through a fault, current flows safely to the earth (and the fuse blows) instead of through a person.

7. Q7 · 2 marks · Short answerHarder exam level

   A glass rod rubbed with silk gets a positive charge. Explain, using conservation of charge, what happens to the silk.

   Hint

   Electrons move.

   Answer

   Electrons move from the glass to the silk, so the silk gets an equal negative charge. Total charge stays zero: charge is transferred, not created.

8. Q8 · 2 marks · Short answerHarder exam level

   How does a lightning conductor protect a tall building?

   Hint

   Safe path.

   Answer

   Its pointed metal top and thick copper strip give the electric charge an easy path down to a metal plate buried in the ground, so the discharge passes to earth without damaging the building.

9. Q9 · 2 marks · Short answerHarder exam level

   Five 10 W LED bulbs are used for 6 hours a day for 30 days. Find the units used.

   Hint

   Multiply by 5.

   Answer

   $\frac{5\times 10\times 6\times 30}{1000}=9$ units.

10. Q10 · 2 marks · Short answerHarder exam level

    How many 1.5 V cells must be connected in series to make a 6 V battery? Draw the arrangement in words.

    Hint

    Voltages add.

    Answer

    4 cells, each positive terminal joined to the negative terminal of the next.

11. Q11 · 3 marks · Short answerHOTS / competency

    A 100 W bulb is replaced by a 10 W LED giving similar light. Both are used 5 hours a day for 30 days. How much money is saved in a month at ₹6 per unit?

    Hint

    Find units for each.

    Answer

    100 W: 15 units; LED: 1.5 units. Saving $=13.5\times 6=₹81$.

12. Q12 · 3 marks · Short answerHOTS / competency

    A charged electroscope's leaves are diverged. Explain what happens when you touch its cap with your hand, and why.

    Hint

    Path to earth.

    Answer

    The leaves collapse. The human body is a conductor connected to the earth, so the charge flows away through the body to the ground and the leaves are no longer charged.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is 1 kWh?Show answer

   The energy used by a 1 kW appliance in 1 hour (1 unit).

2. 2.1 kWh in joules?Show answer

   3.6 × 10⁶ J.

3. 3.In which wire is a fuse placed?Show answer

   The live wire.

4. 4.What is the earth wire connected to in an appliance?Show answer

   Its metal body.

5. 5.What does a gold leaf electroscope detect?Show answer

   Electric charge.

6. 6.Like charges ___; unlike charges ___.Show answer

   Repel; attract.

7. 7.How many 1.5 V cells in series make 9 V?Show answer

   6.

8. 8.What is the purpose of a lightning conductor?Show answer

   To give lightning a safe path to the earth, protecting the building.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Did you divide W × h by 1000?
• ☐Did you convert minutes to hours?
• ☐Can you state the role of each household wire?
• ☐Can you explain a fuse, an MCB and earthing?
• ☐Can you charge an electroscope by induction, step by step?
• ☐Can you list five electrical safety precautions?

Electricity: frequently asked questions