Unit notes and practice questions
Everything here is for unit 4 of the Grade 10 Physics textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.
The cards filed under this chapter on the Grade 10 Physics page. Open one at a time.
When you use it
Use to calculate the electrostatic force of attraction or repulsion between two point charges.
Watch out
Remember to square the distance in the denominator. A negative result indicates attraction and a positive result indicates repulsion.
Drafted from Grade 10 Physics, pages 82-131, then checked twice before it went up
When you use it
Use to calculate the electric field strength at a specific distance from a point charge.
Watch out
Distance r must be measured in metres and squared in the denominator.
Drafted from Grade 10 Physics, pages 82-131, then checked twice before it went up
When you use it
Use when configuring meters to measure current or voltage in an electric circuit.
Watch out
Never connect an ammeter in parallel or a voltmeter in series. An ammeter in parallel draws excess current and gets damaged, while a voltmeter in series prevents current from flowing due to its high resistance.
Drafted from Grade 10 Physics, pages 82-131, then checked twice before it went up
When you use it
Use to relate potential difference, electric current, and resistance across an electrical component.
Watch out
Ohm law applies directly to Ohmic conductors where physical conditions such as temperature remain constant.
Drafted from Grade 10 Physics, pages 82-131, then checked twice before it went up
| Connection | Equivalent resistance | Current | Potential difference |
|---|---|---|---|
| Series | |||
| Parallel |
When you use it
Use when finding the equivalent resistance, branch currents, or voltages in series and parallel resistor circuits.
Watch out
For parallel combinations, solve for 1/R_eq first and take the reciprocal at the end to get R_eq.
Drafted from Grade 10 Physics, pages 82-131, then checked twice before it went up
| Part of the circuit | In normal use | When a fault happens |
|---|---|---|
| Live wire | Brings current to the appliance at about 220 V with respect to the earth. | If it touches the metal body, the body goes live until the supply is cut off. |
| Neutral wire | Returns the same current to the supply and sits near 0 V. | It carries the working current, so it can never stand in for the earth wire. |
| Earth wire | Carries no current at all and holds the metal body at earth potential. | Gives the fault current a low-resistance path to the ground, so a large current flows. |
| Fuse or MCB | Sits in the live wire and never in the neutral. | The large fault current blows the fuse or trips the MCB, and the appliance is disconnected. |
| Two-pin plug | Gives a metal-bodied appliance no earth path. | A fault can leave the casing live. A double-insulated appliance with a plastic casing is built for two pins on purpose. |
| The socket in an Ethiopian wall | European round-pin, 220 V at 50 Hz, with the earth contact often made by clips on the sides. | The three rectangular pins drawn on most revision sheets are the British pattern, where the earth pin is longer and thicker so it makes contact first and breaks last. |
When you use it
Use when a question asks why a metal-bodied appliance such as a refrigerator, an electric mitad or a water heater needs a third wire. The earth wire protects the person touching it: it gives fault current a low-resistance path, so the fuse blows or the MCB trips before anyone is hurt.
Watch out
Students treat the earth wire and the neutral wire as the same thing because both measure near 0 V, and some join the earth pin straight to the neutral to save a wire. They are not the same. In normal use the neutral carries the full working current while the earth carries none, so if a neutral used as an earth breaks anywhere upstream, the metal body is joined through the motor to the live wire and sits at 220 V. The other half of the same mistake is fitting the fuse in the neutral: it still blows, but the appliance stays connected to the live wire and is dangerous to touch. The rule is short: the fuse goes in the live wire, the earth goes to the metal body, and the neutral carries the current back to the supply.
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
According to the law of conservation of charge, what happens to electric charges in an isolated system?
Answer: B They can neither be created nor destroyed, only transferred.
Why
The law of conservation of charge states that electric charges can neither be created nor destroyed, but can only be transferred from one material to another.
Question 2
A student in Hawassa uses a gold-leaf electroscope in the laboratory. What is the primary function of an electroscope?
Answer: C To detect the sign and amount of charge on a body
Why
An electroscope is a device specifically used to detect the sign and amount of electric charge on a body.
Question 3
What is the potential difference between two points when 1 Joule of work is done to move a charge of 1 Coulomb from one point to the other?
Answer: B 1 Volt
Why
By definition, one volt is the potential difference between two points in a conductor when 1 Joule of work is done to move a charge of 1 Coulomb from one point to the other (ΔV = W / Q).
Question 4
Why must an ammeter be connected in series with a resistor in an electric circuit?
Answer: B Because components in series have the same electric current passing through them.
Why
An ammeter is connected in series because objects connected in series have the exact same current flowing through them.
Question 5
According to Coulomb's law, what happens to the electrostatic force between two point charges if the distance between them is doubled?
Answer: A The force becomes 4 times smaller.
Why
Coulomb's law states that the electrostatic force is inversely proportional to the square of the distance (F ∝ 1/r2). Doubling the distance increases the denominator by 22 = 4, making the force 4 times smaller.
Question 6
What is the magnitude of the electric field at a point 2.00 m away from a point charge of 5.00 mC (5.00 × 10-3 C)? (Use k = 9 × 109 N m2/C2)
Answer: C 11.25 × 10<sup>6</sup> N/C
Why
Using the formula E = k · q / r2:
E = (9 × 109 N m2/C2 × 5.00 × 10-3 C) / (2.00 m)2 = 45 × 106 / 4 = 11.25 × 106 N/C.
Question 7
Two copper wires in a workshop in Addis Ababa have the same length. Wire A has twice the cross-sectional area of Wire B. How does the resistance of Wire A compare to Wire B?
Answer: C Wire A has half the resistance of Wire B.
Why
The resistance of a conductor is inversely proportional to its cross-sectional area (R = ρl / A). Therefore, doubling the area reduces the resistance by half.
Question 8
Three resistors of 4.0 Ω, 8.0 Ω, and 12.0 Ω are connected in series across a 24.0 V power source. What is the electric current flowing through the circuit?
Answer: B 1.0 A
Why
For resistors in series, equivalent resistance is Req = 4.0 Ω + 8.0 Ω + 12.0 Ω = 24.0 Ω. Using Ohm's law, I = ΔV / Req = 24.0 V / 24.0 Ω = 1.0 A.
Question 9
Two charges, q1 = +2 × 10-6 C and q2 = -4 × 10-3 C, are separated by 0.3 m. What is the magnitude and nature of the electric force between them? (Use k = 9 × 109 N m2/C2)
Answer: A 800 N, attractive
Why
Using Coulomb's law: F = k · |q1 · q2| / r2 = (9 × 109 × 2 × 10-6 × 4 × 10-3) / (0.3)2 = 72 / 0.09 = 800 N. Since the two charges have opposite signs, they attract each other.
Question 10
A piece of wire is cut into 5 identical pieces. When connected in parallel, their equivalent resistance is 2 Ω. What was the resistance of the original wire before it was cut?
Answer: C 50 Ω
Why
Let each piece have resistance R. In parallel, 1/Req = 5/R, which gives R = 5 × 2 Ω = 10 Ω for each piece. Originally, the 5 pieces were in series, so the total original resistance was 5 × 10 Ω = 50 Ω.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
Unit 4 runs from page 82 to page 131 of the Ministry of Education edition.
Ministry of Education edition, English, 212 pages
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