1True or false
In alternating current the direction in which the charges flow reverses many times every second.
Show answer
Answer: True
With 50 Hz a.c. the current reverses 100 times every second (twice in each cycle).
!Common mistakeLearners sometimes think a.c. just varies in size without reversing; it actually changes direction.
2True or false · ★ Challenge
Distribution lines inside towns carry a higher voltage than the main transmission lines between regions.
Show answer
Answer: False
Transmission lines carry the highest voltages (e.g. 110 kV or 220 kV); the voltage is stepped down at substations before it is distributed in towns (e.g. 15 kV or 30 kV).
!Common mistakeLearners may think lines near people must carry more voltage because they serve many users; in fact the voltage is lowered as power gets closer to users.
3True or false
To measure the output of a transformer, an a.c. voltmeter is connected in parallel across the secondary coil.
Show answer
Answer: True
Voltage is always measured across a component, and the meter must be set to a.c. because the output is alternating.
!Common mistakeLearners sometimes set the meter to d.c.; a d.c. meter shows about zero for an alternating voltage.
4True or false · ★ Challenge
Making a transformer core from one solid block of iron, instead of thin insulated sheets, would reduce the eddy-current loss.
Show answer
Answer: False
A solid core lets large eddy currents circulate; thin insulated laminations break up these current paths and reduce the heating.
!Common mistakeLearners think a solid core is 'stronger' and so better; for eddy currents the laminated core is better.
5True or false
Underground cables need thick insulation because the moist soil around them would otherwise conduct current away from the conductors.
Show answer
Answer: True
Unlike air, soil and water conduct, so buried conductors must be fully insulated and sheathed; this is one reason they cost more.
!Common mistakeLearners sometimes believe soil is a good insulator; moist soil conducts, which is why it is used for earthing.
6Multiple choice · ★ Challenge
Which of these is a distribution line rather than a high-voltage transmission line?
- AThe line on tall pylons linking two provinces of the country
- BThe cable on poles along a street that feeds the houses
- CThe line from a power station's step-up transformer
- DThe line joining Rwanda's grid to a neighbour's grid
Show answer
Answer: B. The cable on poles along a street that feeds the houses
Transmission lines carry power at the highest voltages between power stations and major substations; distribution lines carry lower voltages through towns to consumers.
!Common mistakeChoosing the line from the power station is tempting because it is 'first'; but that line leaves the step-up transformer at the highest voltage, so it is a transmission line.
7True or false
Trees growing under overhead lines are trimmed regularly because branches touching a line can cause faults, fires and electric shocks.
Show answer
Answer: True
A branch can provide a path to earth for current, especially when wet, and can damage or break the line in wind.
!Common mistakeLearners think wood is always an insulator; living, wet wood conducts well enough to be dangerous at high voltage.
8Multiple choice · ★ Challenge
What is the main advantage of linking all the power stations of a country (and those of neighbouring countries) in one grid?
- AIf one station stops, others can still supply the demand
- BEvery station can then run at a much lower voltage than before
- CPower lines no longer need any transformers
- DThe total power lost in the lines becomes zero
Show answer
Answer: A. If one station stops, others can still supply the demand
A connected grid lets power flow from wherever it is available to wherever it is needed, so a breakdown or a dry season at one station does not cause a blackout.
!Common mistakeChoosing 'losses become zero' is wrong: linking stations does not remove line resistance, so I²R losses still occur.
9True or false
Solar panels produce direct current, so an inverter is needed to change it to a.c. before a transformer can step its voltage up for the grid.
Show answer
Answer: True
Transformers work only with a.c.; the inverter makes the solar d.c. into 50 Hz a.c. that can join the grid.
!Common mistakeSome learners think solar panels give a.c. like the mains; photovoltaic cells give d.c.
10Multiple choice · ★ Challenge
For the same power delivered, which ONE change reduces the I²R loss in a line the most?
- ADoubling the cable's cross-sectional area
- BHalving the length of the line
- CDoubling the transmission voltage
- DThey all reduce the loss by the same amount
Show answer
Answer: C. Doubling the transmission voltage
Doubling V halves I, so the loss falls to (1/2)² = 1/4. Doubling the area or halving the length only halves R, so the loss falls to 1/2.
!Common mistakeChoosing 'all the same' forgets that the loss depends on the SQUARE of the current but only on the first power of the resistance.
11True or false
In a transformer the primary and secondary coils are not joined by any wire; energy passes from one to the other through the changing magnetic field in the iron core.
Show answer
Answer: True
The coils are insulated from each other; the alternating flux in the shared core induces the secondary voltage.
!Common mistakeMany learners imagine current flowing from the primary coil into the secondary coil; only magnetic flux links them.
12Fill in the blank · ★ Challenge
The time taken for one complete cycle of a 50 Hz a.c. supply is ______ s.
Show answer
Answer: 0.02
Period T = 1 ÷ f = 1 ÷ 50 = 0.02 s (20 ms).
!Common mistakeLearners sometimes multiply and write 50 s; the period is the reciprocal of the frequency.
13True or false
In an ideal step-down transformer, the current in the secondary coil is larger than the current in the primary coil.
Show answer
Answer: True
VpIp = VsIs, so if Vs is smaller than Vp, Is must be larger than Ip.
!Common mistakeLearners often think a 'step-down' transformer steps everything down; it lowers the voltage but raises the current.
14Multiple choice · ★ Challenge
The primary coil of a transformer is connected to a 6 V battery through a switch, and a voltmeter is connected to the secondary. The switch is closed and left closed. What does the voltmeter show?
- AA steady reading of 6 V for as long as the switch is closed
- BA steady reading worked out from the turns ratio
- CA brief reading as the switch closes, then zero
- DAn alternating reading at a frequency of 50 Hz
Show answer
Answer: C. A brief reading as the switch closes, then zero
A voltage is induced only while the magnetic field in the core is changing, which happens only as the current is switching on. With steady d.c. the field is constant, so the reading returns to zero.
!Common mistakeChoosing 'a steady reading given by the turns ratio' applies Vs/Vp = Ns/Np to d.c.; that equation works only when the primary current keeps changing (a.c.).
15Fill in the blank
The machine that turns the kinetic energy of a spinning turbine into electrical energy is called a ______.
Show answer
Answer: generator
The turbine turns the generator's rotor; electromagnetic induction produces the electrical output.
!Common mistakeLearners sometimes write 'transformer'; a transformer changes voltage but cannot turn motion into electricity.
16Multiple choice · ★ Challenge
What is the main reason why national grids use alternating current rather than direct current?
- AIt always travels much faster than d.c. does along the same wire
- BIts voltage can be raised and lowered easily with transformers
- CIt cannot give anyone an electric shock
- DIt does not heat the wires it flows through
Show answer
Answer: B. Its voltage can be raised and lowered easily with transformers
Transformers need a changing current, so a.c. allows the voltage to be stepped up for transmission and down again for safe use.
!Common mistakeChoosing 'does not heat the wires' is wrong: a.c. heats wires just as d.c. does; the advantage is that its voltage can be transformed.
17True or false
If a transmission line is made twice as long using the same kind of cable, its resistance doubles, so the I²R loss doubles for the same current.
Show answer
Answer: True
Resistance is proportional to length; with the same current, P = I²R doubles when R doubles.
!Common mistakeSome learners think the loss is four times larger; only the current is squared, not the resistance.
18Fill in the blank
A school transformer kit whose coils can be lifted off the C-shaped iron core and swapped is called a ______ transformer.
Show answer
Answer: demountable
Demountable means it can be taken apart, so coils with different numbers of turns can be tried.
!Common mistakeSome learners write 'step-up', but a demountable transformer can be step-up or step-down depending on the coils used.
19Multiple choice · ★ Challenge
In a step-down transformer, the secondary (low-voltage) coil is wound with thicker wire than the primary coil. Why?
- AIt carries a larger current, so it needs lower resistance
- BIt has the higher voltage, so it needs extra insulation
- CThicker wire produces more magnetic flux inside the iron core
- DThick wire has more resistance, so it limits the current
Show answer
Answer: A. It carries a larger current, so it needs lower resistance
The secondary of a step-down transformer carries a bigger current. Thick wire has a lower resistance, so the I²R heating (copper loss) is kept small.
!Common mistakeChoosing 'higher voltage' forgets that in a step-down transformer the secondary has the LOWER voltage and the LARGER current.
20Multiple choice · ★ Challenge
Overhead transmission lines are usually bare aluminium conductors with no plastic insulation on them. Why is this possible?
- AAluminium does not conduct electricity along its outer surface at all
- BThe high voltage stops any current from escaping to earth
- CThe steel pylons carry the current safely to the ground
- DThe air around them insulates them, and insulators hold them up
Show answer
Answer: D. The air around them insulates them, and insulators hold them up
High on the towers, the surrounding air acts as the insulator and the glass or ceramic insulators stop current reaching the towers. Bare conductors are also lighter and lose heat more easily.
!Common mistakeChoosing 'the pylons carry the current to the ground' is the opposite of the truth: the insulators exist to stop current reaching the pylons.
21Fill in the blank
For a transformer, the ratio Vs ÷ Vp is equal to the ratio Ns ÷ ______.
Show answer
Answer: Np
The voltage ratio equals the turns ratio: Vs/Vp = Ns/Np.
!Common mistakeSome learners write Is (current); the current ratio is the inverse, Is/Ip = Np/Ns for an ideal transformer.
22Multiple choice · ★ Challenge
A bird can perch safely on a single high-voltage line, but a worker on a metal ladder who touches the same line is killed. Why?
- AThe bird's feathers are perfect insulators, but human skin is not at all
- BThe line's voltage drops when a bird lands on it
- CThe bird has no p.d. across it; the worker connects the line to earth
- DBirds are too light for the current to pass through them
Show answer
Answer: C. The bird has no p.d. across it; the worker connects the line to earth
Both of the bird's feet are at the same potential, so no current flows through it. The worker is connected between the high-voltage line and the earth, so a large p.d. drives a deadly current through the body.
!Common mistakeChoosing the feathers answer misses the key idea: current needs a potential difference across the body, not just contact with a live wire.
23Multiple choice
A generator in a power station produces electricity by:
- ARubbing two different materials hard against each other
- BTurning magnets and coils relative to each other
- CChemical reactions between two metal plates
- DHeating the junction of two different wires
Show answer
Answer: B. Turning magnets and coils relative to each other
Rotating a magnet inside coils (or coils inside a magnetic field) changes the magnetic flux through the coils and induces a voltage: electromagnetic induction.
!Common mistakeChoosing chemical reactions describes a battery; a generator works by electromagnetic induction, like a transformer.
24Multiple choice
What is the main job of a substation on the edge of a town?
- ATo generate extra electricity when demand is high
- BTo convert the a.c. from the grid into d.c. for all the homes
- CTo step the voltage down and share power among local lines
- DTo measure how much energy each house uses
Show answer
Answer: C. To step the voltage down and share power among local lines
A substation contains step-down transformers, switches and breakers that lower the voltage and send power along different distribution lines.
!Common mistakeChoosing 'generate electricity' mixes up a substation with a power station; a substation only changes and distributes the power.
25Short answer · ★ Challenge
In a transformer experiment, the measured secondary voltage is always slightly less than the value given by Vs = Vp × Ns ÷ Np. Suggest two reasons.
Show answer
Model answer: Any two: not all the magnetic flux from the primary passes through the secondary (flux leakage, especially with a loose demountable core); the windings have resistance, so some voltage is lost when current flows; eddy currents and the repeated magnetising of the core waste energy; the meter may not be perfectly accurate.
!Common mistakeLearners often blame 'human error' only; there are real physical losses in every transformer that make the output smaller.
26Fill in the blank
Energy wasted as heat because of the resistance of the transformer's windings is called ______ loss.
Show answer
Answer: copper
Copper loss = I²R in the windings; it is reduced by using thick copper wire of low resistance.
!Common mistakeSome learners write 'eddy' loss here; eddy currents are in the iron core, not in the copper windings.
27Short answer · ★ Challenge
High-voltage pylons usually have a thin earth wire running along their tops, and each pylon is connected to the ground. Explain two ways this protects the line and people nearby.
Show answer
Model answer: The top earth wire attracts lightning strikes and carries the current safely down the pylons to the ground, protecting the conductors below. Earthing each pylon also means that if a live conductor touches the metal pylon, a large fault current flows to earth and the protective devices switch the line off, so the pylon does not stay live for someone to touch.
!Common mistakeLearners sometimes think the top wire carries part of the power; it carries no power current, only lightning or fault current.
28Fill in the blank
A fenced site containing transformers, switches and circuit breakers, where the voltage is changed and power is sent to different areas, is called a ______.
Show answer
Answer: substation
Substations link transmission lines to distribution lines.
!Common mistakeSome learners write 'power station'; a substation does not generate power, it only transforms and switches it.
29Short answer · ★ Challenge
Families at the far end of a long rural power line notice that their lamps are dimmer in the evening than at midday. Explain why.
Show answer
Model answer: In the evening many households switch on lights, TVs and cookers, so the current in the line is larger. The voltage lost along the line's resistance (V = IR) is therefore larger, so the voltage reaching the last houses is lower and the lamps are dimmer.
!Common mistakeLearners often blame the power station for 'sending less voltage'; the station's voltage is the same, it is the bigger IR drop along the line that lowers the voltage at the far end.
30Multiple choice
Which sequence of voltages is most likely as electricity travels from a hydro power station to a home in Rwanda?
- A230 V → 15 kV → 110 kV → 11 kV
- B110 kV → 11 kV → 230 V → 15 kV
- C11 kV → 230 V → 110 kV → 15 kV
- D11 kV → 110 kV → 15 kV → 230 V
Show answer
Answer: D. 11 kV → 110 kV → 15 kV → 230 V
The generator gives about 11 kV, which is stepped up for transmission (110 kV or 220 kV), stepped down at substations for distribution (30 kV or 15 kV), and finally to 230 V for homes.
!Common mistakeChoosing 11 kV → 230 V → 110 kV steps down before transmission; the voltage must be HIGH on the long lines to keep losses small.
31Short answer · ★ Challenge
To deliver the same power, an engineer raises the transmission voltage 4 times but, to save money, uses a cable with half the cross-sectional area (so twice the resistance). By what factor does the line loss change? Show your working.
Show answer
Model answer: Current falls to 1/4, so I² falls to 1/16. Resistance doubles. Loss = I²R changes by 1/16 × 2 = 1/8, so the new loss is one eighth of the old loss.
!Common mistakeA common slip is to give 1/4 × 2 = 1/2, forgetting to square the change in current.
32Multiple choice
A learner has four transformers, each labelled with its numbers of turns. Which one will raise the voltage of an a.c. supply?
- ANp = 200 turns, Ns = 4000 turns
- BNp = 500 turns, Ns = 50 turns
- CNp = 1200 turns, Ns = 600 turns
- DNp = 300 turns, Ns = 300 turns
Show answer
Answer: A. Np = 200 turns, Ns = 4000 turns
A step-up transformer has more turns on the secondary than on the primary: 4000 > 200, so it raises the voltage 20 times.
!Common mistakeChoosing Np = 1200, Ns = 600 is tempting because 1200 is the biggest number, but it is the SECONDARY that must have more turns.
33Short answer · ★ Challenge
Power stations such as the methane plant on Lake Kivu and the Rusumo hydro plant are far from Kigali, where much of the electricity is used. Explain why power stations are built in such places and what this means for the transmission system.
Show answer
Model answer: A power station must be where its energy source is: methane gas under Lake Kivu, or a river with a large flow and drop. The electricity therefore has to travel a long way to the towns, so it is sent at a high voltage on the grid to keep the current, and so the I²R losses, small.
!Common mistakeLearners sometimes say stations are built far away 'for safety'; the main reason is that they must be next to their energy source.
34Multiple choice
The mains supply in Rwanda has a frequency of 50 Hz. This means that:
- AIt changes its direction only once every 50 seconds
- BIts voltage is always 50 V
- C50 electrons pass each point every second
- DIt goes through 50 complete cycles every second
Show answer
Answer: D. It goes through 50 complete cycles every second
Frequency is the number of complete cycles per second; in each cycle the current flows one way and then the other.
!Common mistakeChoosing 'only once every 50 seconds' turns the idea upside down; 50 Hz means 50 cycles in ONE second.
35Multiple choice · ★ Challenge
An ideal transformer has a turns ratio Np : Ns = 20 : 1. Its secondary supplies 4.0 A to a 12 V load. What are the primary voltage and the primary current?
- AVp = 0.60 V, Ip = 80 A
- BVp = 240 V, Ip = 80 A
- CVp = 0.60 V, Ip = 0.20 A
- DVp = 240 V, Ip = 0.20 A
Show answer
Answer: D. Vp = 240 V, Ip = 0.20 A
Vp = 12 × 20 = 240 V. Power is the same on both sides: Ip = VsIs ÷ Vp = 12 × 4.0 ÷ 240 = 0.20 A.
!Common mistakeChoosing Vp = 240 V, Ip = 80 A multiplies BOTH voltage and current by 20; when the voltage is stepped up the current is stepped down.
36Multiple choice
Power must be supplied to an island on Lake Kivu, 2 km from the shore. Which is the most suitable way?
- AA bare overhead line on floating poles
- BAn uninsulated copper wire under the water
- CA satellite link to the island
- DAn insulated cable laid on the lake bed
Show answer
Answer: D. An insulated cable laid on the lake bed
A fully insulated, waterproof cable can lie under the water, just like an underground cable lies in soil; tall poles in deep water would be very difficult.
!Common mistakeChoosing the uninsulated wire forgets that water conducts, so the current would leak away into the lake.
37Multiple choice
Why is it dangerous to fly a kite near overhead power lines?
- AThe kite could slow down the current in the line and make it overheat
- BThe line attracts the kite magnetically and pulls it in
- CA damp string touching a line can carry current to earth through you
- DThe wind near the lines is always much stronger than elsewhere
Show answer
Answer: C. A damp string touching a line can carry current to earth through you
A wet or wire-reinforced string conducts. If it touches a line (or the high voltage arcs to it), current flows down the string and through the person to earth.
!Common mistakeChoosing the magnetic pull is tempting because currents make magnetic fields, but the real danger is the conducting path to earth.
38Multiple choice · ★ Challenge
A transformer has 1000 turns on its primary coil, which is connected to 230 V a.c. Its secondary coil has connections (taps) after 50, 100 and 150 turns. Which output voltages are available?
- A4600 V, 2300 V and 1530 V
- B50 V, 100 V and 150 V
- C11.5 V, 23 V and 34.5 V
- D5.0 V, 10 V and 15 V
Show answer
Answer: C. 11.5 V, 23 V and 34.5 V
Each turn gets 230 ÷ 1000 = 0.23 V, so Vs = 0.23 × 50 = 11.5 V, 0.23 × 100 = 23 V and 0.23 × 150 = 34.5 V.
!Common mistakeChoosing 4600 V, 2300 V and 1530 V turns the ratio upside down (Vp × Np ÷ Ns); fewer secondary turns must give a smaller voltage.
39Multiple choice
Why are large substation transformers filled with oil and fitted with cooling fins?
- ATo increase the voltage induced in the coils
- BTo remove the heat produced by energy losses
- CTo insulate the core from the magnetic field
- DTo store energy for use during power cuts
Show answer
Answer: B. To remove the heat produced by energy losses
Current in the windings and eddy currents in the core produce heat; the oil carries it to the fins, which pass it to the air, so the insulation does not overheat.
!Common mistakeChoosing 'increase the voltage' confuses cooling with the turns ratio; cooling deals with wasted energy and does not change the output voltage.
40Short answer · ★ Challenge
Generators in power stations usually produce about 11 kV to 15 kV, not the 110 kV or 220 kV used on the transmission lines. Suggest why the voltage is raised by a transformer instead of being generated directly at the high value.
Show answer
Model answer: Very high voltages inside a rotating generator would need enormous amounts of insulation and could cause sparking (arcing) between the moving parts. It is much easier and cheaper to generate at a moderate voltage and then use a step-up transformer, which has no moving parts.
!Common mistakeLearners sometimes say generators 'cannot make high voltage'; the real issue is insulation and safety inside a machine that spins.
41Multiple choice
Which energy chain describes a hydroelectric power station on the Nyabarongo river?
- AGravitational potential → kinetic → electrical
- BChemical → heat → kinetic → electrical
- CKinetic → gravitational potential → light → electrical
- DElectrical → kinetic → gravitational potential
Show answer
Answer: A. Gravitational potential → kinetic → electrical
Water stored behind the dam has gravitational potential energy; it falls and gains kinetic energy, turns the turbine, and the generator changes this into electrical energy.
!Common mistakeChoosing the chemical → heat chain describes a methane, peat or diesel power station, not a hydroelectric one.
42Short answer · ★ Challenge
A pole-mounted transformer near a school steps 15 kV down to 230 V. Calculate its turns ratio Np : Ns and explain why it is placed near the users rather than at the power station.
Show answer
Model answer: Np : Ns = Vp : Vs = 15 000 : 230 ≈ 65 : 1. Placing it near the users means the power travels almost all the way at high voltage and low current, so I²R losses are small; only the last short distance is at 230 V.
!Common mistakeLearners sometimes invert the ratio (1 : 65); a step-down transformer must have MORE turns on the primary.
43Short answer
After a storm, a power line has fallen across a road in a village. Give three things people should do to stay safe.
Show answer
Model answer: Any three: assume the line is live and keep well away (at least about 10 m); keep children and animals away; do not touch anything that the wire is touching, such as a fence, car or puddle; warn other people; report it at once to the electricity company (REG) or the police.
!Common mistakeA dangerous belief is that a line lying still on the ground must be dead; fallen lines can still be live.
44Multiple choice · ★ Challenge
A village solar mini-grid sends 23 kW at 230 V through a distribution cable of total resistance 0.10 Ω. Roughly how much of this, as a percentage, ends up heating the cable?
- A0.43 %
- B4.3 %
- C10 %
- D43 %
Show answer
Answer: B. 4.3 %
I = P ÷ V = 23 000 ÷ 230 = 100 A. Loss = I²R = 100² × 0.10 = 1000 W. Percentage = 1000 ÷ 23 000 × 100 = 4.3 %.
!Common mistakeChoosing 43 % comes from a slip in powers of ten (10 000 ÷ 23 000); check that the loss in watts is compared with the power in watts, not kilowatts.
45Short answer
Copper is a better conductor than aluminium, yet most overhead lines use aluminium conductors (often with a steel core). Give two reasons.
Show answer
Model answer: Aluminium is much lighter than copper, so the long spans do not sag as much and the pylons can be lighter. It is also cheaper. A steel core is added to give the conductor strength.
!Common mistakeLearners often say 'aluminium has less resistance'; for the same size it has more, but its low mass and cost make it the better choice for overhead lines.
46Short answer · ★ Challenge
A student says: 'A step-up transformer gives out more energy than it receives, because it raises the voltage.' Evaluate this statement.
Show answer
Model answer: The statement is wrong. Energy cannot be created. In an ideal transformer VpIp = VsIs, so when the voltage is raised the current is lowered by the same factor and the power stays the same. A real transformer gives out slightly less power than it takes in because some energy is wasted as heat.
!Common mistakeThe error is to look only at the voltage; power depends on voltage AND current, and the current falls as the voltage rises.
47Short answer
Describe a simple, safe experiment to show that a transformer works with a.c. but not with steady d.c.
Show answer
Model answer: Connect the primary of a small school transformer first to the low-voltage a.c. terminals of a power pack and then to a battery or the d.c. terminals. Connect a lamp or a voltmeter to the secondary. With a.c. the lamp stays lit; with d.c. it only flickers at the moment the switch is closed or opened, then stays off.
!Common mistakeLearners sometimes suggest connecting the transformer to the mains; school experiments must use a low-voltage supply for safety.
48Multiple choice · ★ Challenge
With a demountable transformer (primary 200 turns on 4.0 V a.c.), a learner records the secondary voltage for different secondary coils: 100 turns, 1.9 V; 200 turns, 3.9 V; 400 turns, 7.7 V; 800 turns, 15.2 V. Which conclusion is best supported?
- AVs is proportional to Ns, but a little below the ideal value
- BVs is exactly equal to Vp × Ns ÷ Np in every single case tested
- CVs falls as the number of secondary turns increases
- DVs doubles each time, so it does not depend on Ns
Show answer
Answer: A. Vs is proportional to Ns, but a little below the ideal value
Doubling Ns roughly doubles Vs (Vs/Ns ≈ 0.019 V per turn each time), but the ideal values would be 2.0 V, 4.0 V, 8.0 V and 16 V, so every reading is slightly lower because of losses.
!Common mistakeChoosing 'exactly equal' ignores the data: every reading is slightly below the ideal value, which shows that a real transformer is not 100 % efficient.
49Short answer
Describe how a graph of voltage against time for the mains supply differs from the graph for a torch battery.
Show answer
Model answer: The mains graph is a wave (a sine curve) that rises to a positive peak, falls through zero to a negative peak and repeats 50 times a second. The battery graph is a straight horizontal line, because its voltage stays the same and never changes direction.
!Common mistakeLearners sometimes draw the a.c. graph as positive only; the voltage of a.c. goes negative for half of each cycle.
50Short answer · ★ Challenge
A technician needs to know how many turns are on the primary coil of an old transformer connected to the 230 V mains. She winds an extra test coil of 10 turns on the same core and measures 2.0 V across it. Calculate the number of primary turns.
Show answer
Model answer: Np/Ns = Vp/Vs, so Np = Ns × Vp ÷ Vs = 10 × 230 ÷ 2.0 = 1150 turns.
!Common mistakeLearners often divide the wrong way and get 10 × 2.0 ÷ 230 ≈ 0.09 turns; a sensible answer must be much larger than 10 because the primary voltage is much larger.