Domestic uses and safety

遊んで学ぼう

問題に答えてエネルギーを集めたら、釣りや探検を楽しもう。アカウント不要。

教育者の方へ： Domestic uses and safety（Science、Physics）向けのすぐ使えるレッスンスライド, 復習ノート — レッスンで使うか、学習者がライブゲームとして遊ぶインタラクティブなクラス活動としてトピックを実施できます。

レッスンノート

Direct and Alternating Current

Direct current (d.c.) flows steadily in one direction around a circuit, from positive to negative.
A d.c. supply has a fixed positive terminal and a fixed negative terminal, so the current is always in the same direction.
Cells and batteries produce direct current.
Alternating current (a.c.) continuously changes direction, going back and forth around a circuit.
An a.c. supply has two identical terminals that switch between positive and negative.
The frequency of an alternating current is the number of times it changes direction back and forth each second.

Direct current and alternating current graphs

UK Mains Electricity

Mains electricity is generated by power stations and transported around the country through the National Grid.
Mains electricity is an alternating current (a.c.) supply.
In the UK, the domestic supply has a frequency of 50 Hz and a potential difference of about 230 V.
A frequency of 50 Hz means the current changes direction back and forth 50 times every second.
Because mains is a.c., it does not have positive and negative sides; instead the two ends of the circuit are called live and neutral.

Three-Core Cable

Most electrical appliances are connected to the mains using a three-core cable containing a live wire, a neutral wire and an earth wire.
The insulation covering each wire is colour coded: live – brown, neutral – blue, earth – green and yellow stripes.
The live wire carries the alternating potential difference from the supply to the circuit.
The neutral wire completes the circuit and is at, or close to, earth potential (0 V).
The earth wire is a safety wire that stops the appliance from becoming live; it only carries a current if there is a fault.

Potential Difference of the Wires

The live wire has a potential difference of about 230 V and is the most dangerous wire.
The neutral wire is at, or close to, 0 V and is much less dangerous than the live wire, although it can still deliver a small shock.
The earth wire is at 0 V and normally carries no current.
The potential difference between the live wire and earth is about 230 V.

Fuses

A fuse is a safety device that cuts off the flow of electricity if the current becomes too large.
A fuse usually consists of a glass cylinder containing a thin metal wire.
If the current becomes too large, the wire heats up and melts, breaking the circuit and stopping the current.
Fuses are always connected in the live wire so that when a fuse blows, the device is no longer connected to the live supply.
A fuse without an earth wire protects the circuit and cabling for a double-insulated appliance.

Circuit Breakers

A circuit breaker is an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value.
Circuit breakers have a major advantage over fuses because they do not melt and can be reset and used again.
Circuit breakers also work much faster than fuses.
For these reasons, circuit breakers are used in mains electricity in homes, sometimes misleadingly called 'fuse boxes'.

The Earth Wire and Safety

Many electrical appliances have metal cases, which poses a safety hazard.
If a live wire inside the appliance touches the metal case, the case becomes electrified and anyone touching it risks electrocution.
The earth wire provides a low resistance path to the earth if this happens.
This causes a surge of current in the earth wire and hence also in the live wire.
The high current through the fuse causes it to melt and break, cutting off the supply and making the appliance safe to touch.
The earth wire is an additional safety wire that reduces the risk of the appliance becoming live.

Dangers of Mains Electricity

The live wire, at about 230 V, is the most dangerous wire.
Because of the large potential difference between the live wire (230 V) and earth (0 V), connecting them together creates a very large current.
If a person provides the connection between live and earth, a large current can pass through them, giving a potentially lethal shock.
Even if a device is switched off but the mains supply is on, the live wire can still cause an electric shock.
Electricians always switch off the mains supply before working on electrical appliances to avoid contact with live wires.

Switches and Fuses in the Live Wire

Switches and fuses are always connected to the live wire.
This means that when a switch is turned off or a fuse blows, the device is no longer connected to the live supply.
If they were connected to the neutral wire, the device could still be live even when switched off, which is dangerous.

スライド

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

登録 — 無料ログイン

練習問題

無料プレビュー — 60問中8問。すべて見るには登録を。

1.Which of these correctly describes an alternating current?
Easy
- AA current that is steady and flows in one direction only
- BA current that continuously changes its direction, going back and forth around a circuit
- CA current that flows only when a switch is closed
- DA current that flows only through the earth wire
2.Which row about the UK domestic supply is correct?
Easy
- AType of current: a.c., frequency 50 Hz, voltage 230 V
- BType of current: a.c., frequency 230 Hz, voltage 50 V
- CType of current: d.c., frequency 50 Hz, voltage 230 V
- DType of current: d.c., frequency 230 Hz, voltage 50 V
3.Which of these electrical devices provides direct current?
Easy
- AA battery
- BA mains socket
- CA generator at a power station
- DThe National Grid
4.What colour is the insulating cable of the live wire in a UK plug?
Easy
- ABlue
- BGreen and yellow stripes
- CBrown
- DRed
5.An electric kettle is plugged into the mains supply. Which of these is the potential difference between the live and neutral wires?
Easy
- A0 V
- B120 V
- C230 V
- D460 V
6.Which of the following correctly describes where a fuse should be fitted and how the fuse does its job?
Easy
- AWhere: live wire — What it does: reduces the current to the fuse rating
- BWhere: neutral wire — What it does: breaks the circuit
- CWhere: live wire — What it does: breaks the circuit
- DWhere: neutral wire — What it does: reduces the current to the fuse rating
7.A circuit contains a fuse and each ammeter in the circuit reads 1.2 A. Which option correctly describes the fuse that should be fitted in this position?
Medium
- A1.0 A
- B3.0 A
- C5.0 A
- D13.0 A
8.The earth wire carries a current in normal operation of an appliance.
Easy
True or false?

もっと問題

9.A live wire can still give an electric shock even when a switch in the mains circuit is open.
Easy
True or false?
10.Match each wire in a three-core cable to its colour of insulation.
Easy
- Live wire
- Neutral wire
- Earth wire
- Brown
- Blue
- Green and yellow stripes
11.Match each wire to its potential difference relative to earth.
Medium
- Live wire
- Neutral wire
- Earth wire
- About 230 V
- Close to 0 V
- 0 V
12.Which of the following statements about the earth wire are correct? (select all that apply)
Medium
- AIt is a safety wire that stops the appliance becoming live
- BIt carries a current at all times when the appliance is switched on
- CIt provides a low resistance path to earth if a fault occurs
- DIt is connected to the live wire inside the plug
- EIt is at 0 V in normal operation
13.Which of the following are advantages of a circuit breaker over a fuse? (select all that apply)
Medium
- AIt can be reset and used again
- BIt works much faster
- CIt melts to break the circuit
- DIt does not need to be replaced after operating
- EIt reduces the current to a safe value
14.A toaster is fitted with a 13 A fuse. Sam fits a 3 A fuse instead and switches the toaster on. What happens?
Medium
- AThe toaster works normally because the fuse rating does not matter
- BThe 3 A fuse melts immediately because the toaster draws more than 3 A
- CThe toaster draws less current because the fuse rating is smaller
- DThe earth wire melts instead of the fuse
15.Place the steps in order to show how the earth wire and fuse make a metal-cased appliance safe when the live wire touches the case.
Medium
- A large current flows through the earth wire and the live wire
- The live wire touches the metal case and the case becomes live
- The fuse wire heats up and melts, breaking the circuit
- The earth wire provides a low resistance path to earth
16.Why is it dangerous for a person to touch the metal case in this situation?
Medium
- AThe case is at 0 V so no current flows
- BThe case becomes live at about 230 V, so a large current could pass through the person to earth
- CThe case becomes negatively charged and repels the person
- DThe neutral wire melts and stops the current
17.A fuse prevents a person from receiving an electric shock if they touch the live wire directly.
Medium
True or false?
18.A double-insulated appliance has a plastic (non-conducting) outer casing and a fuse but no earth wire. Which statement is correct?
Medium
- AThe fuse protects the circuit and the cabling
- BThe fuse protects the user from all electric shocks
- CThe appliance cannot be connected to the mains without an earth wire
- DThe plastic case becomes live if a fault occurs
19.Which of these is a hazard of damaged insulation on a mains cable?
Medium
- AThe current in the cable becomes too small to work the appliance
- BThe live wire is exposed, so a person could touch it and receive a shock
- CThe frequency of the supply changes
- DThe earth wire stops being at 0 V
20.Which of the following are hazards when using a mains supply? (select all that apply)
Medium
- ADamaged insulation
- BOverheating cables
- CDamp conditions
- DExcess current from overloading sockets
- EUsing a fuse with the correct rating
21.An electric lawnmower draws 4 A normally and is protected by a circuit breaker that operates above 5 A. What happens when the current is 4 A and what happens if the current rises to 14 A?
Medium
- AAt 4 A the circuit does not break; at 14 A the circuit does not break
- BAt 4 A the circuit does not break; at 14 A the circuit breaks quickly
- CAt 4 A the circuit breaks quickly; at 14 A the circuit does not break
- DAt 4 A the circuit breaks quickly; at 14 A the circuit breaks quickly
22.Place the wires in order of their potential difference relative to earth, from highest to lowest.
Medium
- Live wire
- Neutral wire
- Earth wire
23.Why must the outer casing of a metal-cased electrical appliance be earthed?
Medium
- ATo reduce the current drawn by the appliance in normal use
- BTo stop the case becoming live if a live wire touches it
- CTo increase the potential difference across the appliance
- DTo change the supply from direct to alternating current
24.Which of the following statements about direct current is correct?
Medium
- AThe current is only positive or only negative
- BThe current changes direction 50 times each second
- CThe current flows only in the earth wire
- DThe current reverses direction back and forth around the circuit
25.Which graph correctly shows an alternating current?
Easy
- AA sine wave that goes above and below the time axis
- BA horizontal straight line above the time axis
- CA horizontal straight line below the time axis
- DA straight line that increases steadily with time
26.Which graph correctly shows direct current?
Easy
- AA horizontal straight line at a constant positive value
- BA sine wave that goes above and below the time axis
- CA horizontal straight line at a constant negative value
- DA straight line that increases steadily with time
27.Which of the following statements about the UK domestic mains supply are correct? (select all that apply)
Medium
- AIt is an alternating current supply
- BIts frequency is 50 Hz
- CIts potential difference is about 230 V
- DIt has a fixed positive terminal and a fixed negative terminal
- EIt is a direct current supply
28.Match each electrical source to the type of current it provides.
Medium
- Battery
- Mains electricity supply
- Cell
- Direct current
- Alternating current
29.Which of the following are functions of the live wire in a mains circuit? (select all that apply)
Medium
- ACarries the alternating potential difference from the supply to the circuit
- BIs the most dangerous of the three wires
- CCompletes the circuit by forming the opposite end to the live wire
- DIs a safety wire that stops the appliance becoming live
- EHas a potential difference of about 230 V
30.Which of the following statements about the earth wire in a three-core cable are correct? (select all that apply)
Medium
- AIt is a safety wire that stops the appliance becoming live
- BIt is at 0 V
- CIt carries a current only if there is a fault
- DIt completes the circuit in normal use
- EIt is connected to the live terminal of the plug
31.A fuse is fitted in a mains plug. Which row correctly describes the fuse and its operation?
Medium
- AIt contains a thin metal wire that melts and breaks the circuit when the current is too large
- BIt contains a thick metal wire that melts and breaks the circuit when the current is too large
- CIt contains a thin metal wire that reduces the current to a safe value
- DIt contains an electromagnet that melts to break the circuit
32.A lamp is fitted with a 3 A fuse. Which statement describes the role of the fuse in the lamp circuit?
Medium
- AIt breaks the circuit if the current becomes larger than 3 A
- BIt reduces the current to 3 A during normal use
- CIt breaks the circuit if the current becomes smaller than 3 A
- DIt increases the current to 3 A during normal use
33.Place the steps in order to show how a fuse makes a circuit safe when the current becomes too large.
Medium
- The current in the fuse wire becomes too large
- The fuse wire heats up and melts
- The circuit is broken and the current stops
34.An electric lawnmower is fitted with a 5 A fuse. Which row correctly describes where the fuse should be fitted and how it does its job?
Medium
- AIn the live wire — it breaks the circuit when the current is too large
- BIn the neutral wire — it breaks the circuit when the current is too large
- CIn the live wire — it reduces the current to 5 A
- DIn the neutral wire — it reduces the current to 5 A
35.An electric lawnmower draws 4 A under normal conditions and is protected by a circuit breaker. The circuit breaker trips when the current reaches 14 A. Which row correctly describes what happens when the current is 4 A and when it rises to 14 A?
Medium
- AAt 4 A the circuit does not break; at 14 A the circuit breaks quickly
- BAt 4 A the circuit does not break; at 14 A the circuit does not break
- CAt 4 A the circuit breaks quickly; at 14 A the circuit does not break
- DAt 4 A the circuit breaks quickly; at 14 A the circuit breaks quickly
36.A metal-cased kettle has its live wire touching the metal case. Which of the following statements are correct? (select all that apply)
Hard
- AThe case becomes live at about 230 V
- BThe earth wire provides a low-resistance path to earth
- CA large current flows and the fuse melts, cutting off the supply
- DThe case remains safe to touch because it is metal
- EThe neutral wire prevents the case from becoming live
37.Which of the following is a hazard of damaged insulation on a mains cable?
Medium
- AA person may touch the live wire and receive an electric shock
- BThe cable becomes double-insulated
- CThe current in the cable becomes direct current
- DThe frequency of the mains supply changes
38.The live wire may still be dangerous even when a switch in the mains circuit is open.
Easy
True or false?
39.The neutral wire is at, or close to, earth potential (0 V).
Easy
True or false?
40.The earth wire carries a current during normal operation of an appliance.
Medium
True or false?
41.Complete the table of wire functions in a three-core cable.
Easy
The ____ wire carries the alternating potential difference from the supply. The ____ wire completes the circuit. The ____ wire is a safety wire to stop the appliance becoming live.
42.Complete the sentences about UK mains electricity.
Medium
UK mains electricity is an ____ current supply with a frequency of ____ Hz and a potential difference of about ____ V.
43.A label on a kettle says its power output is 2.5 kW. The kettle is connected to the 230 V mains supply. Calculate the current in the kettle. Give your answer to 2 significant figures.
Medium
44.A kettle draws a current of about 11 A from the 230 V mains supply. Calculate the resistance of the kettle. Give your answer to 2 significant figures.
Medium
45.An electric lawnmower draws a current of 4 A under normal operating conditions. A circuit breaker is fitted. The circuit breaker breaks the circuit when the current exceeds 5 A. State the maximum current, in amperes, that the lawnmower can draw before the circuit breaker operates.
Medium
46.Name the electrical device that provides direct current in a circuit.
Easy
47.State the frequency of the UK domestic electricity supply, in hertz.
Easy
48.Complete the sentence about the earth wire.
Easy
The earth wire is at ____ V and only carries a current if there is a ____ in the appliance.
49.State the potential difference of the UK domestic mains supply, in volts.
Medium
50.Complete the sentence about the live wire.
Easy
The live wire carries the ____ potential difference from the supply to a circuit.
51.Name the three wires in a three-core mains cable.
Easy
The three wires are the ____, the ____ and the ____.
52.Explain how the earth wire makes the metal-cased heater safe to use when the live wire touches the case.
Hard
53.Explain why switches and fuses should be connected in the live wire of a domestic circuit.
Medium
54.Explain why it is dangerous to provide any connection between the live wire and the earth wire.
Hard
55.State the function of the neutral wire in a mains circuit.
Medium
56.Explain how the earth wire makes a metal-cased appliance safe to use when the live wire touches the case.
Hard
57.A label on a kettle says its power output is 2.5 kW. The kettle is connected to the 230 V mains supply. Calculate the current in the kettle. Give your answer to 2 significant figures.
Medium
- A10 A
- B11 A
- C12 A
- D13 A
58.A kettle draws a current of about 11 A from the 230 V mains supply. Calculate the resistance of the kettle. Give your answer to 2 significant figures.
Medium
- A21 Ω
- B19 Ω
- C25 Ω
- D23 Ω
59.An electric lawnmower draws a current of 4 A under normal operating conditions. A circuit breaker is fitted. The circuit breaker breaks the circuit when the current exceeds 5 A. State the maximum current, in amperes, that the lawnmower can draw before the circuit breaker operates.
Medium
- A6 A
- B4 A
- C7 A
- D5 A
60.State the potential difference of the UK domestic mains supply, in volts.
Medium
- A276 V
- B230 V
- C253 V
- D207 V

フラッシュカード

What is direct current (d.c.)?

A current that is steady and constantly flows in the same direction around a circuit, from positive to negative. Cells and batteries produce direct current.

What is alternating current (a.c.)?

A current that continuously changes direction, going back and forth around a circuit. It is defined as positive or negative depending on which way it is flowing at that moment.

What is the frequency of an alternating current?

The number of times the current changes direction back and forth each second.

What are the frequency and potential difference of the UK domestic mains supply?

50 Hz and about 230 V. It is an alternating current (a.c.) supply.

What does a frequency of 50 Hz mean for UK mains current?

The direction of the current changes back and forth 50 times every second.

Which device produces direct current?

A cell or battery. Circuits powered by cells or batteries use a d.c. supply.

Which device produces alternating current?

A generator or alternator (as used by power stations for the mains supply). Mains electricity is an a.c. supply.

How does a d.c. power supply differ from an a.c. power supply?

A d.c. supply has a fixed positive terminal and a fixed negative terminal. An a.c. supply has two identical terminals that switch between positive and negative.

What is mains electricity?

Electricity generated by power stations and transported around the country through the National Grid. It is an alternating current supply.

Why does mains electricity have live and neutral instead of positive and negative?

Because it is an alternating current, so it has no positive and negative sides to the power source. Live and neutral are the equivalent, forming either end of the circuit.

What is the purpose of the live wire?

It carries the alternating potential difference from the supply to the circuit. It is the most dangerous of the three wires.

What is the purpose of the neutral wire?

It forms the opposite end of the circuit to the live wire, completing the circuit. It is much less dangerous than the live wire.

What is the purpose of the earth wire?

It is a safety wire that stops the appliance becoming live. It only carries a current if there is a fault.

What are the insulation colours of the three wires in a UK plug?

Live wire – brown; neutral wire – blue; earth wire – green and yellow stripes.

What is the potential difference of the live wire?

About 230 V. It carries most of the power to the circuit and is the most dangerous wire.

What is the potential difference of the neutral wire?

Close to 0 V. It is at, or close to, earth potential.

What is the potential difference of the earth wire?

0 V. It normally carries no current and only carries one if there is a fault.

Why is the live wire dangerous even when a switch in the mains circuit is open?

The live wire is still connected to the 230 V supply, so it can still give an electric shock. A device that is switched off but still plugged in can still be dangerous.

Why are switches and fuses always connected in the live wire?

So that when a switch is turned off, or a fuse blows, the device is no longer connected to the live supply. This makes it safe to touch.

How does a fuse work?

It contains a thin metal wire that heats up and melts if the current becomes too large. This breaks the circuit and stops the current.

How does a circuit breaker work?

It is an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value. It works much faster than a fuse and can be reset and used again.

Why are circuit breakers used instead of fuses in mains electricity in homes?

They do not melt and break, so they can be reset and used again, and they work much faster than a fuse.

How does the earth wire make an appliance with a metal case safe?

If a live wire touches the metal case, the earth wire provides a low resistance path to earth. This causes a surge of current that melts the fuse, cutting off the supply.

Why is it dangerous to provide any connection between the live wire and earth?

There is a large potential difference between the live wire (230 V) and earth (0 V), so a very large current can be created. If a person makes the connection, a potentially lethal current passes through them.

Why do electricians switch off the mains supply before working on appliances?

The live wire is at 230 V while the electrician is at 0 V. This large potential difference would drive a current through their body to earth if they touched it.

Unlock all 60 questions, flashcards & more

無料アカウントを作って、このトピックのすべての問題・スライド・フラッシュカード・復習ノートを見よう。

登録 — 無料ログイン

過去問

このトピックの過去問練習は近日公開。

近日公開