Electricity

Question 1

What are the three required things for electricity to flow?

Answer 1

The source of electricity, a connected complete path for electricity to flow, and a device using electrical energy.

Question 2

Which type of circuit allows electricity to flow through?

Answer 2

Closed circuit

Question 3

List four examples of electrical conductors. State which one(s) are not metals.

Answer 3

Copper, aluminum, iron, and graphite, in which graphite is not a metal.

Question 4

Give three examples of electrical insulators. State their property.

Answer 4

Plastic, glass, and wood. They don’t allow electricity to flow through them.

Question 5

Will electricity flow through a open switch or closed switch?

Answer 5

Closed switch

Question 6

In which direction do the free electrons of the metals flow when the metal is connected to a closed circuit?

Answer 6

The free electrons flow from the negative pole of the electric cell through the wires to the positive pole of the electric cell.

Question 7

In which direction does the conventional electric current flow in a closed circuit?

Answer 7

It flows from the positive pole of the electric cell through the wires to the negative pole.

Question 8

In the water pipe analogy, what do the water pipe and the water turbine symbolise?

Answer 8

The pipe and turbine symbolise connecting wires and the device using electrical energy respectively.

Question 9

In the travelling lorries analogy, what do the lorries and the lunch boxes symbolise?

Answer 9

The lorries symbolise free electrons and the lunch boxes symbolise electrical energy.

Question 10

What is the device for measuring the size of electric current and what unit is used?

Answer 10

Ammeter, ampere (A)

Question 11

Describe how to connect the ammeter to a closed circuit.

Answer 11

The wire connecting the positive pole of the battery should connect the positive (red) pole of the ammeter, and the wire connecting the negative pole of the battery should connect the negative (black) pole of the ammeter.

Question 12

When a low voltage power supply is connected in a closed circuit with a nichrome wire, what energy conversion takes place? What effect of the electric current does this show?

Answer 12

Electrical energy from the power supply is converted into thermal and light energy given out by the nichrome wire (it becomes red hot and glows). This shows the heating effect of electrical current.

Question 13

Which three sets of apparatus (apart from wires, switch, and electric cell) are used to observe the magnetic effect of the electrical current?

Answer 13

Wire loop and compass, iron nail with coil and compass, iron nail with coil and paper clips.

Question 14

Describe how to show the magnetic effect of an electrical current using a compass and a wired loop.

Answer 14

The wired loop is connected in a closed circuit. The compass is placed above it with its needle facing roughly the direction of the wire loop. The needle of the compass will change direction, which indicates that there is a magnetic effect given out by the electrical current.

Question 15

What is the name of a coil with an electric current flowing through it?

Answer 15

Electromagnet

Question 16

Give three examples of daily application of electromagnets.

Answer 16

Electric door lock, maglev trains, fire alarm bell

Question 17

What is the difference between voltage and current?

Answer 17

Voltage measures the amount of energy supply to the free electrons, and current measures the speed of the free electrons.

Question 18

How is voltage related to current? Why?

Answer 18

The higher the voltage, the larger the current, and vice versa, because when voltage increases, more electrical push is applied to the free electrons, so the free electrons move faster, causing more free electrons to pass through a certain point each second, so the current becomes larger.

Question 19

What is the unit of resistance?

Answer 19

Ohm (Ω)

Question 20

Compare the resistances of good electrical conductors, poor electrical conductors, and electrical insulators.

Answer 20

Good conductors have the lowest resistance. (They still have resistance, but very little.) Poor conductors have larger resistance, and insulators have very large resistance that electricity cannot flow through them.

Question 21

What are the factors affecting the resistance of a wire?

Answer 21

The material of the wire, the thickness of the wire, and the length of the wire.

Question 22

How does the length of a wire affect resistance?

Answer 22

The longer the wire, the higher the resistance.

Question 23

How does the thickness of a wire affect its resistance?

Answer 23

The thinner the wire, the higher the resistance.

Question 24

What is the name of a device which resistance can be adjusted?

Answer 24

Rheostat

Question 25

Which type of circuit allows circuit components to be switched on and off independently?

Answer 25

Parallel circuit

Question 26

In which circuit does the current stay the same at every point?

Answer 26

Series circuit

Question 27

In which circuit does the voltage stay the same? At which points?

Answer 27

Parallel circuit. The voltage is the same across every branch of the circuit.

Question 28

Point out one difference between series circuits and parallel circuits.

Answer 28

The electrical devices in the series circuit is switched on or off at the same time, while electrical devices in different branches of parallel circuits and be switched on or off independently.

Question 29

What is the voltage of the mains electricity in Hong Kong?

Answer 29

220 volts

Question 30

When a three-pin plug is put so that the plug is facing you, match the names of the three pins it’s its relative position.

Answer 30

Earth pin (top), Live pin (bottom-left), Neutral pin (bottom-right)

Question 31

Match the colours of the wires in a three-pin plug to their names.

Answer 31

Green-and-yellow (Earth wire), brown (live wire), blue (neutral wire)

Question 32

Match the purposes and voltages the three wires in a three-pin plug carries with its name.

Answer 32

Earth wire: for safety only, 0V
Live wire: provides voltage to electrical appliance, the voltage is identical to mains voltage (220V for HK)
Neutral wire: provides a return path for the current, 0V

Question 33

Match the holes of the main socket with their position.

Answer 33

Earth hole (top), Live hole (bottom-right), Neutral hole (bottom-left)

Question 34

Are domestic circuits connected in series or parallel? Why?

Answer 34

They are connected in parallel because the devices in the domestic circuit must operate independently.

Question 35

What are the hazards of domestic circuits?

Answer 35

Overloading and short circuit

Question 36

What causes overloading?

Answer 36

Overloading takes place when too many electrical appliances are plugged in one mains socket. The current passing through the overloaded socket becomes very large and the circuit becomes very hot.

Question 37

What causes short circuit?

Answer 37

Short circuit is caused when there is a significantly easier path with less resistance for electricity to flow in a circuit. The current of this branch becomes very large and the circuit becomes very hot.

Question 38

List two situations in real life where short circuit may occur.

Answer 38

Short circuits may occur when the plastic casing of a wire loosens and the copper wire touches each other, or when dust is built up in circuit boards.

Question 39

List all safety devices to prevent the circuit from overheating and how they work.

Answer 39

Fuse and circuit breaker. They prevent the circuit from being too hot by regulating the current of the circuit. The fuse blows when the current is above its rating and the circuit breaker automatically closes the circuit when the current is above its rating. The fuse is non-reusable but the circuit breaker is reusable and can be reset.

Question 40

List all safety devices that protect us from getting an electric shock.

Answer 40

Earth wire