18 — Practical electricity

Question 1

Explain why wire leading to switch rather than wire leading to bulb is connected to the live terminal in the plug.

Answer 1

Switch must be connected to the live terminal in the plug so that it can disconnect the supply voltage from the table lamp when it is opened, ensuring that the lamp is at 0V.

Question 2

The cable is connected to a plug which contains a fuse. Explain the purpose and the action of the fuse.

Answer 2

The purpose of the fuse is to prevent excessive current from flowing in an electrical circuit when an electrical fault occurs. When the current flowing thru the circuit exceeds the fuse rating due to an electrical fault, the fuse will melt so that power to the appliance will be cut off.

Question 3

Wire A becomes loose and touches the metal case. The metal case is connected to an earth wire. Explain why a person who touches the case feels no shock and is not harmed.

Answer 3

The earth wire is of negligible resistance when the person touches the “live” metal case of the lamp, large current flows thru the earth wire instead of the person, preventing the person from getting an electric shock.

Question 4

A lamp is doubly insulated. Explain why this lamp is safe to use.

Answer 4

Electric cable is insulated from internal components of the lamp. Besides, the internal components r insulated from the external casing of the lamp.

Question 5

The neutral wire

Answer 5

The neutral wire is connected to zero voltage and provides a return pathway for the current back to the supply

Question 6

Live wire

Answer 6

The live wire is connected to a high voltage and delivers the current to the appliance

Question 7

1kWh

Answer 7

3.6 x10^6J

Question 8

Electrical hazards — damaged insulation

Answer 8

Wires usually covered w insulating materials to prevent electric shocks
Insulation can be damaged with age and prolonged use, such as by tempt and high electric field. Wires become brittle and breaks apart, exposing the live conducting wires, thus there is a risk of electric shock.

Question 9

Hazards — Overheating of cables

Answer 9

Energy can be transferred electrically to internal stores of devices -> rising tempts in electrical circuits.
Occurs when
A. Electrical cable passes current that exceeds its limit
B. A large current flows thru the wires due to too many plugs being connected to a socket -> overloading
C. Contact betw a plug and a socket is poor.

A and B -> too much current drawn causes overloading. Since work done W = I^2Rt, amt of energy transferred to internal store increases rapidly w current, component overheats and can cause fire and explosion.

Question 10

Hazards — Damp environment

Answer 10

Pure water cannot conduct electricity, but water from other sources usually contains charged ions and impurities, mobile ions r present n thus is good conductor of electricity. Electrical shock occurs when an individual is part of an electric circuit and the ground.

Question 11

Measures to prevent electric shock

Answer 11

1. Keep hands dry when using electrical appliances
2. Wear rubber shoes, as they act as insulators to cut off closed circuit to ground, keeping us safe.

Question 12

Fuse

Answer 12

A fuse is a safety device added to an electric circuit to prevent excessive current flow by breaking the circuit

Question 13

Circuit breakers

Answer 13

A circuit breaker is a safety device to prevent excessive current flow by switching off the electrical supply in a circuit when there is an overflow of current

Question 14

Differences betw fuse and circuit breakers

Answer 14

Mechanism:
F: consists of a fusible wire that will melt when current flowing thru exceeds a certain value
CB: current exceeds rating -> CB trips -> faulty appliance has to be identified and removed b4 CB turns back on

Location:
F: found in plugs of electrical appliances
CB: part of the main electrical wiring to an apartment; not added to plug of an appliance

F: cannot be reused
CB: can be reused

Similarity:
Limits the amt of current based on its current rating

Question 15

Why circuit breaker is more advantageous than a fuse

Answer 15

Circuit breakers can be reused by the resetting of a switch and thus need not be replaced but fuse needs to be replaced

Question 16

Advantages and disadvantages of earthed metal casing or double insulation

Answer 16

A - prevent contact w live wires
D - increase cost of appliances

Question 17

Answ technique for earthed metal casing or double insulation

Answer 17

If the live wire comes into contact with metal casing, a live current passing thru the earthed metal casing flows to the ground. Since the earth wire has alm no resistance, a large current flows, exceeding rated value x of fuse, causing CB to trip or fuse to melt, circuit is opened, preventing further current flow, thus preventing electric shock.

Question 18

What happens without earthing

Answer 18

1. Due to mechanical vibration, live wires can become loose and comes into contact w metal casing
2. Metal casing is at high electrical potential
3. Closed path may form when person touches metal casing. Large current flows thru person, causing an electric shock

Question 19

Wires and their colours

Answer 19

Earth wire — yellow + green
Neutral wire — blue
Live wire — brown (to prevent colour blind ppl from unable to distinguish colours)

Question 20

What happens when switch is connected to neutral wire

Answer 20

1. Live wires may become loose and comes into contact w casing
2. Appliance’s casing at high electrical potential
3. Closed path forms when person touches metal casing, even when switch is opened.

Question 21

Switch connected to live wire

Answer 21

1. Switch disconnects live wire from metal casing
2. No closed loop formed, preventing electric shock.

Question 22

Safety features and how they prevent safety hazards

Answer 22

1. Fuses — used in socket plugs to prevent excessive current
2. Circuit breaker — used in distribution box to prevent excessive current
3. Earthed metal casing — to prevent live wire from causing metal casing to be at a high electric potential
4. Double insulation — to prevent contact w live wire
5. Earth wire — to earth the metal casing of appliance

Question 23

Comparing energy conserving appliances, which one is better?

Answer 23

For the same amount of time, x consumes more energy, thus power rating is higher.

Question 24

The electrical fan is not connected to an earth wire. Explain how this will affect the safety of the user.

Answer 24

If the live wire touches the metal frame of the fan, the fan will become live or be at a high potential. The user will get an electric shock when they touch the fan.

Question 25

If the live wire touches the neutral wire inside the fan, state and explain what will happen to the lamps and fuses.

Answer 25

The fan will not work as it will be short-circuited. The xA and ya fuses connected in series will not blow because it is in a parallel connection. The lamps will not light up because the 10A fuse has blown. Hence, the circuit becomes open.4

Question 26

What causes the fuse in the plug to blow?

Answer 26

1. Live wire touches the neutral wire or
2. Live wire touches earth wire

Question 27

Insulation of cable damaged, bare metal wires show. In what cases will u get electric shock?

Answer 27

When you touch any of the wires, ENL

Question 28

Circuit breaker — a current in the circuit causes the contacts to separate. Explain why.

Answer 28

Unmagnetised steel core is magnetised when there is a current flowing thru the coil. Steel core exerts a downward attractive force which separates the contact, forming an open circuit which prevents further current from flowing thru.

Question 29

Using steel instead of soft iron core for circuit breaker -> circuit can only be used once. Explain why.

Answer 29

Steel is a hard magnetic material which is difficult to be demagnetised. Once magnetised to attract the iron bar. It retains magnetism after the circuit breaker is disconnected. Thus the circuit breaker cannot be reset and is only for one-time use.

Question 30

The fuse is a safety feature. State 2 other examples of electrical safety features in household appliances. [2]

Answer 30

1. Earthing
2. Double insulation

Question 31

Suggest which of the 3 fuses is the best to use in the plug and explain why the other fuses r less suitable [2]

Answer 31

5A fuse. Normal operating current of 3.5A exceeds the 3A fuse acting and would blow the fuse. The 13A fuse allows current greater than normal operating current to flow into toaster which may damage the appliance.

Question 32

The efficiency of the solar cells used is 15%. Suggest what happens to the incident energy not converted into electrical energy [1]

Answer 32

Some of the solar power could be reflected off surface of solar cells
Some of the solar power is used to heat up the solar cells.

Question 33

Describe 2 potential hazards of using electricity from a mains socket [2]

Answer 33

1. A fire hazard risk as all of the electrical appliances are connected in the mains sockets, overloading the circuit thus large current flowing thru the circuit overheats the wire.
2. Since all of the appliances r connected to the main socket, other appliances cannot work if any fault occurs in the circuit since fuse will blow and cut off the current in the mains socket.

Question 34

State the relationship between the power P lost in the sections and the length l of the section.

State the relationship between the power P lost in each of these sections and the cross-sectional area A of the section

Answer 34

1. By P = I^2R and R = row x length/Area, power P lost id directly proportional to length l of the section.
2. Power P lost in each section is inversely proportional to the cross-sectional area A of the section.

Question 35

2 resistors r connected in series. Resistance of R1 is larger than resistance of R2. Explain why R1 has a larger power output than R2. [1]

Answer 35

By P = I^2R, since current across R1 and R2 is the same, and resistance of R1> R2, power output of R1 is greater than R2.

Question 36

Alternating current

Answer 36

is an electric current that periodically reverses its direction and changes its magnitude in a circuit

Question 37

Earth wire

Answer 37

A low-resistance wire which is connected to the metal casing of the appliance and the other end is connected to earth so that any leakage of large current will be directed to the ground.

It provides a pathway of low resistance to conduct large current away from the appliance when the appliance is “live”, preventing the user from getting an electric shock.

Question 38

Double insulation

Answer 38

A safety feature in electrical appliances. They normally use 2-pin plug and the electric cables are insulated from the internal components of the appliance. The internal components are also insulated from the external casing.

Question 39

A wind turbine produces an alternating voltage of 600V. Electric cables connect the wind turbine to houses some distance away. Energy is wasted within cables. State and explain one method to reduce energy wasted. [2]

Answer 39

Include a step-up transformer to increase the voltage.
By having a larger alternating voltage, this will result in a smaller current flowing in the wires resulting in less thermal energy being wasted within the cables through P = I2 R losses.
OR
Increase the area of the cable to decrease the resistance as R = (row x L) / A, where
A is the cross-sectional area of the cable.
By having a smaller resistance, this will result in less thermal energy being wasted within the cables through P = I2 R losses.

Question 40

Explain why using a resistor of higher resistance increases battery life. [1]

Answer 40

A larger resistance by I = V/R, results in a lower current flowing through the circuit with the
same voltage at the source hence lesser energy is depleted from the battery to generate the lower current.

Question 41

Explain why the value of R1 cannot increase beyond6k ohm. [1]

Answer 41

When the value of R1 is increased beyond 6000 ohm, total effective resistance increases to such that the current flowing to the electronic device may be smaller than 1.0 mA, which may result in the electronic device not working normally.

(Note: common mistake is to say that it may cause a fire or overheating though
the current is low)