LS2 Science- Electricity

Question 1

Electric current (I)

Answer 1

Refers to the flow of electricity
Symbol: I
SI Unit: Ampere (A)

• Measured by: amount of electric charge moving per unit time past any point in the circuit

Question 2

Conventional current

Answer 2

Current flows from a Positive terminal → Negative terminal

Question 3

Electron flow

Answer 3

Current flows from a Negative terminal → Positive terminal

Question 4

Electric curcuits

Answer 4

(P.S: Refer to notes for drawing of symbols)

- All the components of the electric circuit must be connected correctly for it to work

Question 5

Switch

Answer 5

Turns the current on or off

Question 6

Dry cell

Answer 6

Provides energy for the electric current to flow in the circuit

Question 7

Wires

Answer 7

Connects the various components together, providing a complete circuit for the current to flow.

Question 8

Bulb

Answer 8

Converts electrical energy into light and heat

Question 9

Series circuit

Answer 9

Electrical components are joined one after another to form a single loop.
• Electric current passes through a single path in a series circuit

Question 10

Parallel circuit

Answer 10

• Electric current is divided into BRANCHES
• Electrical components are connected in each branch
• Electric current passes through all branches of a parallel circuit

Question 11

Ammeter (Low resistance)

Answer 11

Used to measure Current.
- Must be connected in SERIES
Drawing: A circle with an ‘A’ in it.

Question 12

Potential Difference (V) [In short ‘p.d.’]

Answer 12

Refers to the energy needed to move one unit of electric charge from one point to another.
Symbol: V
SI Unit: Volt (V)

• One volt of potential difference means that one joule of energy is needed to move one unit of charge
• The more energy needed to move a charge between two points in a circuit, the greater the potential difference between the two points

Question 13

Voltmeter (High resistance)

Answer 13

Used to measure Potential Difference.
- Must be connected in PARALLEL
Drawing: A circle with a ‘V’ in it.

Question 14

Resistance (R)

Answer 14

The resistance of a component is the potential difference over the current flowing through it.
Symbol: R
SI Unit: Ohm (Ω)

Resistance of Object=
Volts/Ampere OR
Potential difference/ Current
R= V/I (Current) [Symbols]

Question 15

Factors of resistance

Answer 15

• Cross sectional area (Narrower= More resistance)
• Length (Longer= More resistance)
• Type of material (Poor conductor= More resistance)

Question 16

Calculating Resistance (16.1) [SERIES Circuit]

Answer 16

SERIES Circuit: Components are joined one after another to form a single loop
(Only 1 path for current to flow)
[Take R₁ and R₂ as bulbs connected in a series circuit]
Total resistance = R₁ + R₂

Question 17

Calculating Resistance (16.1) [PARALLEL Circuit]

Answer 17

PARALLEL Circuit: Components are connected in branches and the current gets divided into branches
[Take R₁ and R₂ as bulbs in separate branches in a parallel circuit]
• The voltage across each branch in parallel is the same.
Therefore, the combined resistance become less than the resistance of either resistors.
Hence, R

Question 18

Fixed resistors

Answer 18

If a circuit contains only of fixed resistors, the resistance of the circuit cannot be changed unless the resistors themselves are changed.

Question 19

Variable resistors (rheostats)

Answer 19

[Refer to notes]

• The resistance can be changed with the long resistance wire (Through sliders)
• Low resistance = Bulb lights up brighter

Question 20

Arrangement of circuit components (Affecting resistance)

Answer 20

[Fixed resistor] Drawing: A rectangle
[Variable resistor- rheostat] Drawing: A rectangle with a arrow in the north- east direction

Examples [Brightness] :
Arranging resistors in parallel (Bulbs in series) > Arranging resistors in series in series (Bulbs also in series)

Question 21

16.2 Chemical effect of an electric current

Answer 21

Able to cause a chemical change when passed through certain liquids
Chemical Effect used in:
• ‘Electrolysis’ and ‘Extraction of metals’ (Breaking down compounds into its elements)
• ‘Electroplating’ (Object coated with a metal, then immersing it in a liquid then passing an electric current through the liquid) , coating objects with a thin layer of metal

Question 22

Magnetic effect of an electric current

Answer 22

• An electric current has an effect on magnets.
- This effect can be seen when an electric current flows through a compass
• Solenoid: Can increase the magnetic effect by making current flow in a coil of wire (Solenoid)
- Gives a magnetic field that can be switched on and off
• Telephones (For the hearing impaired): A changing magnetic effect causes a thin sheet of metal (diaphragm) to vibrate

Question 23

Heating effect of an electric current

Answer 23

• Heating effect
- Wire heats up as a current passes through it
- Converts electric energy into heat
- Greater resistance = Greater heat
• Lighting effect:
(Light bulb)
- The current flows through the highly-coiled filament
- The current causes the filament to heat up, giving out light

Question 24

Power (16.3 Household Electricity)

Answer 24

It is the amount of electrical energy converted to other forms of energy per unit time by the component
Symbol: ‘P’
SI Unit: watt (W)
- The power rating tells us how quickly an appliance converts electrical energy into other forms of energy
FORMULA: Energy = ‘Power (kW or W)’ times ‘TIme’

Question 25

Electrical Power (Equation)

Answer 25

• Power is the rate of work done
  Power = Work done/ Time
  Power = VI
• Electric current is measured by the amount of charge moving per unit time
  I = Q/T
  Q= IT, where ‘Q’ equals to charge

Question 26

Reducing wastage of electricity

Answer 26

Importance:
- Singapore does not have the natural resources such as fossil fuels. The electric energy we need comes from imported fuels
MINIMISING:
- Turing off electrical appliances when not in use
- Ensuring that all electrical appliances are in good working condition
- Using energy efficient appliances

Question 27

Hazards of electricity > Short circuit

Answer 27

Short Circuit: Occurs when the live wire touches the neutral wire, providing a shorter path of low resistance for the current to flow through

Question 28

Hazards of electricity > Electric shocks and electrocution

Answer 28

Occurs when large currents passes through a person’s body

• Leads to serious injuries or death
• This can happen in appliances with metal casing or metallic parts on the outside
• We may accidentally touch the exposed metallic parts that conduct electricity

Question 29

Prevention from hazards

Answer 29

• Switch off and pull out the plug from sockets before cleaning appliances (e.g fans)
• Never use electrical gadgets in wet places/ Do not use with wet hands
• Call an electrician to repair damaged appliances (When wires inside are exposed)

Question 30

Earth wire (Safety features)

Answer 30

Provides a path of low electrical resistance, hence the current flows directly from the live wire into the ground or earth
- Yellow and green (at 0 volts)

Question 31

Applications of earth wire

Answer 31

• People who work with high voltage electrical equipment must wear rubber shoes
  Reason: The high resistance prevents current to flow through the person to the ground/ Safe from electrocution
• Appliances with metal surfaces are earthed
• Appliances without metal surfaces are not earthed

Question 32

Fuse

Answer 32

A short thin wire placed within an insulating capsule

• Makes use of the heating current to switch off a large current
• A large current causes the short, thin wire in the fuse to heat up and melt
• Prevents the burning of the equipment (Electrical fires) as the circuit will be broken
• Needs to be replaced

Question 33

Circuit breakers

Answer 33

Is able to cut off an electric current.
- Does not need to be replaced
- Found in the consumer unit in our homes
- When the current in the circuit becomes too large:
(Trigger mechanism switches it off and breaks the circuit)
- Shuts off the large current before it causes a fire