LS2 Science- Electricity Flashcards
Electric current (I)
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
Conventional current
Current flows from a Positive terminal → Negative terminal
Electron flow
Current flows from a Negative terminal → Positive terminal
Electric curcuits
(P.S: Refer to notes for drawing of symbols)
- All the components of the electric circuit must be connected correctly for it to work
Switch
Turns the current on or off
Dry cell
Provides energy for the electric current to flow in the circuit
Wires
Connects the various components together, providing a complete circuit for the current to flow.
Bulb
Converts electrical energy into light and heat
Series circuit
Electrical components are joined one after another to form a single loop.
• Electric current passes through a single path in a series circuit
Parallel circuit
- Electric current is divided into BRANCHES
- Electrical components are connected in each branch
- Electric current passes through all branches of a parallel circuit
Ammeter (Low resistance)
Used to measure Current.
- Must be connected in SERIES
Drawing: A circle with an ‘A’ in it.
Potential Difference (V) [In short ‘p.d.’]
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
Voltmeter (High resistance)
Used to measure Potential Difference.
- Must be connected in PARALLEL
Drawing: A circle with a ‘V’ in it.
Resistance (R)
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]
Factors of resistance
- Cross sectional area (Narrower= More resistance)
- Length (Longer= More resistance)
- Type of material (Poor conductor= More resistance)
Calculating Resistance (16.1) [SERIES Circuit]
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
Calculating Resistance (16.1) [PARALLEL Circuit]
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
Fixed resistors
If a circuit contains only of fixed resistors, the resistance of the circuit cannot be changed unless the resistors themselves are changed.
Variable resistors (rheostats)
[Refer to notes]
- The resistance can be changed with the long resistance wire (Through sliders)
- Low resistance = Bulb lights up brighter
Arrangement of circuit components (Affecting resistance)
[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)
16.2 Chemical effect of an electric current
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
Magnetic effect of an electric current
• 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
Heating effect of an electric current
• 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
Power (16.3 Household Electricity)
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’
Electrical Power (Equation)
- 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
Reducing wastage of electricity
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
Hazards of electricity > Short circuit
Short Circuit: Occurs when the live wire touches the neutral wire, providing a shorter path of low resistance for the current to flow through
Hazards of electricity > Electric shocks and electrocution
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
Prevention from hazards
- 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)
Earth wire (Safety features)
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)
Applications of earth wire
- 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
Fuse
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
Circuit breakers
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
