Unit 13: Electrical Systems

Question 1

Topics

Answer 1

• Definitions
• Current, Potential Difference, Resistance
• Drawing and Interpretation of Circuits
• Series vs Parallel Circuits
• Effects of Current (Heating, Magnetic, Chemical)
• Household Electricity
• Dangers of Electricity (Electric Shocks & Fires, Fuse, 3-Pin plug)
• Consumption of Electrical Energy (Power)

Question 2

Parts of a Circuit (1pt)

Answer 2

• The parts of an electrical system are called electrical components.
• Electrical systems consist of an electrical source and one or more load.

Question 3

Current, Potential Difference & Resistance
Current
2pt

Answer 3

• Electric current refers to the rate of flow of electrons in a circuit.
• Electrons move from the negative terminal of the cell around the circuit and back to the positive terminal of the cell (opposite of conventional current direction which is from +ve to -ve)

Question 4

Ammeter (3-4pt)

Answer 4

• The magnitude of an electric current can be measured using an ammeter.
• The S.I. Unit for current is Ampere (A).
• An ammeter is connected in series in a circuit (near to the current of bulb to be measured (if any)).
• The positive terminal (red) is connected to the positive side of the cell and the negative terminal (black) is connected to the negative side of the cell.
• Metals are good conductors of electricity as they have free electrons that can move freely.
• Circuit wires are made of metal. To get an electric current, electrons in the metal atoms move from one atom to the next in the wires.
• To measure the current of the circuit with 2 bulbs in parallel with different resistance, take the current of the 2 bulbs and divide by 2! (i.e. the ammeter is in a place where the current will pass through the 2 bulbs)

Question 5

Potential Difference (1-2pt)

Answer 5

• Potential Difference is the work done to drive a unit charge across an electrical component.
• The potential difference (p.d.) between two points is a measure of the amount of energy that is changed into other forms of energy when a unit of charge passes between these two points.
• An electron leaving the negative terminal of an electric cell has a higher (electrical) potential energy. At other points in the circuit, its (electrical) potential energy is lower.
• When a unit charge passes through a light bulb or other electrical components, the electrical energy is converted to other forms of energy.
• E.g. energy conversion in a light bulb: Electrical energy → Light energy + Thermal energy (LED bulb lighter bc no energy converted to TE)
• The term potential difference is used for the voltage between any two points in a circuit.
• The term voltage is commonly used to describe how many volts are in an electrical device.

Question 6

Voltmeter (4-5pt)

Answer 6

• A voltmeter can be used to measure the potential difference of a component.
• The S.I. Unit of potential difference (voltage) is the Volt (V).
• A voltmeter is connected in parallel across the bulb (basically add wire).
• The positive terminal (red) is connected to the positive side of the cell and the negative terminal (black) is connected to the negative side of the cell.
• As the number of electric cells in series increases, the voltage increases.
• The total voltage across all the electric cells is equal to the sum` of the voltages of the individual cells.
• Add dots to indicate junction!!

Question 7

Resistance (3-4pt)

Answer 7

• The resistance of a component is the ratio of the potential difference across it to the current flowing through it.
• The higher the resistance in a component, the higher the potential difference needed to move electric charge through the component.
• The electrical components in a circuit act as an obstacle against the flow of electrons. The components are said to have resistance.
• The SI unit of resistance is the ohm (Ω).
• The greater the ratio of V to I, the greater the resistance.
• The greater the resistance in a circuit, the lower the current.
• The longer the wire, the higher the resistance!!

Question 8

Formula for Resistance, Potential Difference and Current?

Answer 8

R = V / I
V = R I
I = V / R

Question 9

Resistors - Types?

Answer 9

Resistors
* There are two types of resistors: Fixed and Variable resistors.

Fixed Resistors
* A fixed resistor (often called a resistor) has a fixed resistance.
* The resistance of fixed resistors can range from a fraction of an ohm to thousands of ohms.

Variable Resistor (Rheostat)
* A variable resistor called a rheostat is used to change (vary) the resistance in a circuit.
* As the resistance changes, the current also changes.
- When the resistance ↑/↓, the current ↓/↑, hence
the brightness of the bulb ↓/↑.

• When the slider is moved to the left / right, the resistance of the circuit ↓/↑.
• If resistors are joined in series,
• Total resistance (R) = Sum of individual resistance of each resistor
  → R = R1 + R2 + . . . + Rn
• If resistors are joined in parallel,
• Total resistance (R) = Resistance of the individual resistors
  → 1/R = 1/R1 + 1/R2 … +1 / Rn (substitute these values into the R on LHS)

Question 10

Resistors - Fixed resistors (1pt)

Answer 10

Fixed Resistors
* A fixed resistor (often called a resistor) has a fixed resistance.
* The resistance of fixed resistors can range from a fraction of an ohm to thousands of ohms.

Question 11

Resistors - Variable Resistor - Rheostat (3pt)

Answer 11

Variable Resistor (Rheostat)
* A variable resistor called a rheostat is used to change (vary) the resistance in a circuit.
* As the resistance changes, the current also changes.
- When the resistance ↑/↓, the current ↓/↑, hence
the brightness of the bulb ↓/↑.

• When the slider is moved to the left / right, the resistance of the circuit ↓/↑.

Question 12

How to calculate resistance for resistors joined in series and in parallel?

Answer 12

• If resistors are joined in series,
• Total resistance (R) = Sum of individual resistance of each resistor
  → R = R1 + R2 + . . . + Rn
• If resistors are joined in parallel,
• Total resistance (R) = Resistance of the individual resistors
  → 1/R = 1/R1 + 1/R2 … +1 / Rn (substitute these values into the R on LHS)

Question 13

How to calculate resistance for resistors joined in series?

Answer 13

• If resistors are joined in series,
• Total resistance (R) = Sum of individual resistance of each resistor
  → R = R1 + R2 + . . . + Rn

Question 14

How to calculate resistance for resistors joined in parallel?

Answer 14

• If resistors are joined in parallel,
• Total resistance (R) = Resistance of the individual resistors
  → 1/R = 1/R1 + 1/R2 … +1 / Rn (substitute these values into the R on LHS)

Question 15

Electric Circuits - Closed Circuits, Open Circuits (2pt each)

Answer 15

• When we join various electrical components together, they form an electric circuit (or simply a circuit).
• Electricity flows in a circuit when there is:
• a source of electrical energy
• a closed circuit
  Closed Circuit
• When electricity flows from one end of the electric cell, through the connecting wires and the light bulb, to the other end of the electric cell, a closed (or complete) circuit is formed.
• Because the circuit is closed, the bulb lights up.

Open Circuit
* When there is a gap in the circuit, electricity cannot flow from one end of the electric cell to the other. This is called an open (or incomplete) circuit.
* Electricity cannot flow in an open circuit. Hence the bulb does not light up.

Question 16

Drawing Circuit Diagrams

Answer 16

• An electric circuit can be drawn as a circuit diagram.
• Instead of actual components, a circuit diagram uses circuit symbols.

Electric circuits
* All the components of the electric circuit must be connected correctly for it to work.
* A circuit diagram helps us see if the electrical components are connected correctly.

Drawing circuit diagrams
* We use common symbols when drawing circuit diagrams.
* Ensure that rulers are used for wires/straight lines
* Junctions should be represented with a dot!

Question 17

Arrangement of Circuits

Answer 17

Arrangement
Series
Parallel

Current (A, I)
* Same throughout the circuit

• I = I1 + I2 + … In (find average)
• Branch with the least resistance will have the largest current flowing through it

Voltage (p.d, V)
* E = V1 + V2 … Vn (find average)
* Component with the largest resistance has the largest voltage across it

• Same across the parallel branches

Resistance (R)
* Total resistance (R) = Sum of individual resistance of each resistor
→ R = R1 + R2 + . . . + Rn

• Total resistance (R) = Resistance of the individual resistors
  → 1/R = 1/R1 + 1/R2 … +1 / Rn (substitute these values into the R on LHS)

Bulbs
* Brightness of each bulb is equivalent to the current divided by the number of each bulb (its equivalent to the current passing through it, A)
* If one bulb is removed or broken, no current flows, and the other bulbs do not light up because the circuit is now open.
* Brightness of each bulb is equivalent to the current
* If one bulb is removed or broken, no current flows, and the other bulbs do not light up because the circuit is now open.

Question 18

Effects of Electric currents (3pt)

Answer 18

• An electric current can produce three effects:
• Heating Effects
• Magnetic Effects
• Chemical Effects

Question 19

Heating Effect of an Electric current (2-3pt)

Answer 19

• When an electric current flows through a wire, the wire heats up.
• This is called the heating effect of an electric current.
• Energy change: electrical energy  thermal energy
• The greater the resistance of the wire, the greater the amount of heat produced.

Question 20

Applications of the Heating Effect of an Electric current (2-3pt)

Answer 20

• The heating effect is used in common electrical appliances, such as electric kettle, toaster and iron.
• Each appliance has a heating ‘element’ such as a high-resistance nichrome wire which becomes hot when a current flows through.
• Nichrome wire = good conductor of TE, metal = high resistance, high melting point = ↑EE converted to ↑TE (which comes from friction opposing the current, since there is a limit to the amount of current that can pass through, the extra EE would hence be converted to TE). – has similar properties to tungsten
• Heating element and connecting wire are made out of electrical conductors to allow electric current to flow.
• Heating element is made out of high resistance metal as the wire would become very hot when current passes through it.
• Connecting wire is made out of a low resistance metal to prevent a large amount of heat from being produced when current flows through it.

Question 21

Magnetic Effect of an Electric current (3-4pt)

Answer 21

• A bar magnet will cause the needle of a compass to be deflected. This illustrates the magnetic effect of a bar magnet.
• When an electric current flows through a wire, the compass needle is also deflected. This shows that an electric current also has a magnetic effect.
• Instead of a straight wire, a coil of wire can also be used. This increases the strength of the magnetic effect.
• A coil of wire, usually wound around a piece of iron, is called an electromagnet.
• When current flows through the coil, the coil acts like a bar magnet. When the current stops flowing, the electromagnet loses its magnetism.
• The magnetic effect can be increased by making current flow in a coil of wire called a solenoid.
• Advantages of electromagnet over a bar magnet: Can vary the strength of the magnet and can turn on and off its magnetism.

Question 22

How to increase strength of electromagnet?

Answer 22

• How to increase strength of electromagnet:
  1. Increase the number of coils
  2. Increase the number of batteries (cells)
  3. Add a piece of metal (solenoid)

Question 23

Applications of the Magnetic Effect of an Electric current

Answer 23

Used in
How it works

Magnetic cranes
* Used in cranes to lift iron and steel objects
* Objects are released by switching off the current
* Can also be used to separate iron and steel objects from non-magnetic material

Electric bells
* Electromagnet switches turn on and off rapidly causing a hammer to vibrate and hit a metal gong, producing sound.
Electric motors
* Make use of electromagnet to convert electrical energy into kinetic energy
* Can be found in fans, vacuum cleaner, washing machines and in electric train

Example
* When the bell button is pressed, an electric current flows through the coil of copper wire. It produces a magnetic effect that magnetises the iron core. The magnetised iron core attracts the iron armature which in turn causes the hammer to hit the gong. The circuit is broken and the coil loses it magnetism. The iron armature springs back. The above cycle repeats.
* Electric bell would not work if iron armature is replaced with a
plastic one as plastic is not a ferromagnetic material.

Electric motors
* Make use of electromagnet to convert electrical energy into kinetic energy
* Can be found in fans, vacuum cleaner, washing machines and in electric train

Question 24

Chemical Effect of an Electric current
Electrolysis 3pt

Answer 24

• Electrolysis is the decomposition of a compound by an electric current.
• When electricity passes through a solution of copper chloride, the copper chloride decomposes into copper and chlorine gas.
• The chemical effect of a current produces new compounds or causes compounds to break down.

Question 25

Chemical Effect of an Electric current - Extraction of Metals 2pt

Answer 25

• Reactive metals, such as sodium and aluminium, are extracted from their compounds.
• Electric current is passed through the molten compound of the metal, which breaks down to give the metal.

Question 26

Chemical Effect of an Electric current - Electroplating 4-5pt

Answer 26

• Electroplating is the process in which a metallic object is covered with a thin layer of another metal.
• Objects can be plated with metals such as copper, chromium, gold, silver and nickel.
• The metal being used would shrink by a little as it would be deposited on the metallic object.
• The diagram on the left shows the set-up to carry out electroplating. An iron spoon and a silver bar, used as the electrodes, are dipped into a silver nitrate solution.
• The surface of the spoon became more shiny when an electric current was passed through the solution as silver from the silver electrode and silver nitrate solution was deposited onto the surface of the spoon.
• When the iron spoon is replaced with a plastic spoon, there is no change in the plastic spoon (no electroplating) as plastic does not conduct electricity.

Question 27

Household Electricity
Dangers of Electricity
2 main effects, 4 dangers

Answer 27

• 2 main effects with main circuits: Electrical Shock and Electric Fire
• 4 Dangers:
• Touching frayed and damaged wires.
  o Insulation around a wire is damaged or worn out
  o When a person touches a bare wire, current flows through the body resulting in an electric shock.
  o Solution: Replace wires / call a technician / never touch the bare or broken wires.
• Overloading of appliances to a main socket.
  o Occurs when too many electrical appliances are connected to a mains socket using an electrical adaptor.
  o A large current flows in the house wiring which causes the wires to get hot.
  o When the current is too large, the circuit is said to be overloaded. This can cause a fire in the house wiring or the adaptor.
  o Solution: Never overload a circuit. If many appliances are connected to an adaptor, use only one appliance at a time.
• Wet Conditions; Touching a damaged wire or appliance with wet hands.
  o Water can conduct electricity even though it is not a good conductor.
  o When a person touches a damaged wire or appliance with his wet hands, an electric current may flow through his body, giving him an electric shock.
  o Solution: Never use appliances in wet places / never touch appliances, plugs or switches with wet hands.
• Short circuit occurs when a broken or bare wire touches another wire in the circuit.
  o Short circuit is a path of very low resistance between 2 points in a circuit. (↑ current = ↑ TE, hot = ↑catch fire)
  o Solution: Do not push anything into sockets or electrical appliances / call an electrician

Question 28

• 4 Dangers of electricity:

Answer 28

• Touching frayed and damaged wires.
  o Insulation around a wire is damaged or worn out
  o When a person touches a bare wire, current flows through the body resulting in an electric shock.
  o Solution: Replace wires / call a technician / never touch the bare or broken wires.
• Overloading of appliances to a main socket.
  o Occurs when too many electrical appliances are connected to a mains socket using an electrical adaptor.
  o A large current flows in the house wiring which causes the wires to get hot.
  o When the current is too large, the circuit is said to be overloaded. This can cause a fire in the house wiring or the adaptor.
  o Solution: Never overload a circuit. If many appliances are connected to an adaptor, use only one appliance at a time.
• Wet Conditions; Touching a damaged wire or appliance with wet hands.
  o Water can conduct electricity even though it is not a good conductor.
  o When a person touches a damaged wire or appliance with his wet hands, an electric current may flow through his body, giving him an electric shock.
  o Solution: Never use appliances in wet places / never touch appliances, plugs or switches with wet hands.
• Short circuit occurs when a broken or bare wire touches another wire in the circuit.
  o Short circuit is a path of very low resistance between 2 points in a circuit. (↑ current = ↑ TE, hot = ↑catch fire)
  o Solution: Do not push anything into sockets or electrical appliances / call an electrician

Question 29

Electric Shocks & Electrocution (4-5pt, 2subpt)

Answer 29

• An electric shock occurs when a person touches an exposed or damaged wire carrying a current.
• Electrocution occurs when a person is injured or killed by a large current passing through the body.
• This can happen if the insulation of the wires has been damaged or if the earth wire of an appliance is not properly connected.
• When a person touches the live wires, too much current flows through his body, giving him an electric shock.
• A current of just 0.1 A is often enough to kill a human.
• Dangers:
• Touching frayed and damaged wires.
• Touching a damaged wire or appliance with wet hands.

Question 30

Electrical Fires (4-5pt, 2subpt)

Answer 30

• An electric fire occurs when a large current flows in electric wiring and the wires get too hot.
• A short circuit occurs when the live wire touches the neutral wire, providing a shorter path of low resistance for the current to flow through.
• The large current flowing through the wires may cause the wires to overheat and start a fire.
• Electrical sparks from a short circuit may occur and start a fire when they come into contact with flammable materials.
• Connecting too many electrical appliances to the same output socket or forgetting to switch off an appliance may cause overheating of wires. This may cause a fire.
• Dangers:
• Overloading of appliances to a main socket
• Short circuit when a broken or bare wire touches another wire in the circuit

Question 31

13A 3-pin plug (fused plug) 2pt

Answer 31

• The cable grip/cord grip is used to hold the cable in place.
• Electricity flows through the live and neutral wires but not the earth wire.

Question 32

Fuse (3-4pt)

Answer 32

• The fuse blows and opens the circuit when the current flowing through the live wire exceeds the rated value.
• The fuse makes use of the heating effect of an electric current to stop a large current.
• A large current causes a short, thin wire in the fuse to heat up and melt.
• A fuse is labelled or rated with the current that will melt the wire in it.
• When the wire in a fuse melts, we say the fuse is ‘blown’ and the circuit is open.
• A blown fuse should be replaced after an electrical fault has been rectified.

Question 33

Consumption of Electrical Energy
Power - 4pt

Answer 33

● The power of an electrical appliance is the amount of electrical energy it converts to other forms of energy in one second.
● The SI Unit of power is the watt (W).
● The SI Unit of energy is Joules (J). 1W = 1J
● (One) kilowatt-hour (kWh) is the amount of electrical energy consumed by a (1) kW appliance in one hour.
● Energy consumed in (kWh) = Power (kW) x Time (h)

Question 34

How to calculate power?

Answer 34

● Energy consumed in (kWh) = Power (kW) x Time (h)

Question 35

How to convert:
1W to J?
1kW to W?
1MW to kW & W?

Answer 35

1 W = 1 J
1 kW = 1000 W
1 MW = 1000 kW
1 MV = 1 000 000 W

Question 36

Calculations for Science: General rules applied to mathematical manipulation of values (numbers) in terms of their d.p. or/and s.f. (2 impt pt - 3 pt)

Answer 36

• Round the answer to the least precise measurement used in calculation (incl. exact numbers)
• For addition, subtraction & average: Leave answers to the least number of decimal places! (Eg. 1.2 + 0.456 = 1.7 (1 d.p.))
• For multiplication, division & rest of operations: Leave answers to the least number of significant figures! (Eg. 5.25 x 10 = 53 (2 s.f.))