electricity

Cell/battery

Provides the circuit with a source of potential difference. A battery is two or more cells

Switch

Turn the circuit on (closed), or off (open)

Resistor

A resistor limits the flow of current. A fixed resistor has a resistance it cannot change

Variable resistor

A resistor with a slider that can be used to change its resistance. These are often used in dimmer switches and volume controls

Thermistor

The resistance of a thermistor depends on its temperature. As its temperature increases, its resistance decreases and vice versa

Light-dependent resistor (LDR)

The resistance of a LDR depends on the light intensity. As the light intensity increases, its resistance decreases and vice versa.

Diode

Diode allows current flow in one direction only. They are used to convert AC to DC current

Light Emitting Diode (LED)

This is equivalent to a diode and emits light when a current passes through it. These are used for aviation lighting and displays (TVs, road signs)

Ammeter

Used to measure the current in a circuit. Connected in series with other components.

Voltmeter

Use to measure the potential difference of an electrical component. Connected in parallel with the relevant components.

Sources of potential difference

• A cell
• Batteries
• Electrical Generator

Potential Difference

The energy transferred per unit charge flowing from one point to another. This is known as voltage and the symbol is V.

Potential difference equation

V = W/Q \Rightarrow \text{Potential difference} = \text{Work Done} / \text{Charge}

Ohm's Law Equation

V = I \times R \Rightarrow \text{Potential Difference} = \text{Current (Amps, A)} \times \text{Resistance (Ohms)}

Electric current

The flow of electrical charge. It is measured in Amps, A and the symbol for current is I.

Electric Current Equation

Q = I \times t \Rightarrow \text{Charge (C)} = \text{Current (A)} \times \text{Time (s)}

Electrons

Negatively charged, they flow away from the negative terminal if a cell towards the positive terminal.

Conventional Current

The flow of positive charge from the positive terminal of a cell to the negative terminal

Circuit in a closed loop

The current is the same value at any point.

• This is because the number if electrons per second that passes through one part of the circuit is the same number through any other part. All components in a closed-loop have the same current.

Resistance

The opposition to flow of current. The higher the resistance of a circuit, the lower the current. Good conductors have a low resistance and insulators have high resistance. Symbol for Resistance is R and is measured in Ohms.

The greater the resistance…

The lower the current for a given potential difference across the component.

The lower the resistance…

The greater the current for a Gutenberg potential difference across the component.

Required Practical: Investigating Resistance - Equipment

• Power Supply/Cell/Battery: Source if potential difference to the circuit.
• Wires: To connect all components in the circuits
• Crocodile Clips: To connect different lengths of the resistance wire.
• Ammeter: To measure the current through the circuit (0.01 A)
• Voltmeter: To measure the potential difference through the resistors. (0.1 V)
• 2 or more resistors: To measure the resistance of the length of a wire at a constant temperature
• Thin resistance wire: To measure the resistance of the length of wires at a constant temperature
• Metre ruler: To measure the length of the resistance wire( 1mm)

Required Practical: Investigating Resistance - Aim

To investigate how the length of a wire at a constant temperature affects the resistance of electrical circuits.

Required Practical: Investigating Resistance - Variables

• Independent Variable: Length of resistance wire
• Dependent Variable: Resistance
• Control Variables:
  • Potential difference of the power supply
  • Temperature of the wire

Required Practical: Investigating Resistance - Method

1. Set up the apparatus by connecting two crocodile clips to the thin resistance wire a distance of 10 cm apart and setting the power supply to 1.5 V
2. Connect the wire, using the clips, to the rest of the circuit
3. Record the potential difference from the voltmeter and current from the ammeter
4. Move the clips in 10 cm intervals further apart
5. Take new measurements from the voltmeter and ammeter for each length reading
6. Continue until the crocodile clips are a length of 1 m apart

Combinations of Resistors in Series and Parallel - Aim

The aim of this experiment is to investigate how combinations of resistors in series and parallel affect the total resistance in electrical circuits

Combinations of Resistors in Series and Parallel - Variables

• Independent Variable: Number of Resistors
• Dependent Variable: Total Resistance
• Control Variables:
  • Potential Difference of the power supply
  • Temperature of resistors

Required Practical - Investigating Resistance - Systematic Errors

• The first crocodile clip should start at 0 on the ruler, since it would cause a zero error in measurements of the length.
• Both the ammeter and voltmeter should be checked to start from 0

Required Practical - Investigating Resistance - Random Errors

• Only allow small currents to flow through the wire, this keeps the temperature of the wire constant, so it doesn't change its resistance.
• Current should be switched off between readings so its temperature doesn't change its resistance.
• Repeat the experiment by reducing the length of the wire 10cm each time down to a length of 10cm.

Required Practical - Investigating Resistance - Safety Considerations

• Make sure never to touch the wire directly when the circuit is switched off.
• Switch off the power supply right away if burning is smelled
• Make sure there are no liquids close to the equipment, as this could damage the electrical equipment if spilled.

Two types of resistors

• Fixed Resistor
• Variable resistors
• Once the resistance is set, it will stay at this value no matter how the current changes..
• Resistance if components such as lamps, diodes, thermistors and LDRs changes with the current through the component.

Fixed Resistor

• Have a resistance that remains constant

Variable Resistor

• Can change the resistance through the circuit, this can vary the amount of current through the circuit

Ohm's Law

The current through a conductor is directly proportional to the potential difference across it. Electrical conductors the obey Ohm's Law are referred to an ohmic conductors, it is relevant only at constant temperatures. E.g:

• Fixed Resistors
• Wires
• Heating Elements

Filament Lamps

• As the current increases, the temperature of the filament in the lamp increases
• The higher temperature causes the atoms in the metal lattice of the filament to vibrate more
• This causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through
• Resistance opposes the current, causing the current to increase at a slower rate

Resistance and Temperature

• The higher the temperature, the faster these atoms vibrate
• An increase in temperature causes an increase in resistance

Diodes

• A component that lets current flow in one direction only (forward bias)
• In the reverse direction, the diode has a very high resistance, therefore no current flows (reverse bias)
• An LED is a specific type of diode that emits light and works the same way as a normal diode.

Linear and Non-Linear Graphs

Linear components are said to obey Ohm's Law and have a constant resistance, whilst non-linear do not.

Linear Elements

• Fixed Resistors
• Wires
• Heating Elements

Non-Linear Elements

• Filaments Lamps
• Diodes & LEDs
• LDRs
• Thermistors

Thermistors

• As temperature increases the resistance of a thermistor decreases and vice versa..
• A thermistor is a temperature sensor and is regularly used as a thermostat, this means it automatically regulates temperature or activates when the temperature reaches a certain point.

Thermistors are found in…

• Ovens
• Refrigerators
• Fire alarms
• Digital thermometers
• Boilers
• They are commonly used to regulate and monitor the temperature in environments where it must by carefully controlled e.g. food and beverage factories.

Light-Dependent Resistors

• As the light intensity increases the resistance of an LDR decreases and vice versa.

Application of LDRs

• Light sensor: It automatically regulates the amount of light intensity on it or activates a device when the light intensity reaches above or below a certain point

LDRs are found in…

• Lights that switch on when it gets dark
• Alarm clocks
• Burglar alarm circuits
• Light intensity meters
• Security lights
• The advantage of an LDR is that these circuits are automatic therefore nit needing any human time and intervention to function correctly everyday.

Required Practical: Investigating I-V Characteristics - Aim

The aim of the experiment is to use circuit diagrams to construct appropriate circuits to investigate the I-V characteristics of a variety of circuit elements. These include a fixed resistor at a constant temperature, a lamp and diode.

Required Practical: Investigating I-V Characteristics - Variables

• Independent Variable: Potential Difference
• Dependent Variable: Current
• Control Variables:
  • Potential difference of the power supply
  • Use of the same equipment e.g. wires, diodes

Required Practical: Investigating I-V Characteristics - Equipment List

• Ammeter =0.01 A
• Voltmeter = 0.1V
• Variable Resistors = 0.005 ohms
• Fixed Resistor (between 100 and 500 ohms)
• Filament Lamp
• Diode
• Voltage Supply
• Wires

Required Practical: Investigating I-V Characteristics - Method

1. Set up the circuit as shown with the fixed resistor
2. Vary the voltage across the component by changing the resistance of the variable resistor, using a wide range of voltages (between 8-10 readings). Check the appropriate voltage reading on the voltmeter
3. For each voltage, record the value of the current from the ammeter 3 times and calculate the average current
4. Increase the voltage further in steps of 0.5 V and repeat steps 2 and 3
5. Make sure to switch off the circuit in between readings to prevent heating of the component and wires
6. Reverse the terminals of the power supply and take readings for the negative voltage (and therefore negative current)
7. Replace the fixed resistor with the filament lamp, then the diode, repeating the experiment for each

Required Practical: Investigating I-V Characteristics - Systematic Errors

• The Voltmeter and ammeters SHIULD start from zero, to avoid zero error in the readings

Required Practical: Investigating I-V Characteristics - Random Errors

• The voltmeter and ammeter will still have some resistance, therefore the voltages and currents displayed will be slightly inaccurate.
• The temperature of the equipment should effect its resistance
• Taking multiple readings of the current for each component will provide a more accurate result and reduce uncertainties.

Required Practical: Investigating I-V Characteristics - Safety considerations

• When there is a high current and a thin wire the wire will become very hot, make sure to never touch the wire directly when the circuit is switched on
• Switch off the power supply right away if burning is smelled
• Make sure there are no liquids close to the equipment, as this could damage the electrical equipment
• The components will get hot especially at higher voltages
• Make make sure there are no liquids close to the equipment, as this could damage the electrical equipment
• Disconnect the power supply in between readings to avoid the components heating up too much

Series Circuit

• Consists of a string of two or more components, connected end to end.
• The current is the same at all points, through each component
• The total potential difference of the power supply is shared between the components.
• The total resistance of the two components is the sum of the resistance of each component

Parallel circuit

• Consists of two more components attached along separate branches of the circuits.
• The current through the whole circuit is the sum of the currents through the separate components.
• The potential difference across each component is the same
• The total resistance of two resistors is less than the resistance of the smallest individual resistor

Resistors in series circuits

• When two or more resistors are connected in series, the total resistance is equal to the sum of their individual resistances.
• Increasing the number of resistors increases the overall resistance, as the charge now has more resistors to pass through.

Resistors in parallel circuits

• When two or more resistors are connected in parallel, the combined resistance increases.
• This happens because each resistor creates an extra paste along which the charge can flow, this allows more charge to flow over all and smaller overall resistance.
• The advantages of this kind of circuit are:
  • The components can be individually controlled, using their own stitches.
  • If one component stops working the others will continue to function

Comparing series and parallel circuits - Current

• In a series circuit, the current is the same at all points
• In a parallel circuit, the current splits at junctions - some of it going one way and the rest going the other

Comparing series and parallel circuits - Potential Difference

• In a series circuit, the voltage if the power supply is shared between the components
• In a parallel circuit, the voltage across each component is the same

Comparing series and parallel circuits - Resistors

• In a series circuit, the total resistance is the sum of the resistance. Two resistors will have a larger overall resistance than just one, This is because the charge has to push through multiple components when flowing around the circuit.
• In a parallel circuit, the total resistance decreases and is less than the resistance of any of the individual components. Two resistors in parallel will have a smaller over all resistance than one. This is because the charge has more than one pathway to take, so only some charge will flow along each path

DC Series Circuits

• A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative. The potential difference across a cell in a d.c circuit is in one direction only and has a fixed positive and negative terminal

Alternating Current (AC)

• A current that continuously changes its direction, going back and forth around a circuit. It has two identical terminals.

Mains Electricity

• The electricity generated by power stations and transported around the country through the National Grid.
• It is an alternating current supply.
• In the UK, the domestic electricity supply has a frequency of 59Hz and a potential difference of about 230V

3 Core Cables

• In the UK, most electrical appliances are connected to the mains using a three-core cable consisting of:
  • A live wire - Brown
  • A neutral wire - Blue
  • An Earth wire - Green and Yellow

Live Wire

• Carries the alternating potential difference from the mains supply to a circuit.
• It is the most dangerously if the three wires
• If it touches the appliance without the Earth wire, it can carouse electrocution
• It has a voltage of 230 V

Neutral Wire

• Forms the opposite end of the circuit to the live wire to complete the circuit
• Because if it's lower voltage, it is much less dangerous than the live wire
• It has a voltage close to 0 V

Earth wire

• Acts as a safety wire to stop the appliance from becoming live,
• This prevents electric shocks from occurring if the appliance malfunctions or the live wire breaks off and touches the case of the plug
• It has a voltage of 0 V

Dangers of Mains Electricity

• If a live wire came into contact with the case, inside the appliance, the case would become electrified. So if anyone touched it they would risk electrocution.
• The earth wire provides a low resistance paste to the earth
• This causes a surge of current in the Earth wire and hence also in the live wire
• The high current through the fis causes it to melt and break
• This cuts off the supply of electricity to the appliance, making it safe to touch.

Dangers if Mains Electricity Current

• Because of the large potential difference between the live (230 V) and the earth (0 V), if the two are connected together, a very large current can be created
• If a person provides the connection between live and earth then a large current can pass through them, providing a potentially lethal shock
• Electricians will always switch off the mains electricity supply to the whole house, or section of a house when they are working with electrical appliances
• Thus is because they will come into contact with live wires when they are working
• The potential difference of the live wire is 230 V and the potential of the electrician is 0 V
• Therefore, there is a large potential difference between the live wire and the electrician, so, a current would pass through the electrician's body to reach the earth
• Even if a device is switched off but the mains supply is on, the live wire can still cause an electric shock

Power

• The rate of energy transfer or the amount of energy transferred per second.
• The power of a device depends on the voltage and current of the device.

Power Equation

P = V \times I \Rightarrow \text{Power (Watts)} = \text{Current (A)} \times \text{Potential Difference/Voltage (V)}

Power and Resistance

• Voltage depends upon the current and resistance of that device
  P = I^2 \times R \Rightarrow \text{Power (Watts)} = \text{Current}^2 \text{(A)} \times \text{Resistance (Ohms)}

Energy and Power

• Everyday appliances transfer energy electrically from the mains to energy stores within the appliance: In a heater, energy will be transferred to the thermal store of the heating element, and then to the thermal store of the surroundings.

Amount if energy transferred to and from an appliance depends on

• How long the appliance is switched on for
• The power if the appliance
• A 1kW iron uses the same amount of energy in 1 hours as a 2kW iron uses in 30 mins
• A 100 W heater uses the same amount of energy in 30 hours as a 3000 W heater does in 1 hour.

Energy Transfers in Appliances

• Some domestic appliances, such as a remote control, transfer energy electrically from DC cells and batteries
• Most larger household appliances transfer energy electrically from the AC mains
• This energy can often be transferred to the kinetic energy store of an electric motor
• Motors are used in:
  • Vacuum cleaners - to create the suction to suck in dust and dirt off carpets
  • Washing machines (or tumble dryers) - to rotate the drum to wash (or dry) clothes
  • Refrigerators - to compress the refrigerant chemical into a liquid to reduce the temperature

Work Done

equal to energy transferred

Calculating Energy Transferred

• E = P \times T \Rightarrow \text{Energy (J)} = \text{Power (W)} \times \text{Time (s)}
• E = I \times V \times T \Rightarrow \text{Energy (J)} = \text{Current (A)} \times \text{Potential Difference (V)} \times \text{Time (s)}
• \text{Energy Transferred Electrically} = E = Q \times V \Rightarrow \text{Energy (J)} = \text{Charge (Coulombs)} \times \text{Potential Difference (V)}

Power Ratings

• Power Ratings for domestic appliances are usually given on a label this includes:
• The potential difference required to make the device work
• The frequency of the supply
• The power rating in Watts
• The higher the power rating, the faster the energy is transferred

The National Grid

• Distributes electricity across the UK
• It consists of a system of cables and transformers linking power stations to consumers (houses, factories and buildings).
• Electrical power is transferred from power stations to consumers using the National Grid
• Transformers include:
  • Step-up transformers which increase the voltage and reduces the current through the wires
  • Step-down transformers which decrease the voltage and increases the current through the wires

Benefits if the National Grid

• Efficient way to transfer energy due to the use of step-up/down transformers
• The current that is generated is greater than which is required for homes and other buildings so it must pass through a network of wires.
• When electricity is transmitted over large distances, the resistance in the wires causes heating

use of transformers in the high voltage transmission of electricity

• Transformers are used to increase and decrease the potential difference of the current before and after transmission across the National Grid
• A step-up transformer has more turns on the secondary coil than the primary
• A step-down transformer has more turns on the primary coil than the secondary

Charging by friction

When certain insulating materials are rubbed against each other and become electrically charged; negatively charged electrons are rubbed off one material and on to the other.

• An example of this is a plastic or polythene rod being charged by rubbing it with a cloth

Static Electricity

• Caused by a build up of stationary charge on a surface this occurs on the surfaces of insulators. E.g
• The accumulation of dust particles on surfaces
• Hair sticking up after combing it with a plastic comb or going down a plastic slide
• Rubbing a balloon and sticking it to s wall
• Sparking

Demonstrating Forces between Charges

• When charged objects are brought near each other, forces of attraction or repulsion can be observed.
• When two objects with different types of charge come together, a force of attraction is observed.
• When two objects of the same type of charge come together, a force of repulsion is observed.

Sparking

• A spark occurs between two objects when:
• There is a large potential difference between the two objects which causes current to flow between them
• Sparking often occurs between a charged insulator and an earthed conductor
• An earthed conductor is a wire, usually made from copper, that allows a current to flow to the Earth
• A current will always take the path of lower resistance
• Since copper has a lower resistance than, for example, a person, the current will flow from the insulator to the Earth through the copper wire rather than the person

Dangers of Sparking

Sparking can become dangerous in certain situations such as:

• Electrocution e.g. by lightning
• Ignition of a fire or explosion by a spark

Lightning

• In a storm, ice crystals in clouds rub against each other causing a movement of electrons between them
• The top of the cloud becomes positively charged, and the bottom becomes negatively charged
• The electrons on the ground are strongly repelled by the negative charge on the cloud, which causes it to become positively charged
• The potential difference between the cloud and the ground becomes increasingly large (\sim 10^6 \text{ V})
• Eventually, the cloud discharges a large spark as the negative charges jump to meet the positive charges on the ground

Ignition by sparking

• may ignite an explosion or fire when close to a flammable gas or liquid
• For example, when refuelling aeroplanes, a build-up of static charge can pose a significant danger
• As the fuel passes through a pipe, the friction between them causes static charge to build up
• If the potential difference becomes too large, it could cause a spark
• A spark could then ignite the fuel and cause an explosion

Electric Fields

• Objects in an electric field will experience an electrostatic force
• Since force is a vector, the direction of this force depends on whether the charges are the same or opposite
• The force is either attractive or repulsive
• If the charges are the same (negative and negative or positive and positive), this force will be repulsive and the second charged object will move away from the charge creating the field
• If the charges are the opposite (negative and positive), this force will be attractive and the second charged object will move toward the charge creating the field

Size of the force

• The size of the force depends on the strength of the field at that point
• This means that the force becomes:
  • Stronger as the distance between the two charged objects decreases
  • Weaker as the distance between the two charged objects increases

Strength and distance of the force

• The relationship between the strength of the force and the distance applies to both the force of attraction and force of repulsion
• Two negative charges brought close together will have a stronger repulsive force than if they were far apart

Electric Field Definition

A region where an electric charge experiences an force

• Electric fields are represented by electric field lines that are always in the direction of positive to negative
• The electric field lines for a charged, isolated sphere, such as a spherical conductor:
  • Point away from the centre of a positive sphere
  • Point towards the centre of a negative sphere
• A uniform electric field, such as that between two parallel plates, are straight parallel lines from positive to negative

Electric Field

• The electric field helps to explain the non-contact force between charged objects since the electric field cannot be seen, but can be detected by another charged object that moves within that field due to the electric force
• This is a non-