topic 2 - electricity

state the symbol for an open switch

state the symbol for a closed switch

state the symbol for a cell

state the symbol for a battery

state the symbol for a diode

state the symbol for a resistor

state the symbol for a variable resistor

state the symbol for an LED

state the symbol for a lamp

state the symbol for a fuse

state the symbol for a voltmeter

state the symbol for an ammeter

state the symbol for a thermistor

state the symbol for an LDR

state what is needed for electrical charge to flow through a closed circuit

source of potential difference

state what electrical current is

rate of flow of electrical charge

explain how the size of electrical current is related to electrical charge

size of electrical current is the rate of flow of electrical charge

state the equation that links current, charge and time

charge(C) = current (A) x time (s)

state the symbol equation that links current, charge and time

Q (C) = I (A) x t (s)

state what the current through a component depends on

• resistance of the component

• potential difference across the component

state the effect of increasing resistance of a component on the current through it with a given resistance

• increasing resistance decreases current

• they have an inversely proportional relationship

state the equation linking potential difference, current and resistance

potential difference (V) = current (A) x resistance (Ω)

state the symbol equation linking potential difference, current and resistance

p.d (V) = I (A) x R (Ω)

core practical - investigate how the length of a wire at constant temperature affects the resistance of an electrical circuit (method)

1. set up the apparatus by connecting two crocodile clips to the thin resistance wire a distance of 10cm apart

2. set the power of the power supply to 1.5V

3. connect the wire, using clips, to the rest of the circuit

4. record the initial current from the ammeter and the potential difference from the voltmeter

5. move the clips apart further in 10cm intervals

6. take new potential difference and current readings

7. continue until the crocodile clips are 1m apart

core practical - investigate how the length of a wire at constant temperature affects the resistance of an electrical circuit (results)

• as the distance between the crocodile clips increases (length of wire increases), the resistance will increases

• therefore resistance is directly proportional to the length of the wire

state the independent variable in the investigation of how the length of a wire at constant temperature affects the resistance of an electrical circuit

length of resistance wire

state the dependent variable in the investigation of how the length of a wire at constant temperature affects the resistance of an electrical circuit

resistance

state the control variable in the investigation of how the length of a wire at constant temperature affects the resistance of an electrical circuit

• potential difference of power supply

• temperature of wire

core practical - investigate how the combination of resistors affects the resistance of an electrical series circuit (method)

1. connect the first circuit with a battery of 4V with a resistor, a voltmeter connected in parallel to the resistor and an ammeter connected in series to the resistor

2. close the switch in the circuit and record the reading on the voltmeter and ammeter

3. open the switch in the circuit

4. repeat step 2 with another resistor in series with the other resistor

core practical - investigate how the combination of resistors affects the resistance of an electrical series circuit (results analysis)

• resistance across one resistor is the same as the resistance of 2+ resistors combined

• this is because the electrons flow through just one path, through both resistors, so the current does too

core practical - investigate how the combination of resistors affects the resistance of an electrical parallel circuit (method)

1. set up the circuit with a power of 4V with a resistor, another resistor in parallel, a voltmeter in parallel and an ammeter in series to both resistors

2. close the switch in the circuit and record the readings from the voltmeter and ammeter

core practical - investigate how the combination of resistors affects the resistance of an electrical parallel circuit (results analysis)

• the resistance of the resistors in parallel will be less than the total resistance of one or more resistors in series

• this is because the electrons are split between the different paths. but the resistors still have the same potential difference across them

explain the relationship between current through an ohmic conductor and potential difference across it

• directly proportional

• meaning resistance remains constant as current changes

explain what the graph showing the relationship between current through an ohmic conductor and potential difference across it looks like

• linear

• goes through origin

• positive correlation

explain the relationship between current through a filament lamp and potential difference across it

• directly proportional until a certain point

• as when the current increases, the potential differences increases

• however, when electrons flowing through the wire collide with ions in the wire, the temperature increases

• as the kinetic energy of the wire increases

• causing the resistance of the circuit to increases, decreasing the current through the circuit

explain what the graph showing the relationship between current through a filament lamp and potential difference across it looks like

• not linear

• goes through origin

• positive correlation

explain what the graph showing the relationship between current through a diode and potential difference across it looks like

• sharp increases on right side of the graph

• when the direction of the diode reverses, reverse bias occurs

state what happens to the resistance of a thermistor when temperature increases

resistance decreases

state an example of when the application of thermistors in a circuit is needed

thermostat

state what happens to the resistance of an LDR as light intensity increases

resistance decreases

state an example of when the application of LDRs in a circuit is needed

switching lights on when it gets dark

state the method of how to investigate the I-V characteristics of a filament lamp/diode/resistor at constant temperature in a circuit

1. set up a circuit with a power supply, a filament lamp/diode/resistor, an ammeter in series, a variable resistor in series and a voltmeter in parallel

2. vary the voltage across the filament lamp/diode/resistor by changing the resistance of the variable resistor

3. take a wide range of voltage readings

4. for each voltage reading taken, record the current reading 3 time and calculate an average current

5. increase the voltage by 0.5V and repeat steps 2 and 3

6. switch off the circuit between readings to prevent heating of components and wires

7. reverse the terminals of the power supply to get negative voltage and current readings

state the expected results of the investigation to determine the I-V characteristics of a filament lamp at constant temperature in a circuit

• the I-V graph should display a non-linear line

• as the filament lamp is a non-ohmic conductor

• meaning the resistance of the filament lamp doesn’t remain constant due to the changes in voltage

state the expected results of the investigation to determine the I-V characteristics of a diode at constant temperature in a circuit

• the I-V graph should display a non-linear line

• as the diode is non-ohmic conductor

• meaning the resistance of the diode doesn’t remain constant due to the changes in voltage

state the expected results of the investigation to determine the I-V characteristics of a resistor at constant temperature in a circuit

• the I-V graph should display a linear line

• as the resistor is an ohmic conductor

• meaning the resistance of the resistor remains constant despite the changes in voltage

state what is observed in series circuits

• there is the same current through each component

• the total potential difference is shared between components

• the resistance across components is equal

state the equation to calculate the total resistance across components in a series circuit

Rₜₒₜₐₗ = R₁ + R₂

state what is observed in parallel circuits

• the potential difference across each component is the same

• the total current through the circuit is the sum of current through each component

• the total resistance of 2+ resistors is less than the resistance of the smallest individual resistor

state how to calculate the total current in a parallel circuit

Iₜₒₜₐₗ = I₁ + I₂

state what the supply of mains electricity in the uk is

ac supply

state what the frequency of domestic electricity supply in the uk is

50Hz

state what the voltage of domestic electricity supply in the uk is

230V

what is the difference between direct (dc) and alternating (ac) potential difference

• ac - direction of flow of electrons constantly changes

• dc - direction of flow of electrons doesn’t change

state how most electrical appliances are connected to the mains supply

using three-core cables

state what the brown insulation in wires means

the wire is a live wire

state what the blue insulation in wires means

the wire is a neutral wire

state what the green and yellow stripes insulation in wires means

the wire is an earth wire

state what the purpose of a live wire is in a circuit

carries the alternating (ac) potential difference from the supply

state what the purpose of a neutral wire is in a circuit

completes the circuit

state what the purpose of an earth wire is in a circuit

safety wire to stop the appliance becoming live

state what the potential difference between live wires and earth wires is

230V

state the voltage of an earth wire

0V

state the voltage of a neutral wire

~0V

state the only time when an earth wire carries current

when there is a fault in the circuit

explain why live wires are dangerous even when a switch in the mains circuit is open

• if the circuit became complete, a large potential difference would start

• this could electrocute someone

explain the dangers of providing any connection between the live wire and earth

• a shock or fire will occur

• as a connection between the live wire and earth completes the circuit between them

state the equation linking power, potential difference and current

power (W) = voltage (V) x current (A)

state the symbol equation linking power, potential difference and current

P (W) = V (V) x I (A)

state the equation linking power, current and resistance

power (W) = (current)² (A) x resistance (Ω)

state the symbol equation linking power, current and resistance

P (W) = (I)² (A) x R (Ω)

state the reason for the design of electrical appliances

to bring about energy transfers

state what the amount of energy an appliance transfers depends on

• how long the appliance is switched on for

• power of the appliance

describe how a torch transfers energy from batteries to the kinetic energy of heating devices

• energy is initially in the chemical store of the torch’s batteries

• when the torch is switched on, the circuit is completed

• the kinetic energy of the electrons flowing in the current is transferred to the thermal store of the bulb

describe how a vacuum cleaner transfers energy from ac mains supply to the kinetic energy of electric motors

• energy is initially stored in the chemical store of the vacuum’s power station

• when the vacuum is connected to the ac mains supply, the circuit is completed

• the kinetic energy of the electrons flowing in the current is transferred to the kinetic store of the motor and the thermal store of the surroundings

state when work is done in a circuit

when charge flows

state the equation linking energy, power and time

energy (J) = power (W) x time (s)

state the symbol equation linking energy, power and time

E (J) = P (W) x t (s)

state the equation linking energy, charge and voltage

energy (J) = charge (C) x voltage(V)

state the symbol equation linking energy, charge and voltage

E (J) = Q (C) x V (V)

explain how the power of a circuit device is related to the potential difference across it

if the potential difference increases, the power increases

explain how the power of a circuit device is related to the current through it

if the current increases, the power increases

explain how the power of a circuit device is related to the energy transferred over a given time

if power increases, the rate of energy transfer increases

state what the power of an appliance is

amount of energy it transfers by electrical work every second

describe the relationship between the power ratings for domestic electrical appliances and the changes in stored energy when they are in use

• power rating of an appliance is how much energy it needs to work

• the more energy transferred per second by the appliance, the higher the power rating

state what the national grid is

• system of cables and transformers

• linking power stations to consumers

state how electrical power is transferred from power stations to consumers

national grid

state why step-up transformers are used

• to increase potential difference

• and reduce current

• from the power station

• to the transmission cables

state why step-down transformers are used

• to decrease potential difference

• and increase current

• for domestic use

explain why the national grid system is an efficient way to transfer energy from power stations to transmission cables

• its use of step-up and step-down transformers

• as the current generated by power stations is greater than needed for domestic use

• causing it to need to be transferred across the uk by cables

• and as electricity is transferred over large distances, resistance in the wires causes heating

• which increases unwanted energy transfers

• so increasing the potential difference transfers a larger amount of power using a smaller current

• which results in less unwanted energy transfers

explain why the national grid system is an efficient way to transfer energy from transmission cables to homes and buildings

• its use of step-up and step-down transformers

• as the current generated by power stations is greater than needed for domestic use

• causing it to need to be transferred across the uk by cables

• and as electricity is transferred over large distances, resistance in the wires causes heating

• so decreasing the potential difference transfers a smaller amount of power using a larger current

• which results in more energy transfers by heating

• allowing the power to reach the level found in homes and buildings

explain how insulating materials can become charged through friction

• through friction, negatively-charged electrons are transferred

• the material that loses electrons will now have a net positive charge

• the material that gains electrons will now have a net negative charge

state what happens when two electrically-charged objects are brought close together

they exert a force on each other

state what happens when two objects with like charges are brought close together

they repel

state what happens when two objects with opposite charges are brought close together

they attract

state what type of force attraction and repulsion between electrically-charged objects is

non-contact force

state how static electricity is produced

static electricity is caused by the build up of stationary charge on a surface

explain how sparks are produced by rubbing surfaces

• build up of electrostatic charge causes sparks

• sparks occur when there is a large potential difference between surfaces

• when there is contact/friction between surfaces, current flows between them

explain how magnets are evidence that charged objects exert force on each other when not in contact

• a magnet has two poles; north and south

• each pole is charged opposite to the other

• like poles will repel each other

• opposite poles will attract

• a magnetic force is experienced by any magnetic material in a magnetic field

• when two magnets are brought together in a magnetic field, the poles will attract or repel even though there is no contact between them

explain how the transfer of electrons between objects can explain static electricity

• when there is friction between surfaces, there is a transfer of electrons

• electrons building up on a surface produces static electricity

• creating an excess net negative charge on the surface receiving electrons

state when a charged object is created

when the objects creates an electrical field around itself