Topic 10 - electricity and circuits

the structure of the atom

positively charged nucleus surrounded by negatively charged electrons

subatomic particle	relative mass	relative charge
proton	1	+1
neutron	1	0
electron	0 (0.0005)	-1

symbol drawing for diode

symbol drawing for ammeter

symbol drawing for LDR

symbol drawing for thermistor

symbol drawing for battery

symbol drawing for voltmeter

symbol drawing for fixed resistor

symbol drawing for variable resistor

symbol drawing for filament bulb

symbol drawing for cell

symbol drawing for motor

symbol drawing for LED

series circuit

• closed circuit

• the current is the same everywhere

parallel circuits

• branched circuit

• current splits into multiple paths

• total current into a junction = total current in each of the branches

• voltage is the same across each “branch”

potential difference (represented as ‘V’)

• potential difference is measured in Volts

• energy transferred per unit charge, Joule per Coulomb

  • measured across two points, as it is the amount of energy per unit charge to move from one point to the next

• measured with a voltmeter, placed in parallel across a component

• there can be a voltage across a component, in a closed or open circuit

• when it is in a closed circuit, and there is a potential difference (voltage), current will always flow

E = QV
energy transferred (joule, J) = charge moved (coulomb, C) x potential difference (volt, V)

where is a voltmeter connected

a voltmeter is connected in parallel with a component to measure the potential difference (voltage), in volt across it

where is an ammeter connected

an ammeter is connected in series with a component to measure the current, in amp, in the component

current (represented by ‘I’)

• current is measured in amps

• it is the rate of flow of charge (the flow of electrons in the wires)

  • measured by any single point on the circuit

• measured with ammeter which is placed in series

• V = IR

  potential difference (volt, V) = current (ampere, A) x resistance (ohm, Ω)

• when a closed circuit includes a source of potential difference there will be a current in the circuit

• current is conserved at a junction in a circuit

equation for charge

charge (coulomb, C) = current (ampere, A) x time (second, s)

Q = I x t

resistance

• greater resistance, the harder it is for change to flow through the component, therefore the current is smaller

• variable resistor changes the amount of resistance of the component, changing the amount of current that flows in the circuit

• if two resistors are in series, the net resistance is increased, whereas with two in parallel the net resistance is decreased

series

• components are connected end to end

• all the current flows through all the components

• can only switch them all off at once

  • PD (potential difference) is shared across the whole circuit

    • PD of power supply = sum of PD across each component

  • current is the same through all parts of the circuit

    • current at one point = current at any other point

  • total resistance is the sum of the resistance in each component R₁ + R₂ = R

    • resistance of two components is bigger than just one of them, because the charge has to push through both of them when flowing round the circuit

parallel

• components are connected separately to the power supply

• current flows through each one separately

• you can switch each component off individually

  • PD is the same across all branches

    • PD of power supply = PD of each branch

    • because charge can only pass through any one branch

  • current is shared between each of the branches

    • current through source = sum of current through each branch

  • total reistance is less than the branch with the smallest resistance

    • two resistors in parallel will have a smaller overall resistance than just one - 1/R₁ + 1/R₂ = 1/R

    • because charge has more than one branch to take, so only some charge will flow along each branch

how resistance changes: with current

• as current increases, electrons (charge) has more energy

• when electrons flow through a resistor, they collide with the ions in the resistor

• the current here is doing work against the resistance

• this transfers energy to the ions, causing them to vibrate more (heating resistor)

• this makes it more difficult for electrons to flow through the resistor

  • so resistance increases, and current decreases

• this may be a benefit, as some appliances like a toaster use heating filaments that have a high resistance to get hot easily

how resistance changes: with temperature

• normal wires - see above, the same process occurs as atoms vibrate when hot

• THERMISTER ONLY

  • hotter temperatures, resistance is lower

  • used in temperature detectors/thermostats

how resistance changes: with length

• greater length, the more resistance, and the lower the current

• electrons make their way through more resistor atoms, so it is harder to get through than if you were using a shorter wire

how resistance changes: with cross sectional area

• thinner wires give greater resistance

• because less overall room for electrons to pass through between atoms

how resistance changes: with light

• LDR (light dependent resistor) ONLY

  • greater the intensity of light, the lower the resistance

    • so resistance greatest when dark

  • used in automatic night lights

how resistance changes: with voltage

• DIODIE ONLY

  • diode allows current to flow freely in one direction

  • in the opposite direction, it has a very high resistance, so no current can flow

Core practical: construct electrical circuits to:

a) investigate the relationship between potential difference, current and resistance for a resistor and a filament lamp

b) test series and parallel circuits using resistors and filament lamps

A:

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 0.5V 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 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 and repeat the experiment from step 1

8. put your results in a table and plot a graph

B:

1. set up the circuit as shown with the single fixed resistor

2. record the voltage using the voltmeter and the current using the ammeter

3. for each pair of voltage and current, calculate the resistance and record this

4. change the resistor and repeat step 2 and 3

5. arrange the two resistors in series as shown in the image, then repeat step 2

6. arrange the two resistors in parallel as shown in the image, then repeat step 2

7. replace the fixed resistor with a filament lamp and repeat the experiment from step 1

8. record your results in a table

a resistor at constant temperature

a filament lamp

a diode

explain how the design and use of circuits can be used to explore the variation of resistance in the following devices: filament lamps, diodes, thermistors, LDRs

• filament lamps

  • connected to DC of 2,4,6…,10,12V

  • connect the filament lamp to Ammeter in series and Voltmeter in parallel

  • measure the current for each voltage

  • plot a graph to show relationship between the pd and current

  • non-linear shows resistance varies

• diodes

  • connected to DC of 1,5,2,4,6,…,10,12V

  • connect to an Ammeter in series and Voltmeter in parallel

  • measure the current for each voltage

  • switch the diode the other way round to record current for -1,-1.5,-2,-4V

  • plot graph for the positive and negative potential differences to show the relationship

• thermistor

  • constant velocity of 12V

  • connect to an Ammeter

  • place in ice water with thermometer

  • measure current at 0 degrees

  • add hot water and stir, measuring current at 10,20,….,60 degrees

  • calculate the resistance

  • plot a graph of resistance against temperature

• LDR

  • constant voltage of 12V

  • connect to an Ammeter

  • shine lamp immediately onto LDR and measure current

  • keep doing this until 50cm

  • calculate resistance at each light intensity

  • plot graph of resistance against light intensity

what happens when there is an electrical current in a resistor

• when there is an electric current in a resistor, there is an energy transfer which heats the resistor

• as the result of collisions between electrons and the ions in the lattice

what happens to electrical energy when an electrical current does work against electrical resistance

• electrical energy is dissipated as thermal energy in the surroundings when an electrical current does work against electrical resistance

ways of reducing unwanted energy transfer

• low resistance wires made from metals such as copper, reduce unwanted energy transfer to thermal stores as the current flows between components

• thicker wires can also be used as they have a lower resistance

• also cooling metals reduce the resistance so the lattice ions are not vibrating as much

describe the advantages and disadvantages of the heating effect of an electric current

advantages

• fuses use the effect to protect circuits - they melt and break the circuit if the current gets too high

• the heating effect of the electric current is good if you want to heat something. there is a coil of high resistance wire which gives off infrared radiation, which transfers energy to the item needed to be heated (food)

disadvantages

• heating up a component generally reduces its efficiency - less is transferred to useful energy stores as more is being transferred to the thermal store of the component

• if the temperature gets too high, this can cause components in the circuit to melt. this will stop the circuit from working or work properly

equation for energy transferred

energy transferred (joule, J) = current (ampere, A) x potential difference (volt, V) x time (second, s)

E = I x V x t

what is power

• power is the energy transferred per second and it measured in watt

power (watt, W) = energy transferred (joule, J)/ time taken (second, s)

P = E/t

• power is directionally proportional to current and voltage, so doubling current doubles power

P = IV

electrical power (watt, W) = current (ampere, A) x potential difference (volt, V)

P = I² x R

electrical power (watt, W) = current² (ampere², A²) x resistance (ohm, Ω)

• power loss is proportional to resistance, and to current²

• energy is transferred from chemical potential in batteries to electrical energy in wires to any form of useful energy in the device they power

AC/DC

• AC is alternating current, which come from the mains

  • current continuously varies, from positive to negative (charge changes direction)

• DC, direction current, is the movement of charge in one direction only

  • cells and batteries supply direct current

• in the UK, mains supply is at 50Hz and 230V

plug

• in a plug there a 3 different wires: live wire, neutral wire and earth wire

• live wire:

  • this is a brown colour

  • it carries voltage from mains to appliance

  • this may be dangerous even if mains circuit is off, as current may still be flowing through it

• neutral wire:

  • this is a blue wire

  • completes the circuit

• earth wire:

  • this has green and yellow stripes

  • it is the safety wire used to stop the appliance becoming live

  • it is connected to the earth and to the casing

  • if the live wire touches the metal casing of the appliance, it will become live (you’ll get a serious electric shock if you touch it, as current flows through you to the ground)

  • the earth wire is connected to the metal casing, and its low resistance means the current will go from the casing through the earth wire and to the ground

• fuse

  • connected to the live wire

  • if a large current passes through live wire, fuse heats up and melts, breaking the circuit - preventing a fire or damage

explain the difference between direct and alternating voltage

• in alternating voltage, the positive and negative ends of the p.d keep alternating

• but in direct voltage, that is when the p.d is only positive or negative, not both

explain why switches and fuses should connected in the live wire of a domestic circuit

switches and fuses are connected in the live wire of the circuit so that the circuit can be broken - the appliance becomes isolated and the risk of an electric shock is reduced

recall the potential differences between the live, neutral and earth mains wires

potential difference between:

• live and neutral - 230 V

• live and earth - 230 V

• earth and neutral - 0 V

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

• the power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use

• eg. a microwave with a 500W will take longer to cook food than one with a power rating of 750W. this is because the 500W transfers less energy per second to the thermal energy store of the food, so it takes longer to cook