Topic 10 - Electricity and circuits

Question 1

What are the relative masses of each subatomic particle?

Answer 1

Proton = 1
Neutron = 1
Electron = Negligible

Question 2

What are the relative charges of each subatomic particle?

Answer 2

Proton = +1
Neutron = 0
Electron = -1

Question 3

Describe the structure of the atom

Answer 3

A positively charge nucleus surrounded by negatively charged electrons

Question 4

Draw the symbols for a:
-Cell
-Switch
-Voltmeter
-Ammeter
-Resistor
-Variable resistor
-Lamp
-Motor
-Diode
-Thermistor
-LDR
-LED

Question 5

Describe the differences between series and parallel circuits

Answer 5

Series Circuits:
-Closed circuit
-The current is the same everywhere
-The voltage varies
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’

Question 6

How are voltmeters connected?

Answer 6

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

Question 7

What Is potential difference?

Answer 7

Potential difference (voltage) is the energy transferred per unit charge passed and hence that the volt is a joule per coulomb

Question 8

What is the equation for E = Q*V?

Answer 8

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

Question 9

How are ammeters connected?

Answer 9

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

Question 10

How does an electric current vary from the current in metals?

Answer 10

An electric current is the rate of flow of charge while the current in metals is a flow of electrons.

Question 11

What is the equation for charge?

Answer 11

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

Q = I ⋅t

Question 12

What causes a current in a closed circuit?

Answer 12

The closed circuit including a source of potential difference.

Question 13

Where is current conserved?

Answer 13

Current is conserved at a junction in a circuit

Question 14

Explain how changing the resistance in a circuit changes the current and how this can be achieved using a variable resistor

Answer 14

The greater the resistance, the harder it is for charge to flow through the component, therefore the current is smaller.

A variable resistor changes the amount of resistance of the component, changing the amount of current that flows in a circuit.

Question 15

What is the equation for potential difference?

Answer 15

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

V = I ⋅ R

Question 16

What are the main features of a series circuit?

Answer 16

-Components are connected end to end
-All the current flows through all the components
-Can only switch them all of at once

Question 17

How is potential difference affected by being in a series circuit?

Answer 17

Potential Difference is shared across the whole circuit.

Potential difference of power supply = sum of potential difference across each component

Question 18

How is current affected by being in a series circuit?

Answer 18

Current is the same through all parts of the circuit.

Current at one point = current at another point

Question 19

How is total resistance affected by being in a series circuit?

Answer 19

Total resistance is the sum of the resistance in each component.

Resistance of two components is bigger than just 1 of them, because the charge has to push through both of them when flowing round the circuit.

Question 20

What are the main features of a parallel circuit?

Answer 20

-Components are connected separately to the power supply
-Current flows through each one separately
-You can switch each component off individually

Question 21

How is potential difference affected by being in a parallel circuit?

Answer 21

Potential difference is the same across all branches.

Potential difference of power supply = potential difference of each branch
Because charge can only pass through any one branch

Question 22

How is current affected by being in a parallel circuit?

Answer 22

Current is shared between each of the branches.
Current through source = sum of current through each branch

Question 23

How is total resistance affected by being in a parallel circuit?

Answer 23

Total resistance is less than the branch with the smallest resistance.

Two resistors in parallel will have a smaller overall resistance than just one.
Because charge has more than one branch to take, so only some charge will flow along each branch.

Question 24

Describe the core practical to construct electrical circuits to
a investigate the relationship between potential difference, current and resistance for a resistor and a filament lamp

Answer 24

-Connect the circuit as shown in the first diagram.
-Adjust the variable resistor so that the potential difference is very low or zero at the start.
Record the reading on the voltmeter and ammeter.
-Use the variable resistor to increase the potential difference.
-Record the new readings on the voltmeter and ammeter.
-Repeat steps three to four, each time increasing the potential difference slightly.
-Reverse the power supply connections and repeat steps two to six.
-Repeat the experiment but replace the fixed resistor with a bulb.
-Plot a graph of current against potential difference for each
component.

Question 25

What does the core practical on the relationship between potential difference, current and resistance show for the fixed resistor?

Answer 25

For a fixed resistor, the potential difference is directly proportional to the current. Doubling the amount of energy into the resistor results in a current twice as big through the resistor. This relationship is called Ohm’s Law and is true because the resistance of the resistor is fixed and does not change. A resistor is an ohmic conductor.

(The graph has current as y-axis and voltage as x-axis and is a diagonal line left-to-right.)

Question 26

What does the core practical on the relationship between potential difference, current and resistance show for the filament bulb?

Answer 26

In a filament bulb, the current does not increase as fast as the potential difference. Doubling the amount of energy does not cause a current twice as big.
The more energy that is put into the bulb, the harder it is for the current to flow - the resistance of the bulb increases. As the potential difference increases, so does the temperature of the thin wire inside the bulb, the filament. The increased vibrations of the
ions in the filament because of the increased temperature make it harder for the electrons to get past.

(The graph has current as y-axis and voltage as x-axis and is a diagonal line left-to-right that curves at the end)

Question 27

Describe the core practical to construct electrical circuits to test series and parallel circuits using resistors and filament lamps

Answer 27

-Set up the circuit as shown in figure 1, turn the power supply on and close the switch.
-Record the voltmeter and ammeter readings and calculate the resistance of the resistor using R = V/I, where R is resistance, V is potential difference and I is current.
-Change the resistor and repeat step two to find the resistance of a second resistor.
-Arrange the two resistors in series as shown in figure 2 and close the switch.
-Record the voltmeter and ammeter readings once again and determine the total resistance of both resistors in series using R = V/I.
-Arrange the two resistors in parallel as shown in figure 3 and close the switch.
-Record the voltmeter and ammeter readings once again and calculate the total resistance of both resistors in parallel.

Question 28

What are the results of the core practical testing series and parallel circuits with resistors and filament lamps.

Answer 28

In series, the resistance of the network is equal to the sum of the two individual resistances.
In parallel, the resistance of the network is less than either of the two individual resistances.

Placing the resistors in series causes the resistance to be double that of a single resistor because there is only one path for the electrons to follow - the supply must drive current through one resistor and then the other.
Placing the resistors in parallel causes the resistance to be half that of a single resistor.
The outcomes of this experiment are the same using filament lamps, or combinations of resistors and lamps, as the lamps act as resistors.

Question 29

How does current vary with resistance?

Answer 29

As current increase, electrons gain 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. This makes it more difficult for electrons to flow through the resistor so resistance increases and current decreases.

Question 30

Describe how the resistance of a light-dependent resistor (LDR) varies with light intensity

Answer 30

Greater the intensity of light, the lower the resistance so resistance is greatest when dark.
Used in automatic night lights.

Question 31

Describe how the resistance of a thermistor varies with change of temperature

Answer 31

Hotter temperatures, resistance is lower.

Used in thermostats

Question 32

Explain how the design and use of circuits can be used to explore the variation of resistance in filament lamps

Answer 32

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 the relationship between the potential difference and current. Non-linear shows R varies

Question 33

Explain how the design and use of circuits can be used to explore the variation of resistance in diodes

Answer 33

Connected to DC of 1, 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, -1.5m -2, -4V.
Plot a graph for the positive and negative potential differences to show the relationship.

Question 34

Explain how the design and use of circuits can be used to explore the variation of resistance in thermistors

Answer 34

Constant voltage of 12V. Connect to 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 graph of resistance against temperature.

Question 35

Explain how the design and use of circuits can be used to explore the variation of resistance in LDRs

Answer 35

Constant voltage of 12V. Connect to ammeter. Shine lamp immediately onto LDR and measure current.
Move the lamp ~10cm away and measure current.
Keep doing this until 50cm.
Calculate resistance at each light intensity.
Plot graph of resistance against light intensity.

Question 36

What happens when there is an electric current in a resistor and why?

Answer 36

When there is an electric current in a resistor, there is an energy transfer which heats the resistor. This is as a result of collisions between electrons and the ions in the lattice.

Question 37

What happens when an electrical current does work against electrical resistance?

Answer 37

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

Question 38

Explain ways of reducing unwanted energy transfer through low resistance wires

Answer 38

A smaller resistance will mean there are fewer collisions, therefore less energy will be wasted through heating.

Question 39

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

Answer 39

-The loss of energy as heat energy can make an appliance inefficient
-If an appliance overheats it can catch fire or overheat, which could ruin the device or injure the user

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

Question 40

What is the equation for E = IVT?

Answer 40

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

E = I ⋅V ⋅t

Question 41

Describe power

Answer 41

The energy transferred per second ( measured in watt)

Question 42

What is the equation for Power?

Answer 42

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

Question 43

Explain how the power transfer in any circuit device is related to the potential difference across it and the current in it

Answer 43

Power is directly proportional to current and voltage (P = IV) so doubling current doubles power

Question 44

What is the equation for P = I*V?

Answer 44

electrical power (watt, W) = current (ampere, A) × potential difference (volt, V)
P = I ⋅V

Question 45

What is the equation for P = I* R^2?

Answer 45

electrical power (watt, W) = current squared (ampere2, A2) × resistance (ohm, Ω)
P = I ⋅ R 2

Question 46

Describe how, in different domestic devices, energy is transferred from batteries and the a.c. mains to the energy of motors and heating devices

Answer 46

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

Question 47

Describe direct current

Answer 47

In direct current the movement of charge is in one direction only.
Cells and batteries supply direct current (d.c.)

Question 47

Descrive alternate current

Answer 47

In alternating current the movement of charge changes direction

Question 48

What is the domestic supply in the UK and what is its frequency and voltage?

Answer 48

In the UK the domestic supply is a.c., at a frequency of 50 Hz and a voltage of about 230 V

Question 49

Describe the live wire

Answer 49

Is a brown wire that carries voltage from mains to appliance. This may be dangerous even if mains circuit is off, as current may still be flowing through it.

Question 50

Explain the earth wire

Answer 50

A wire with green and yellow stripes.
It is the safety wire used to stop the appliance becoming live therefore is connected to the earth and the metal casing. Its low resistance means the current will go from the casing through the earth wire and to the ground

Question 51

Explain the function of fuses or circuit breakers in ensuring safety

Answer 51

A fuse is connected to the live wire. If a large current passes through the live wire, the fuse heats up and melts, breaking the circuit to prevent a fire or damage.

Question 52

What happens if the live wire touches the metal casing of the appliance?

Answer 52

It will become live. If you touch it you will get a serious electric shock as current flows through you to the ground.

Question 53

Describe the neutral wire.

Answer 53

It is a blue coloured wire that completes the circuit.

Question 54

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

Answer 54

Between the live and neutral wire there is a potential difference of 230V.

Between the live and earth wire there is a potential difference of 230V.

Between the neutral and earth wire there is a potential difference of 0V.

Question 55

Explain power ratings

Answer 55

A power rating is the power of an appliance when in use.
The greater the power rating the greater the energy consumption per second.
So it uses more energy in a given time.