P1 Electricity

Question 1

Define electrical current

Answer 1

• flow of electrical charge (electrons)

- higher flow = higher current

Question 2

How is an ammeter connected to a circuit? What does it measure?

How is a voltmeter connected to a circuit? What does it measure?

Answer 2

• Ammeters measure current, connected in series

- Voltmeters measure voltage, connected in parallel

Question 3

How does resistance effect?

• flow of charge
• current

Answer 3

• more difficult for charge to flow

- so current is lower

Question 4

Define potential difference

Answer 4

The difference in electrical potential between two points in a circuit, like and electrical “push”

Question 5

How does increasing potential difference effect

• flow of change
• current t

Answer 5

• greater flow of charge through component

- bigger current

Question 6

In a V-I graph, what does the gradient show?

Answer 6

• Resistance
• steep gradient = low resistance (large current will flow for a small pd)
• shallow gradient = high resistance (large of needed to produce a small current)

Question 7

What does the graph for a resistor look like? What does this indicate?

Answer 7

• For resistors e.g. Ohmic Conductors
• current is directly proportional to potential difference (at a constant temperature), linear graph, resistance is constant as current changes

Question 8

What does the graph for a filament lamp look like? What does this indicate?

Answer 8

• current through a filament lamp increases -> temperature increases
• so resistance increases as current does so graph is curved

Question 9

What does the graph for a diode look like? What does this indicate?

Answer 9

• current only flows in one direction
• high resistance in curved direction
• horizontal line when no current flows

Question 10

How does resistance effect temperature in a thermistor? What is this useful for?

Answer 10

• resistance decreases as temperature decreases

- good for temperature control and response (e.g. thermostat that turns on at a specific temperature)

Question 11

How does resistance effect light in a LDR? What is this useful for?

Answer 11

• resistance decreases as light intensity increases

- useful for automatic lights (e.g. dusk til dawn lights)

Question 12

In a series circuit, what happens to current around the circuit?

Answer 12

It is the same through each component

Question 13

In a parallel circuit, what happens to current around the circuit?

Answer 13

The current drawn from the power supply is split between each component. Adding them all up = current from power supply

Question 14

In a series circuit, what happens to potential difference around the circuit?

Answer 14

Shared between components

Question 15

In a parallel circuit, what happens to potential difference around the circuit?

Answer 15

It is the same across each component

Question 16

What happens to resistance in a series circuit?

Answer 16

• Total resistance of two components is the sum of resistance of each component
• R1 + R2 = Rtotal
• Adding components in series increases resistance

Question 17

How is resistance effected in a parallel circuit?

Answer 17

• Total resistance of two resistors is less than the smallest individual resistor
• In parallel, there are more paths for current to take, so current flows more easily
• So adding resistors in parallel decreases total resistance

Question 18

What does power in a circuit depend on?

Answer 18

• potential difference and current ( P=VI)

- higher pd or current will use more energy/second than lower pd or current, which means it is more powerful

Question 19

Describe direct current

in terms of potential difference, current and display on an oscilloscope

Answer 19

• PD is always positive or always negative
• Current direction is always the same
• Supplied by cells and batteries
• Straight line on an oscilloscope

Question 20

Describe alternating current

in terms of potential difference, current and display on an oscilloscope

Answer 20

• PD alternated between positive and negative
• Current direction changes
• Used in mains electricity
• Wave/alternating on an oscilloscope

Question 21

What is the potential difference and frequency of mains electricity?

Answer 21

• 230V

- AC - changes direction 50 times a second = 50Hz

Question 22

Describe a three-core cable (voltage, colour etc)

Answer 22

• live wire, brown, 230V - carries current to appliance
• neutral wire, blue, (close to) 0V earth potential - carries current away from appliance
• earth wire, yellow/green stripes, 0V

Question 23

What happens in a three core cable during operation?

Answer 23

• PD causes current to flow through live and neutral wires
• Live wire carries alternating potential from supply
• Neutral wire completes the circuit
• Current will only flow in earth wire if there is a fault connecting it to a non 0 potential (live wire)
• Earth wire is for safety to stop exterior of appliance becoming live

Question 24

Why is touching a live wire dangerous?

Answer 24

• Our bodies are 0v
• creates a large pd across the body
• large current flows through body
• can be fatal

Question 25

Why is it dangerous if a tv is switch if but still plugged in and switched on at the wall?

Answer 25

- live wire ire between the wall and switch is still at an alternating potential - Just needs a path for the electricity to flow through - Path could be provided by a damaged cable exposing the live wire - If someone touches the live wires creating a pd from the live to the earth causing the current to flow -> electric shock

Question 26

1J of energy transfer =

Answer 26

1W of power

Question 27

How can you increase efficiency?

Answer 27

Reduces wasted energy transfers

Question 28

Define power

Answer 28

Rate at which energy is transferred

Question 29

What happens when charge flows and has to overcome resistance in a circuit?

Answer 29

- work is done depending on amount of charge that flows and potential difference

Question 30

Explain how the National Grid works

Answer 30

Power Station - 25000V - transfer energy supply in to electrical energy - more efficient than many local power stations as they are more efficient (steam turbines are more efficient at higher steam temperatures) Step up transformers - Increase Pd - Reduces current so reduces heat loss so more efficient Transmission cables - transfer electricity Step down transformers - reduces pd for domestic use (230V)

Question 31

How is static electricity produced?

Answer 31

- Insulators rubbed together - friction moves negative electrons between the objects - gain electrons - negative charge - loose elections - positive charge - insulators means charge remains and builds up

Question 32

What does it mean for an object to be isolated?

Answer 32

No conducting path to earth

Question 33

How is an electrical charge produced?

Answer 33

- As charge on an isolated object increases, potential difference between object and earth increases - Potential difference becomes too high - Spark jumps across the gap to any earthed conductor nearby - Spark discharges the object and can be a source of ignition - Lightning = charge build up in clouds in a thunderstorm producing a spark

Question 34

What type of force is an electrostatic force?

Answer 34

- non contact (attraction or repulsion - if a charge object is brought near an uncharged object, it can attract it (e.g charged ruler near water flowing from a tap)

Question 35

What does the strength of an electric field depend on?

Answer 35

- distance from the object (further distance = weaker field) | - amount of charge ( higher charge = stronger field )

Question 36

How can field lines show electrostatic forces?

Answer 36

- Pointing away = positive - Pointing towards centre = negative - Arrow direction = direction of positive charge (opposites attract) - Closer lines = stronger field

Question 37

Describe the resistance required practical

Answer 37

- how does length of wire effect resistance? - record voltage and current at varying lengths (use crocodile clips to grip at certain points) - calculate resistance (r=v/i) - use a variable resistor to control current - only turn current on when taking a reading

Question 38

Describe the V-I characteristics practical

Answer 38

- use variable resistor to adjust PD across component - measure voltage and current for a range of voltages - calculus a mean - measure with power supply off to check for 0 errors