P1 Electricity Flashcards
Define electrical current
- flow of electrical charge (electrons)
- higher flow = higher current
How is an ammeter connected to a circuit? What does it measure?
How is a voltmeter connected to a circuit? What does it measure?
- Ammeters measure current, connected in series
- Voltmeters measure voltage, connected in parallel
How does resistance effect?
- flow of charge
- current
- more difficult for charge to flow
- so current is lower
Define potential difference
The difference in electrical potential between two points in a circuit, like and electrical “push”
How does increasing potential difference effect
- flow of change
- current t
- greater flow of charge through component
- bigger current
In a V-I graph, what does the gradient show?
- 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)
What does the graph for a resistor look like? What does this indicate?
- 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
What does the graph for a filament lamp look like? What does this indicate?
- current through a filament lamp increases -> temperature increases
- so resistance increases as current does so graph is curved
What does the graph for a diode look like? What does this indicate?
- current only flows in one direction
- high resistance in curved direction
- horizontal line when no current flows
How does resistance effect temperature in a thermistor? What is this useful for?
- resistance decreases as temperature decreases
- good for temperature control and response (e.g. thermostat that turns on at a specific temperature)
How does resistance effect light in a LDR? What is this useful for?
- resistance decreases as light intensity increases
- useful for automatic lights (e.g. dusk til dawn lights)
In a series circuit, what happens to current around the circuit?
It is the same through each component
In a parallel circuit, what happens to current around the circuit?
The current drawn from the power supply is split between each component. Adding them all up = current from power supply
In a series circuit, what happens to potential difference around the circuit?
Shared between components
In a parallel circuit, what happens to potential difference around the circuit?
It is the same across each component
What happens to resistance in a series circuit?
- Total resistance of two components is the sum of resistance of each component
- R1 + R2 = Rtotal
- Adding components in series increases resistance
How is resistance effected in a parallel circuit?
- 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
What does power in a circuit depend on?
- 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
Describe direct current
in terms of potential difference, current and display on an oscilloscope
- PD is always positive or always negative
- Current direction is always the same
- Supplied by cells and batteries
- Straight line on an oscilloscope
Describe alternating current
in terms of potential difference, current and display on an oscilloscope
- PD alternated between positive and negative
- Current direction changes
- Used in mains electricity
- Wave/alternating on an oscilloscope
What is the potential difference and frequency of mains electricity?
- 230V
- AC - changes direction 50 times a second = 50Hz
Describe a three-core cable (voltage, colour etc)
- 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
What happens in a three core cable during operation?
- 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
Why is touching a live wire dangerous?
- Our bodies are 0v
- creates a large pd across the body
- large current flows through body
- can be fatal
Why is it dangerous if a tv is switch if but still plugged in and switched on at the wall?
- 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
1J of energy transfer =
1W of power
How can you increase efficiency?
Reduces wasted energy transfers
Define power
Rate at which energy is transferred
What happens when charge flows and has to overcome resistance in a circuit?
- work is done depending on amount of charge that flows and potential difference
Explain how the National Grid works
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)
How is static electricity produced?
- 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
What does it mean for an object to be isolated?
No conducting path to earth
How is an electrical charge produced?
- 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
What type of force is an electrostatic force?
- 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)
What does the strength of an electric field depend on?
- distance from the object (further distance = weaker field)
- amount of charge ( higher charge = stronger field )
How can field lines show electrostatic forces?
- Pointing away = positive
- Pointing towards centre = negative
- Arrow direction = direction of positive charge (opposites attract)
- Closer lines = stronger field
Describe the resistance required practical
- 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
Describe the V-I characteristics practical
- 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
