P2 Flashcards
What are in circuits?
- Switches (open or closed)
- Cell
- Battery
- Diode
- Resistor
- Variable resistor
- LED
- Lamp
- Fuse
- Voltmeter
- Ammeter
- Thermistor
- LDR
(NOTE see page 179 for how they look as symbols)
What is needed for an electrical charge to flow through a closed circuit?
The circuit must include a source of potential difference
What is electric current?
Electric current is a flow of electrical charge
When will an electric charge only flow?
- Electrical charge will only flow round a complete (closed) circuit if there is a potential difference
- So a current can only flow if there’s a source of potential difference
What is the unit of current?
The unit of current is the ampere, A
What is the equation for charge flow?
Charge flow (C) = Current (A) x Time (s)
Q = It
No matter what, what does a current always have?
The same value at any point in a single closed loop
What does the current depend on?
- The current (1) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component
- The greater the resistance of the component the smaller the current for a given potential difference (p) across the component
State the equation for potential difference
Potential difference (V) = Current(A) x Resistance(Ω)
Practical 15:
Give three examples for resistors
- Ohmic conductor
- Filament lamp
- Diode
State how the ohmic resistors work
- The resistance of ohmic conductors (e.g. a wire or a resistor) doesn’t change with the current
- At a constant temperature, the current flowing through an ohmic conductor is directly proportional to the potential difference across it
State how the filament lamps work
- When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament, which is designed to heat up
- Resistance increases with temperature, so as the current increases, the filament lamp heats up more and the resistance increases
State how diode works
- For diodes, the resistance depends on the direction of the current
- They will happily let current flow in one direction, but have a very high resistance if it is reversed
Out of ohmic conductors,filament lamps and diodes which ones have a constant resistance and which change
Stay constant : Ohmic conductors
Change : Diode and filament lamp
From the graph of ohmic conductor, what can you see
The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference so you get a straight line
From the graph of filament lamp, what can you see?
- As the current increases, the temperature of the filament increases, so the resistance increases
- This means that the less current can flow per unit PD,so the graph get shallower - hence the curve
From the graph of diode, what can you see?
- Call Roni flow for a diode in one direction, as shown
- The diode has a very high resistance in the reverse direction
Why does the resistance of a thermistor decrease?
Because the temperature decreases
What is required in a circuit?
The applications of thermistors e.g a thermostat
Why does the resistance of an LDR decrease?
Because the light intensity increases
Note:
The application of LDRs in circuits, e.g. switching lights on when it gets dark as required
What is an LDR
A resistor that is dependent on the intensity of light
What is the use and design of a circuit?
- To measure the resistance of a component by measuring the current through, and potential difference across, the component
- The circuit is shown in a diagram with symbols
Note:
AQA may ask to draw a circuit diagram using correct circuit symbols
Note:
AQA says that students should be able to use graphs to explore whether circuit elements are linear or nonlinear, and relate the curves produced to their function of properties
What are the two ways of joining electrical components
Series and in parallel, though some circuits include both series and parallel parts
Characteristics for components connected in series circuits
- There is the same current through each component
- The total potential difference of the power supply is shared between the components
- The total resistance of two components is the sum of the resistance of each component
How do you find the resistance in series circuits?
R (total) = R1 = R2
Characteristics for components connected in parallel circuits
- The potential difference across each component is the same
- The total current through the whole circuit is the sum of the currents through the separate components
- The total resistance of two resistors is less than the resistance of the smallest individual resistor
Note:
Check page 183 and 184 and memorise what the diagram for series and parallel circuits are
Differences between series and parallel circuits
Find answer and put here
Why does the resistance increase in a series circuit when adding resistors?
- In a series circuit, adding resistors will increase resistance
- The current is the same in every single component in the circuit, so the more resistors we add, the harder it is for current to flow
- This means that the overall resistance has increased in a series circuit
Why does the resistance decrease in a parallel circuit when adding resistors?
- In a parallel circuit, the net resistance decreases as more components are added, because there are more paths for the current to pass through
- The two resistors have the same potential difference across them
- The current through them will be different if they have different resistances
What is a DC series circuit?
- When all the resistive components of a DC circuit are connected end to end to form a single path for flowing current
- The manner of connecting components end to end is known as a series connection
What are DC series circuits used for?
DC power is widely used in low voltage applications such as charging batteries, automotive applications, aircraft applications and other low voltage, low current applications
Why are DC series circuits used for measurement and testing purposes
Unknown
What is the equation for potential difference?
Potential difference(V) = Current(A) x Resistance(Ω)
What is meant by equivalent resistance?
If a single resistance can replace the combination of resistances in such a manner that the current in the circuit remains unchanged, then that single resistance is called the equivalent resistance
Note:
AQA wants students to solve problems for circuits which include resistors in series using the concept of equivalent resistance
What is an example of an AC supply in the UK and what does the Domestic electricity supply have a frequency of and how much voltage does it have? (NOTE:)
Mains electricity is an AC supply in the United Kingdom. Domestic electricity supply has a frequency of 50 Hz and is about 230 V
What is Direct potential difference?
- A direct potential difference, also known as a DC potential difference, is a constant potential difference that always flows in the same direction.
- This type of potential difference is produced by direct current (DC) sources, such as batteries
What is alternating potential difference?
- An alternating potential difference, also known as an AC potential difference, is a potential difference that changes direction regularly
- This type of potential difference is produced by alternating current (AC) sources, such as power stations
Most electrical appliances are connected to the mains using three-core cable,the insulation covering each wire is colour coded for easy identification, what are they?
Live wire - Brown
Neutral wire - Blue
Earth wire - Green and yellow stripes
Characteristics of the live wire
The live wire provides the alternating potential difference (at about 230 V) from the mains supply
Characteristics of the neutral wire
The neutral wire completes the circuit - when the appliance is operating normally, current flows through the live and neutral wires. It is around O
Characteristics of the Earth wire
- It is for protecting the wiring, and for safety - it stops the appliance casing from becoming live
- It doesn’t usually carry a current - only when there’s a fault.
- It’s also at 0 V
Why is the live wire dangerous?
- The live wire causes shocks
- Since the live wire has a very large potential difference compared to the human body, the live wire can cause shocks
- When you touch a live wire, current flows through your body and causes an electric shock
- Open circuits are still dangerous
Why is it dangerous to provide a connection with the live wire and earth wire?
- Any connection between live and earth can be dangerous
- If the link creates a low resistance bath to earth, a huge current will flow, which could result in a fire
What are the two equations for power?
Power(W) = Potential difference(V) x Current(A)
P = VI
Power(W) = Current^2(A) x Resistance(Ω)
What are electrical appliances designed to do?
Transfer energy to components in the circuit when a current flows
What does the amount of energy an appliance transfers depend on?
- How long the appliance is switched on for
- The power of the appliance
State examples of how kettles transfer energy
Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle
State examples of how batteries transfer energy
Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor
When is work done in a circuit?
When charge flows
State the equations for energy transferred
Energy transferred(J) = Power(W) x Time(s)
E = Pt
Energy transferred = Charge flow(C) x potential difference(V)
How are current and potential difference related to power?
- Current and potential difference are related to power in that power is equal to the product of current and potential difference
- The more current flowing through a circuit, and the greater the potential difference, the more power is transferred
How are current and potential difference related to the energy transferred?
The greater the power, the greater the energy transferred
What is the power rating?
- The power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use
- Appliances are often given a power rating - they’re labelled with the maximum safe power that they can operate at
- You can usually take this to be their maximum operating power
Why does a higher power not truly not matter?
- A higher power doesn’t necessarily mean that it transfers more energy usefully
- An appliance may be more powerful than another, but less efficient, meaning that it might still only transfer the same amount of energy (or even less) to useful stores
What is the national grid?
- The national grid is a giant system of cables and transformers that covers the UK and connects power stations to consumers (anyone who is using electricity)
- The national grid transfers electrical power from power stations anywhere on the grid (the supply) to anywhere else on the grid where it’s needed (the demand) - e.g. homes and industry
What are step-up transformers used for?
- Step up transformers are used to increase the potential difference from the power station to the transmission cable than step down
- Transformers are used to decrease too much lower value,The potential difference for domestic use
Why is the national grid an efficient way to transfer energy?
- The transfer of electrical energy via the grid is very efficient
- When currents in a cable are higher, more energy is dissipated to the surroundings through heating
- As high currents waste more energy than low currents, electrical power is transported around the grid at a high voltage and a low current
What is the equation for…
Potential difference across primary coil x current in primary coil = potential difference across secondary cool x current in secondary coil
(As in equation sheet)
