SP10 Electricity and Circuits Flashcards
What is the role of the cell?
Provides the circuit with a potential differece, ie: energy per unit of charge
What is the role of the switch?
Turns the circuit on/off
What is the role of the resistor?
Limits the flow of current
What is the difference between a fixed resistor and a variable resistor?
A fixed resistor has a resistance that cannot change, whereas a variable resistor has adjustable resistance
What is the role of the thermistor?
Resistance based on the temperature. As the temperature increases, the resistance decreases
What is the role of the light dependant resistor?
Resistance depends on light intensity. As light intensity increases, resistance decreases
What is the role of the motor?
A device that converts electrical energy to mechanical energy
What is the role of the diode?
Allows the current to flow in one direction only. Used to convert AC into DC
What is the role of the ammeter?
Used to measure the current in a circuit. Connected in series with the rest of the components
What is the role of the voltmeter?
Used to measure the potential difference of an electrical component. Connected in parallel to the relevant component
Describe the direction of current flow
From the positive terminal to the negative terminal of the power supply
Define potential difference?
The amount of energy per unit of charge passing through the terminals
What is 1V =?
1J/C
What is the equation used to calculate potential difference?
energy transferred (J)/charge moved (Q)
Define electric current?
The rate of flow charge
What is the equation used to calculate current?
charge/time
What is needed in a circuit for the current to flow?
- a source of potential difference
- the circuit is closed: there are no gaps in the circuit
Define series circuit
The current can only take one path
Define parallel circuit
There are several junctions that allow the circuits to take different routes
What is current in electrical wires?
The flow of electrons
What equation links potential difference, current and resistance?
V = IR
State the difference between the direction of electron flow and the direction of conventional current
conventional current: positive terminal to negative terminal
electron flow: negative to positive
What does a large resistance cause?
The greater the resistance, the harder it is for charge to flow through the component, therefore the current is smaller
What happens to current at a junction?
It is conserved, which means that the current flowing in a junction is the same as the current flowing out of it
Compare series and parallel circuits
Series:
- current flows through all components together
- cannot switch individual components on and off
- potential difference is shared across the whole circuit
- current is the same throughout the circuit
Parallel
- current flows through each component separately
- can switch off each component individually
- potential difference is the same across all the branches
- current is shared between each of the branches
How do we calculate total resistance in a series circuit?
Sum of each of the individual resistors
How do we calculate total voltage in a series circuit?
Sum of the voltages shared across each of the resistors
Why does the total resistance decrease in a parallel circuit when two or more circuits are connected?
Each additional resistor creates an extra pathway for the current to travel, which allows for more charge to flow overall and a smaller overall resistance
What are the drawbacks of using a series circuit?
- If one component breaks all others will stop working
- The components cannot be controlled separately
What is the relationship between current and a fixed resistor on an I-V graph?
Current is directly proportional to the fixed resistor
What is the relationship between a filament lamp and current on an I-V graph?
- Not directly proportional
- because the resistance increases as temperature increases (of the filament)
- resulting in the gradient to be a bit curved at either ends of the graph
- current will increase proportionally, but much slower
Why aren’t current and the filament lamp directly proportional?
- as current increases, so does the temperature of the filament lamp
- the higher temperature causes the atoms in the metal lattice of the filament to vibrate more
- which causes an increase in the resistance, making it harder for free electrons to pass through
- resistance opposes current, causing current to increase at a slower rate
Describe the uses of light dependent resistors
- street lights
- garden lights
Describe the uses of a thermistor
- temperature sensors in ovens
- fire alarms
- digital thermometers
Describe the energy transfer that occurs when
a current passes through a resistor
Electrical energy is disappated as thermal energy via conduction, convection and radiation
What does the amount of heat produced by electrical energy depend on?
- Current: the greater the current, the more heat that is produced
- Resistance: the higher the resistance, the more heat is produced
Why is electrical energy converted to heat when it passes through a component?
Due to collisions between:
- electrons flowing in the conductor
- the lattice of ions within the metal conductor
As the electrons pass through the metal lattice, they collide with the ions, which resist the flow of electrons.
When the electrons collide, they lose some energy by giving it to the ions, which start to vibrate more, making the metal heat up
How can the amount of heating in wires be reduced?
- reduce the current in the wires
- use wires with a lower resistance
How can the heating element of an electric current be an advantage?
- In appliances that are meant to heat up, such as:
- electric kettles
- electric ovens
- domestic heating
- electric toasters
What are the disadvantages of the heating effect of an electrical current?
- results in energy loss, which reduces efficiency
- can pose a fire hazard
What is the equation for electrical energy transfer?
energy transfer = current x potential difference x time
Define power
Energy transferred per second
What are the units for power?
Watts
What is the equation to calculate power?
energy transferred/ time
What does the electrical power of a device rely on?
- the voltage of a device
- the current of a device
What is the equation for electrical power that links current and potential difference?
current x potential difference
What is the equation for power that links resistance with current?
current^2 x resistance
Define direct current
A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative
Define alternating current
A current that continuously changes its direction, going back and forth around a circuit
What produces a d.c?
- electrical cells
- batteries
What produces an a.c.?
- electrical generators
- such as our UK mains electricity
What is the difference between the terminals of ac and dc?
- d.c.: has a positive and negative terminal
- a.c.: has two identical terminals
What is the frequency of the UK mains energy?
50Hz
What is the potential difference of UK mains electricity?
230V
What are the three wires in a cable in the UK?
- live
- earth
- neutral
What does the live wire do?
- carries the alternating potential difference from the supply to the circuit
- most dangerous of the three wires
- can cause electrocution if touched without the Earth wire
What is the role of the Earth wire?
Acts as a safety wire to stop the appliance from being live
What is the role of the neutral wire?
- Forms the opposite end of the circuit to the live wire to complete the circuit
- much less dangerous than the live wire due to its low voltage
What is the colour of the live wire?
Brown
What is the colour of the neutral wire?
Blue
What is the colour of the Earth wire?
Green
How does the fuse act ensure safety in a circuit?
- Fuses usually consist of a glass cylinder containing a thin metal wire
- if the current in the circuit becomes too large, the wire heats up and melts
- this causes the wire to break, breaking the circuit and stopping the current
How does a circuit breaker ensure safety?
Consists of an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value
Why are circuit breakers preferred as compared to fuses?
- doesn’t melt and break and so can be reused again
- works much faster
Explain the danger of a connection between the live and the earth wires
There is a large potential difference between the live wire (230V) and the earth wire (0V) and if the two are connected together, a large current can be created. If a person happens to be this connection, it can lead to a potentially lethal shock
