2. Electricity Flashcards
Unit to measure current (I)
Amps (A) - using an ammeter
Unit to measure voltage (V)
Volts (V) - using a voltmeter
Unit to measure resistance (R)
Ohms (Ω) using an ohmmeter
Unit to measure power (P)
Watts (W) - using a Wattmeter
Unit of charge (Q)
Columbus (C)
What is current
The rate of flow of charge
What do electrons have
Charge
What is static electricity
Involves the build up of charge held in place on an insulator
What is current electricity
Involves the flow of charge through a conductor
Equation to find current using charge and time
Current (I) = charge (Q) / time (t)
What is voltage
The energy transferred per unit charge
Equation to find voltage using energy transferred and charge
Voltage (V) = energy transferred (E) / charge (Q)
What is potential difference
Voltage
What does it mean if a circuit is in series
Single path for current to flow (one loop)
What does it mean if a circuit is in parallel
Multiple paths for current to flow (multiple branches)
Current rules in series
- current is the same value at any point
- current is the same through any component
- This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part
Current rules in parallel
- at a junction in a parallel circuit the current splits
- so that the current through the branches is equal to the total current through the cell
- however the current in each branch may not always be the same, as this depends on the amount of resistance in each branch
What 2 things does current flowing round a circuit depend on
- voltage of the power source
- number (and type) of components in the circuit
Increasing the voltage of the circuit means the current ….. so the electrons move around the circuit …..
Increases
Faster
In a circuit, Increasing the number of components does what to the overall resistance
Increases it
The higher the resistance, the ….. the current
Lower ( inversely proportional)
Electrons are physical matter meaning they cannot be …… or ……
This means that total number of electrons in a circuit ………..
Created or Destroyed
Must remain the same
Direction of current flow
From positive (+) to negative (-) terminal
Rules for voltage in series
Voltage from the power supply is shared between the components
Rules for voltage in parallel
Voltage is the same across each branch of a parallel circuit as across the power supply
Advantages and disadvantages of series circuit
Advantages:
- All of the components can be controlled by a single switch
- Fewer wires are required
Disadvantages:
- The components cannot be controlled separately
- If one component breaks, they will all stop working as well
Advantages and disadvantages of parallel circuits
Advantages:
- The components can be individually controlled, using their own switches
- If one component breaks, then the others will continue to function
Disadvantages:
- many more wires involved so much more complicated to set up
- All components have the same voltage as the supply, so harder to control if components need to have different voltages
Rules about resistors in series
When two or more resistors are connected in series, the total (or combined) resistance is equal to the sum of their individual resistances
Resistors on total voltage
The total voltage is also the sum of the voltages across each of the individual resistors
(In a series circuit, the voltage of the power supply is shared between all components)
Effect of increasing voltage on the current
As voltage increases, current also increases as they are directly proportional
How is current measured
Connected in SERIES using an ammeter
How is voltage measured
Connected in PARALLEL using a voltmeter
Equation linking resistance voltage and current
Resistance (R) = voltage (V) / current (I)
The overall resistance of a parallel circuit is …….than the smallest resistance
Smaller
Resistance definition
The ration between potential difference across a component and the current going through it
What is power
The rate of transferring energy
Units for power (P)
Watts (W)
Equation linking power, energy and time
(P)Power(W) = (E)energy(J)/ (t)time(s)
Equation linking power, voltage and current
Power = voltage x current
P=VI
Equation linking energy transferred, current, voltage and time
Energy transferred = current x voltage x time
E=IxVxt
E=(Power)x t
What does ohms law state
The current through a conductor is directly proportional to the potential difference across it - provided its temperature remains constant
Equation for ohms law
V ∝I
V= I x k(constant)
What is a component that obeys ohms law called
An ohmic component ( the resistance of ohmic compounds is constant)
What is conventional current
The incorrect assumption that scientists made thinking that it was the positively charged electrons moving - so that the current was positive to negative. This is INCORRECT, however we still mostly use this when dealing with circuits
What is the correct passage of flow in a circuit
From negative to positive as it is the negatively charged electrons (negative charges) flowing
What is direct current
Current that flows in one direction and is produced when using batteries/cells
What is alternating current
Current that switches direction repeatedly and produced when using the mains supply
What determines if the current in the circuit is dc or ac
The voltage supply / power supply
Name of graph used to show ac/dc currents
Oscilloscope (CRO) which produces an image called a trace
Average for mains supply voltage and frequency in the UK
Around 230V
Frequency = 50Hz
Facts about AC only
- charges move one way and then another
- comes from the mains
- has a frequency
- graph looks like a sine wave
Facts about DC only
- the charges flow in one direction
- comes from batteries
- graph is a flat lime
Facts about both AC and DC
- both are the flow of charge
- potential difference (voltage) can be measured by an oscilloscope
The hair dryer contains a coil of wire which is used to heat up air passing though the hair dryer. Explain why the coil of wire heats up when there is a current in it
- due to electrical resistance in the wires (naturally) they resist the motion of electrons
- the electrons collide into atoms on the outside of the wire
- some of their kinetic energy is transferred into thermal energy causing the wire to heat up
What does the brown wire in a plug do
Live wire - carries a current that alternates between a negative and positive voltage
What does the green wire in a plug do
Earth wire - a safety wire that is needed to earth appliances with
a metal case. This makes it safer to touch the appliance if it develops a fault
What does the blue wire in a plug do
Neutral wire - completes the electrical circuit
Name of the thing that holds the wires/ cables in place in a plug
Cable grip
What is a fuse
An electrical safety feature including a a thin wire, which melts if the current is too high. Because it is connected to the LIVE wire, this breaks the circuit and so electricity is unable to flow through the appliance.
The fuse needed should be slightly higher than the normal current
How does Earthing as a safety feature work
The earth wire is attached to metal casing and has LOW resistance.
▶ If the live wire become loose and touches the metal case, a large current flows through the earth wire (instead of the person). This surge in current triggers the fuse to blow, breaking the circuit until it is replaced (danger averted!).
Double insulation as a safety feature
Double insulated appliances have plastic cases, without any wires connected to the case.
• This means that the case cannot become live, because plastic does not conduct electricity.
• So, no need for an Earth wire.
How do circuit breakers work (RCDs)
A circuit breaker uses electromagnets (or bimetallic strips) and performs a similar job to a fuse. If the current flowing through a device is too large then a switch opens making the circuit incomplete.
- The switch can be reset instead of needing replacement (like fuses)
Used to control mains supplies
Opposite charges
Attract
Things with the same charge
Repel
What are electrical insulators
Materials that don’t conduct charge very well so the current can’t flow
What are electrical conductors
Materials that conduct charge easily (the electrons of the material are delocalised - free to move) so a current can flow through them
What is a static charge
A charge which builds up in one place and is not free to move - more common on insulators where current CANNOT flow than on conductors
What usually causes a build up of static charge
Friction
What happens when two insulating materials are rubbed together
The ELECTRONS are transferred from one material to the other, leaving a positive electrostatic charge on one material and a negative electrostatic charge on the other
Why would something (a material) have a positive electrostatic charge
It has LOST electrons ( electrons have been transferred from that material to elsewhere)
How can a charged conductor be discharged safely
By connecting it to the earth with a metal strap. The electrons flow down the strap to the ground if the charge is negative and flow up the strap from the ground if the charge is positive
What happens to the voltage on an isolated object as the electric charge builds up
- The voltage between the object and the earth (zero volts) INCREASES
- if the voltage gets large enough, electrons can jump across the gap between the charged object and the earth (this is a spark)
- they can also jump to any earthed conductor that is nearby
- this usually happens when the gap is fairly small
Jake removes his jumper in a dark room. As he does so, he hears a crackling noise and sees tiny sparks of light between his jumper and his shirt. Explain the cause of this
- as the jumper rubs against his shirt, a static charge builds up on both the jumper and the shirt.
- This is due to electrons being removed from one material and being deposited onto the other
- the charge becomes large enough for the electrons to ‘jump’ across the small air gap between the shirt and the jumper, causing sparks
How to test whether an object is charged (positively or negatively)
Looking for attraction or repulsion
When using a polyethene rod and rubbing it with a duster, what happens in terms of electron transfer
Electrons are transferred from the duster to the rod, leaving the rod with a negative charge and the duster with a positive charge
When using a acetate rod and rubbing it with a duster, what happens in terms of electron transfer
Electrons are transferred from the acetate rod to the duster, leaving the duster with a negative charge and and the rod with a positive charge
