Topic 9 - Electricty & Circuits and Static Electricity Flashcards
voltage/potential difference
the pressure used to push the free electrons around the circuits
-> amount of potential energy the cell transfers to each coulomb of charge flowing through it
which way does the conventional current direction go
goes from the positive terminal to the negative terminal of the battery
series circuit
only one route
parallel circuit
circuit that contains different junctions (branches) for the current take different routes
unit for electric current
amps (A)
ammeter
measures the current passing through a component or circuit
what is a source of potential difference
cell or battery
what happens to the current if the potential difference is increased
the current increases
unit for potential difference
volts
voltmeter
measures the voltage
where is a voltmeter connected?
a voltmeter is always connected in parallel to measure the potential difference across the component or circuit
what is the unit for electric charge
coulombs (C)
current
rate a which electrons flow past a point in an electrical circuit
what is the current in a metal
flow of electrons
What size of the current at any point tell you about the charge?
tells you how much charge flows past that point each second
electrical current
The rate of flow of charge
charge equation
charge = current x time
potential energy
The energy stored in the circuits electrons due to their position in the electric field
how do you get energy to a circuit
The cell transfers energy to the charge so the charge then has a potential to transfer energy to other components in the circuit
Energy transferred equations
- Energy transferred = charge moved x potential difference
- Energy transferred = current x potential difference x time
units for resistance
ohms
potential difference equation
Potential difference = current x resistance
Why is there a greater potential difference across some resistors but not others?
because resistors with higher resistance have greater potential difference
what happens in a parallel circuit when resistors are added?
The total resistance of the circuit is less than the resistance of the individual resistors because there are now more paths for the current
resistance
how hard it is for current to flow
what happens in a series circuit when a resistor is added
The total resistance of the circuit increases the pathway becomes harder for current to flow
ohms law
- V = I x R
- the electric current through a conductor between two points is directly proportional to the voltage across the two points
Explain if a resistor obey ohms law
A resistor obeys ohms law because the two variables in direct proportion because the resistance stays the same
filament lamp in a circuit - current, potential difference, resistance, does it obey ohms law?
- The current causes the filament to heat up and glow
- The greater the potential difference the more current flows and the hotter and whiter the filament gets
- resistance increases as temp of filament increases
- goes against ohms law
doide in a circuit - current, potential difference, resistance, does it obey ohms law?
no diodes do not obey ohms law because the current can only flow in one direction and blocks in in the other direction
Light dependent resistor (LDR) - resistance?
As high resistance in the dark, but the resistance get smaller when the light intensity increases
thermistors - resistance?
Higher resistance at low temperatures, but as the temperature increases, the resistance decreases
Why does the resistor become warm?
- because when a current passes through a resistor energy is transferred because electrical work is done against the resistance
- the energy is transferred by heating and resistor becomes warm
explain how a resistor works and causes resistance
- as the electrons flow through the lattice of vibrating ions, they collide with the ions
- The more collisions they make with the ions the harder is for them to pass through to the higher the electrical resistance
- When the electrons collide with the ions they transfer energy to them
How can resistance be reduced? (2)
- By using wires made from metals with low resistance
- by cooling metals so that the lattice ions are not vibrating as much
What is a metal with low resistance?
copper
Power
The energy transferred per second
-> Rates at which energy is delivered
how to work out power
Power = energy transferred / by time taken
electrical power equations
Electrical power = Current x potential difference
Electric power = current squared x resistance
electrical power
The rate at which electrical energy is transferred in a ciruit
Direct current
- An electric current that is in one direction so the flow of charge is always in the same direction
- Cells and batteries have a positive and negative terminal and the direction of the movement of charge stay the same
how is mains electricity produced?
Mains electricity is produced using generators that rotate causing the direction of the current keep changing
Alternating current
a current in which the direction of the flow of electrons which is back-and-forth at regular intervals or cycles
-> the voltage also changes increase into a peak voltage then decreasing into 0 - it then increases to peak in the opposite direction before decreasing back to 0 - this cycle repeats
what is the power rating of an appliance measured in
watts
Earth wire
- Connected to metal parts of the appliance to a large metal spike or metal tubing that is pushed into the ground
- It is for safety and is at 0 V if the circuit is correctly connected
neutral wire
The return path to the power station
fuse
A safety device marked with the current it can carry
Live wire
Connect the appliance to the generators at the power station
fuse function
- when the current passes through the wire the wire gets hotter
- If the current exceeds a certain value (certain temp.) then the wire melts this breaks the circuit and stops the current
reason for an Earth wire
if the metal part are at high voltage and you touch the metal, you don’t get an electric shock because the current goes to the ground instead of through you
circuit breakers
- alternative to fuses
- They detect to change in the current and safely switch off the supply
advantages of circuit breakers and disadvantages of fuses
- once the fault is fixed, they can be switched back on whereas fuse has to be replaced
- they work very quickly so I can save lives as a fuse take some time to me and will not prevent you getting a shock if for example you touch a wire
what happens if a faulty appliance draws too much current?
It can cause overheating of the wire and can cause a fire, however a fuse stops from happening
-> if an appliance develops a fault it’s metal parts can be a high voltage and if you touch the metal you might get a dangerous electric shock; for this reason the metal parts of the appliances are connected to earth wire so that the current goes into the ground instead of through you
Insulators
A material or an object that does not easily allow electricity to pass through it
what happens when you rub an acetate roll with a dry duster?
Some of the electrons move from the acetate onto the duster
what happens when you rub a polythene rod with a duster
Electrons move from the duster onto the polythene rod
what happens if two objects have opposite charges
There is a force of attraction between the two object
What happens if two objects have the same charge?
The objects repel each other
Static electricity
When the charge is not able to flow away from the surroundings - they all have the same charge and they all repel each other
Charging by induction
A process that charges an object without touching it to another charged object
-> it involves bringing a charge object close to an uncharged conductive material that is grounded. The result is that the conductor has a charge opposite to the charge object.
How do you prevent a spark between a fuel pipe and an aircraft?
There’s a bonding line connected to the aircraft to Earth it before fuelling begins
Why do buildings have lightning conductors?
To discharge clouds safely - clouds have static electricity due to the friction between the particles of ice or water moved by air currents
what happens in electric static spraying for crops?
- when spraying crops with insecticide, electrodes on the sprain nozzle charge the spray droplets as they pass
- The droplets spread out because they repel each other and then they’re attractive to the plants by induction
- this means that the spray spreads around the plant even underneath it
- less spray falls in the ground and farmers don’t use as much
force field
the volume of space around an object in which another object can experience a force
magnetic field
a magnet has a force field
electric field
A charge of an object has a force field around it
field lines
- never cross
- show where the field is strongest (which is where the field lines are closest together)
- show the direction of the force on a charge in the field
- start on a positively charged object and ends on a negatively charged object
point charge
a theoretical electric charge that is located at a single point in space
electric field between two parallel points
it’s uniform - it is the same in all places between the two points
