Flashcards in Electricity Deck (66):
What is current?
The rate of flow of an electrical charge
When will current flow?
When the circuit is complete as a potential difference is needed
What is current measured in
Is there change in current in a closed circuit?
No the current stays the same everywhere in the circuit
What is potential difference?
It is a force that pushes the charge around
What is Potential difference measured in?
What is resistance?
The opposite of potential difference it slows the rate of flow of charge
What is resistance measure in
What affects the amount of current flowing through a component?
The potential difference across the circuit and the resistance of the component
Define the relationship for the amount current between potential difference and resistance
The greater the resistance across the component, the smaller the current that flows (for a given potential difference across the component)
What is the equation for charge flow past a particular point in a circuit
Charge flow (coulombs, C)=Current(A) x Time (S)
What is the formula linking potential difference, current and resistance?
potential difference=current x resistance
How could you investigate factors that effect resistance FOR EXAMPLE LENGTH?
attach crocodile clip to the wire where the length is at 0cm on the ruler
then write down the distance you place the second crocodile clip on the ruler and place it
close the switch and record the PD and current for that length
then open the switch replace the 2nd CC and close the switch again and record PD and current
you can then use v=ir to get the resistance for the wires
then you can plot a graph for length against resistance
should be proportional and going up
What does a voltmeter do?
It measures the potential difference across a wire
MUST BE PLACED IN PARALLEL AROUND WHATEVER YOUR INVESTIGATING
What does an resistor always have at a constant temperture
a constant resistance within a circuit
so current is directly proportional to PD
What resistors do change with change in current?
diodes and filament lamps or bulbs
Why does a bulb change its resistance?
as current flows through the lamp the current also has to increase so the hotter the lamp gets the higher the resistance
Why does a bulb change its resistance?
as current flows through the lamp the current also has to increase so the hotter the lamp gets the higher the resistance as the hotter something is the higher the resistance
Why does a diode change its resistance?
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
How would you find out a components IV characteristics?
get a battery an ammeter and a voltmeter, and variable resistor and the component your investigating set up in a circuit
vary the resistors resistance which alters the current flowing through the circuit and the PD across the component
take several pairs of reading from the ammeter and the voltmeter to see how PD changes along with C. repeat each reading twice and take an average of both readings
then swap over the wires so that the current is reversed
then plot graphs using readings (current against voltage)
What should and ohmic conductor/ resistor IV graph look like
They should be directly proportional
What should a filament lamp/bulb IV graph look like?
it should curve up at the bottom and level off at the top
What should a diodes IV graph look like?
It should stay level then curve upward suddenly
What is a LDR?
It is a light dependent resistor
in bright light the resistance lowers
in dark the resistance goes higher
used in burglar alarms, night lights and outdoor lighting
What is a thermistor?
It is a resistor that changes resistance depending on temperature
in hot conditions the resistance drops
in cold the resistance gets higher
used in temperature detectors car engine temperature sensors and in electronic thermostats
Where can LDR'S and thermistors also be use?
in sensing circuits!!
the fixed resistor and fan will have the same pd across them as they are connected in parallel
the pd from the power supply is shared out between the thermistor and the loop
as the temperature rises the resistance of the thermistor decreases meaning more pd can be given to the fan and the resistor making the fan spin faster
(same with an LDR)(you can reverse the effect by moving the resistor and the adjustable resistor)
What is a series circuit?
The circuit is all connected in a singular closed circuit where the current only flows along one wire with components one after another
What happens if you disconnect a component in a series circuit or if one fails?
The flow of current will stop and the circuit will not be able to carry charge
What is the PD rule in a series circuit?
The PD is shared between components, so the components PD adds up to the total PD
Give the equation for PD
total PD= PD1 + PD2 +....
What is the rule for current in a series circuit?
The same current flows through all the components, so the current is the same everywhere.
What is the equation for current in a series circuit?
What equation determines the current in a series circuit?
What is the rule for resistance in a series circuit and why?
The resistance adds up, as adding a resistor in series the resistors have to share the total PD and resistance is the opposite of PD
Why does adding a resistor in a series circuit reduce current?
It lessens the PD (as is opposite of it) going across each component which means less current is being pushed around, and as the rule for a series circuit is that current is the same everywhere the total current in the series circuit is reduced
What is the rule for cells in a series circuit?
The cell PD's add up, if theyre all connected the same way,
What is a parallel circuit?
It is when each component is seperatley connected to the positive and negative of the supply
What advantage is there to having a parallel circuit?
If a component is removed or disconnected it will hardly effect the rest of the circuit
What is the rule for PD in a parallel circuit?
The PD is the same across all the components
So identical bulbs connected in parallel will have the same brightness
What is the rule for current in a parallel circuit?and equation
Current is shared between branches of the components
if two components in parallel are connected the current will flow the same through each component
What happens if you add a resistor in a parallel circuit?why
if two or more resistors are connected in parallel
it reduces total resistance
as both resistors have same PD in parallel
so pushing force is the same as the source pd for each resistor that you add
by adding another loop current has more than one direction to go in
This increases the total current around the circuit
so in parallel an increase in current means a decrease in total resistance
How can you investigate resistance in a series circuit?
get two or more identical resistors
build a circuit using one of the resistors and a battery and a ammeter(take note of the PD of the battery)
measure the current going through the ammeter and calculate the resistance of the first resistor use R=V/I
Then add the other resistor in series with the first
Measure the current again and use the pd of the battery to find the total resistance
repeat these steps with all the resistors and then plot a graph with number of resistors against total resistance
How could you investigate resistance in a parallel circuit?
If done after test for series use same equipment for fair test but if not get a number of identical resistors, ammeter and a battery (note PD) and build the same initial circuit and measure the resistance using R=V/I
set up the circuit so the resistor is in parallel with the other
then measure the current and the PD and use equation to get resistanc
repeat steps until all resistors are measured in parallel
plot a graph with number of resistors against total resistance
What should happen when you add resistors in series
increases the resistance, and the more you add the more resistance there is. Directly proportional
What should happen when you add resistors in parallel
The total current through the circuit increases so the total current decreases. The more resistors you add the smaller the total resistance. graph should curve
What is alternating current (ac)
Current is constantly changing direction. Produced by alternating voltages where the positive and negative ends keep alternating.
What kind of power is in your home? and at what voltage?
230volts and its AC
What is direct current (DC)
Current is always flowing in the same direction and created by a direct voltage (that's why batteries are never ac)
What colour is the live wire in a plug? and what does is do?
provides the AC PD of 230v from the mains supply
What colour wire is the neutral wire in a plug? and what does this do?
It completes the circuit and carries away current - electricity normally flows in the live wire and out the neutral wire with a 0v
What colour is the earth wire in a plug? and what does this do?
green and yellow
protecting the wiring against the appliance so that if the wire hits a conductive surface on the appliance it will not shock you. At 0v
What happens if yo touch the live wire or the earth wire when there is a fault?
It will shock you as there is a large PD of 230v in the wire and 0v in you
Still danger even if a switch is off as there is still a live wire that can never turn off unless so at the fuse box
What are electrical appliances designed to do
transfer energy to components when a current flows
The more current the....
more energy is transferred thermally
What is the equation for energy transfer?
Power (w) x TIme (s)
What does the power rating on an appliance tell you?
the amount of energy transfer per second
What happens when an electrical charge goes through a change in PD?
Energy is transferred
Energy is supplied to the charge at the power sourceto raise it through potential
Charge gives up its energy when it falls through and potential drop in components
What is the equation for PD when energy is transferred?
E(j)=Q(c) x V(v)
charge flow x potential difference
The more PD the more...
energy will be supplied to the circuit for every coulomb of charge as it is raised higher at the start
what is th eequation for power involvining PD and a current
Power(w)=PD(V) x Current(A)
What is the national grid?
a system of cables and transformers that cover the UK and connects power stations to the consumers
transfers electricity from a power station to wherever it is needed
How does power out put on the grid change
more used in mornings and evenings
more used when cold or dark
more when popular events happen on TV
power stations have headroom for these increases and small stations on standby just incase
How is large amount of power transmitted
high Pd or high Current
Why is high current a problem
power is lost as wires heat up and energy is lost to thermal store
How does NG transmit power
high pd low current