Electricity 1 Flashcards
circuits
What is (electric) current?
-Electric current is the rate of the flow of electrical charge.
-The bigger the current the faster the rate
Define Charge
Charge is a measure of the total current that flowed within a period of time.
Define Potential Difference (V)
The force driving the flow of electrons around a circuit.
Define Resistance (Ohms)
Resistance is everything that resists or opposes the flow of electrons in a circuit.
What is the equation linking pd, current, and resistance?
V=IR
What is the equation linking Charge flow, current, and time?
Q=IT
For components connected in parallel:
- 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.
How should we connect an ammeter in a circuit?
In series with the component we are measuring the current of.
For wirers and resistors, why does increased temperature mean increases resistance?
As temperature increases, ions in the metal wire gain kinetic energy and vibrate faster.
This causes more collisions with electrons flowing through the circuit, hence increased resistance to current flow.
How should we connect a voltmeter in a circuit?
in parallel across the component we are measuring the potential difference of.
For components connected in series:
- 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.
(R total = R₁+ R₂)
Adding a resistor in parallel with an existing resistor will…
…decrease the overall resistance.
Components with a greater resistance will…
…always have a greater share of the voltage.
Alternating current.
-Potential difference changes from + to -
-Because the direction of current flow continually changes at a specified frequency.
Direct current.
Potential difference is constant, hence charge always flows through a circuit in the same direction.
Frequency
A measure of the number of waves per second.
1 ÷ time for one full wave.
UK- 50Hz.
A ‘convectional current’ runs from a…
…positive to negative terminal.
What is the purpose of a cell? (physics)
Provides a potential difference.
What is the purpose of a battery?
Provides a potential difference.
What is the purpose of switches?
To allow the current flow to be switched on or off.
What is the purpose of a voltmeter?
Measures potential difference across a component.
What is the purpose of an ammeter?
Measures the current flowing through a component.
What is the purpose of a fixed resistor?
Provides a fixed resistance to the flow of current.
What is the purpose of a variable resistor?
Provides a changeable resistance.
(Allowing us to easily adjust resistance.)
What is the purpose of a lamp?
Converts electrical energy to light energy as a useful form.
What is the purpose of a fuse?
The wire melts if the current in the circuit gets too high, breaking the circuit.
What is the purpose of a diode?
Allows current to flow in one direction only.
What is the purpose of a thermistor?
Resistance decreases when temperature increases.
What is the purpose of an LDR?
Resistance decreases when the light intensity increases.
What is the purpose of an LED?
(A diode that) gives out light when current flows through it.
Only allows current to flow in one direction.
Describe how an LED is different from a lamp.
An LED will only allow electrical current to pass through it in one direction.
Why do circuit diagrams use standard (agreed) symbols and conventions?
So that they can be understood by everybody across the world.
Name the four components that are commonly used to change or control the amount of resistance in a circuit.
-thermistor
-fixed resistor
-variable resistor
-LDR
What are output devices in electronic circuits?
Devices which transform electrical energy into another type of energy.
Lamp, LED.
What must a closed circuit include in order for electrical charge to flow through it?
A source of potential difference.
What is the unit for charge flow?
Coulombs, C
What is the symbol for charge flow?
Q
What is the symbol for current?
I
What is the unit for current?
Amperes, A
In a single closed loop, what happens to current?
A current has the same value at any point in a single closed loop.
What is the symbol for time?
t
What is the unit for time?
seconds, s
What is the symbol for potential difference?
V
What is the unit for potential difference?
Volts, V.
What is the unit for resistance?
Ohms, Ω
What is the symbol for resistance?
R
What does the current through a component depend on?
-The resistance (R) of the component
-The potential difference (V) across the component
The greater the resistance of the component
the smaller the…
…current for a given potential difference (pd) across the component.
What is potential difference the same as?
Voltage
What does Ohm’s law state?
The current flowing through an ohmic conductor is directly proportional to the potential difference across the component, provided temperature remains constant.
How is the resistance of a variable resistor changed?
Using a slider or a knob.
What does the resistance of a whole circuit depend on?
The resistances of the individual components in the circuit.
The higher the total resistance…
…the smaller the current.
What are components that obey ohm’s law called?
Ohmic conductors
Give examples of ohmic conductors.
-Silver
-Copper
-Aluminium
-Iron
-Wiring used in homes
How do we tell if a resistor is ohmic from an IV graph?
The lines will be straight.
On an IV graph, what does a steeper straight line tell you?
That resistor has a lower resistance than the other.
What happens for resistors as current changes?
-For some, the value of R remains constant
-For others, the value of R can change as the current changes
What happens to the resistance of a filament lamp as the temperature of the filament increases?
It increases
The current through an ohmic conductor (at a constant temperature) is ______ ____________ to the potential difference across the resistor.
directly proportional
This means that the resistance remains
constant as the current changes.
How do LDRs work to switch lights on when it gets dark?
The resistance of an LDR increases as light intensity decreases.
Eventually there will be less resistance in the bulb than the LDR, so current will flow through the bulb and switch it on.
This is because current always takes the path of least resistance.
The resistance of components such as lamps, diodes, thermistors and LDRs is…
… not constant; it changes with the current through the component.
Why does the current through a diode only flow in one direction?
The diode has a very high resistance in the reverse direction.
How could a voltmeter be used in a circuit to show how the resistance of an LDR changes with light intensity?
-Connect a voltmeter across the LDR
-As the light levels increase, the potential difference across the LDR will decrease since the resistance has decreased and a greater share of e.g. the 6V will be across the fixed resistor.
How do thermistors work in thermostats?
The resistance of a thermistor increases as temperature decreases.
This means that less current can flow through the thermistor.
Explain how a thermistor and an LDR could be used together in a circuit. (4)
~steps 1 and 2
-They could be used for a circuit that turns a light and a heater on or off in a greenhouse.
-Set up a parallel circuit, with a thermistor leading to a heater and an LDR leading to filament lamps.
Describe how a circuit could be used to explore the variation in resistance as light levels change.
-The amount of light being shone on an LDR can be changed by varying the intensity of the light it is exposed to from a torch or a lamp.
-The size of the current flowing in the circuit can then be recorded for each of these light levels.
-You’d expect the output current to be highest when the light level is greatest
Explain how a circuit could be used to explore how the resistance of a thermistor changes.
-Set up a series circuit, containing an ammeter, battery, and thermistor
-You could immerse the thermistor in a water bath heated from 10 to 80 °C.
-Record the current flowing in the circuit at each temperature over this range.
-As temp increases, current would increase because the thermistor’s resistance decreases as it gets warmer.
Explain how a thermistor and an LDR could be used together in a circuit. (4)
~ steps 3 and 4
If it gets too hot (temperature increase) the resistance of a thermistor decreases. This allows more current to flow to the heater. I would use a NOT gate to reverse the action, so the heater would switch off.
If it gets too bright (light intensity increase) the resistance of the LDR decreases. This means more light would flow through the LDR than the filament lamps (current always takes the path of least resistance), so they would dim.
What are the two ways we can join electrical circuits?
-In series
-In parallel
Explain why a cell goes flat more quickly when lamps are arranged in parallel.
More bulbs in parallel means a higher current is drawn from the cell, so finite amount of stored energy decreases more rapidly.
Why does adding resistors in parallel
decreases the total resistance?
-There are more paths for the current to flow
- The total resistance of two resistors is less than the resistance of the smallest individual resistor.
Why does adding resistors in series increases the total 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
Explain why car lights are not connected in series. (3)
Current would stop flowing if one bulb broke.
Therefore a parallel arrangement is used so that lights stay on if one or more bulbs break.
Explain the design and use of dc series circuits for measurement and testing purposes.
-Current measurement using an ammeter, since it is the same everywhere in the circuit
-Component testing
-To test the performance of batteries by measuring their voltage and current under load.
What is equivalent resistance?
The total resistance of the circuit for the resistors connected in series or parallel.
What does dc stand for?
Direct current.
What does ac stand for?
Alternating current.
What is one full wave on an oscilloscope?
From a point, rollercoaster, alllll the way down, smiley face, back to the point.
Is mains electricity an ac or dc supply?
ac
In the United Kingdom what is the frequency of the domestic electricity supply?
50 Hz
In the UK, what is the voltage at which electricity is supplied to homes?
230 V.
Give the equation linking current, potential difference, and power.
power = current ÷ potential difference
At a constant voltage, replacing a resistor with one of a higher value has what affect on the reading on the ammeter?
It decreases.
At a constant voltage, a circuit with 2 identical resistors has ______ the resistance of a circuit with 1 resistor.
double
Describe how a student could use a circuit to investigate how the current through a filament lamp affects its resistance. [4]
-use an ammeter to measure current and a voltmeter to measure potential difference
-alter the resistance of a variable resistor to change the current in the circuit
-calculate the resistance of the filament lamp using the equation R= V ÷ I
-Repeat at a wide range of different currents, getting enough readings for a valid conclusion of the relationship to be made
What is effective resistance?
A measure of the total resistance of every resistor in a circuit.
Explain why a graph of resistance against the length of a wire may not produce a straight line that goes through the origin. [4]
The line will not pass through the origin if there are errors introduced when collecting the data or taking measurements for example:
-Errors due to heating of the wire, leading to bigger than expected resistance values
-Zero errors of ammeter or voltmeter
Random errors
Explain why the total effective resistance is greater when resistors are arranged in series compared with being arranged in parallel. [4]
When resistors are added in series, the current has only one route to travel down so it has to go through all (three) resistors and resistance is higher.
When resistors are connected in parallel, there are more routes for the current to travel down (the current splits).
Therefore, the current only has to go through one resistor and the resistance is lower
Explain how we know the relationship between current and resistance is constant from a graph.
The product of current and resistance = a constant
e.g…36x0.08=2.88 and 24x0.12=2.88
Why does increasing voltage increase current?
Increasing the voltage means that the electrons moving around the circuit are given a bigger ‘push’.
They will move faster so a higher number of them will pass the same point every second.
