Electricity Flashcards
Definition of power?
the rate at which electrical energy is transferred to other forms
Explain how you would use an LDR, a variable resistor and a transistor to make a light switch on when the light level in a room drops.
The variable resistor and LDR are placed in series to create a potential divider circuit. In bright light, the resistance of the LDR is low, and therefore the p.d of the transistor is low so the transistor remains off. But, when the light level decreases, the transistor’s input p.d increases due to the fact that the resistance of the LDR increases. This causes the transistor to turn on, allowing current to flow to the lamp.
How can a circuit containing a transistor be used as a switch?
A circuit containing a transistor can be used as a switch because transistors turn on when their input p.d reaches a certain value and when this happens, the transistor allows current to flow to the rest of the circuit.
Explain how potential dividers can be used to run a component that requires a low p.d from a high p.d source.
potential dividers are two resistors which are placed in series in order to split voltage up based on each resistor’s respective resistance.
Why does adding more resistors in parallel make the total resistance of the circuit decrease?
because it causes an increase in current and as current goes up resistance goes down.
Where is an electric field created?
around any Electrically charged object
What happens to the electric field strength as you get closer to the charged object?
It gets stronger
What type of force does a charged object feel in another’s electric field?
Attraction or repulsion
What is the relationship between opposites and like charges?
Opposites attract, like charges repel
Where does the attraction/repulsion force come from?
due to the electric fields of each object interacting
How does distance affect the force between two charged objects?
Greater distance means weaker force
How can electric fields be shown in diagrams?
by drawing electric field lines
Where do electric field lines go from and to?
Positive to negative and are always drawn at a right angle to the surface of a charged object
What is the relationship between line closeness and field strength?
Closer lines indicate stronger field
Field lines show the direction that a force would act on a ——— charge placed at each point and the ——- charge would feel an opposite force.
positive
negative
What can field lines be used to evaluate?
the path that a charge would take if placed in the field.
What is electric current?
term which describes the electrical charge transferred per unit of time
What is needed for current to flow?
Source of potential difference
How is current measured?
Amperes (A)
Who is the electric current unit named after?
André-Marie Ampère
What are the charge carriers in metals?
Electrons
What is the conventional flow of current?
Positive to negative (old idea, incorrect) actually goes from -ve to +ve
What is a characteristic of a closed series circuit?
Current stays the same
What materials allow easier flow of electric charge?
Conductors
What are materials that do not allow electric charge flow called?
Insulators
What does resistance measure?
opposition to flow of current
What is the unit of resistance?
Ohms
What is the formula for Ohm’s law?
V=IR
How can you measure resistance?
Using ohmmeter or multimeter
What component allows changes to be done to current without using a variable supply?
Variable resistor
What is the relationship between potential difference and current in a circuit with a constant resistance?
the potential difference is directly proportional to the current
What are components which follow Ohm’s law known as? (when the resistance is constant and the p.d and current are directly proportional, then it follows ohm’s law)
Ohmic, (opposite is non-ohmic). a resistor with a constant temperature is ohmic.
What happens to resistance typically with temperature?
Increases
What causes a component to heat up?
because when an electrical charge has to flow, it does work against resistance, causing an electrical transfer of energy, some of which is turned into heat.
What happens to resistance as temperature increases?
It increases. when a component gets too hot, its resistance is so high that current can’t flow at all.
What is the behavior of thermistors with temperature?
Resistance decreases when temperature increases
Where must ammeters be placed?
In series
Where must voltmeters be placed?
In parallel
What verifies a resistor follows Ohm’s Law when drawing an I-V graph?
Straight line through origin (you can also calculate the resistance from the gradient)
How is resistance calculated for an ohmic conductor?
Gradient = 1/R (R should stay the same if the resistor is ohmic)
What I-V graphs do non-ohmic components have?
curved ones showing that the resistance isn’t fixed
What should be done to maintain ohmic conditions during experiments?
Use low currents, cool-downs between repeats
When do LEDs emit light?
when Current flows only in one direction
How do LEDs compare to other lighting types?
More efficient as they use much smaller currents
What do LEDs indicate in a circuit?
Presence of current
Where are LEDs commonly used?
Home appliances(often to show when turned on), digital clocks, traffic lights, remote controls.
What do photovoltaic cells do?
Transfer light into electrical energy
What can small solar cells (or photovoltaic cells)power?
Calculators
What can large solar cells power?
Entire satellites
What is a relay?
A switch using an electromagnet called a relay coil
What is an electromagnet?
a magnet which only becomes magnetic when an electric current is flowing through it
What do relays connect?
Two isolated circuits, so that when one is turned on, the other will too
Why are relays used?
To isolate low p.d from high p.d
What are the advantages of using relays?
Protects switch circuit and user
What is the purpose of a relay in a car’s starter motor?
To control high current safely and not allow it to get into bits where it is dangerous to have a high current, like where you turn your key
What happens when the switch in the low current circuit is closed?
Electromagnet activates and generates a magnetic field which then causes an attraction force to close the other switch
What happens in the high current circuit when the switch in the low current circuit is opened?
The electromagnet turns off and stops pulling causing the high current switch to open and the circuit breaks
What type of material is usually used in a relay?
Iron contacts
What do microphones do?
convert sound energy into electrical energy
What energy does a motor convert?
Electrical energy into the kinetic energy of a rotating coil, within the motor
What is a disadvantage of motors?
Waste energy as heat and sound
What happens if one component is removed in a series circuit?
Loop is broken, everything stops
In series circuits, total voltage equals what?
Sum of all voltage at components
How is voltage shared in series circuits?
Shared across components
What is the current like in series circuits?
Same from every point
How do you calculate total resistance in series?
Sum of respective resistances
What happens when a resistor is added in series?
Voltage is shared, current lowers
What happens to potential difference with higher resistance?
More potential difference
What increases voltage in a series circuit?
More cells
How are house electrics connected?
Parallel circuits
What is used to connect all sockets in a building?
Ring main
Benefit of a ring main?
Sockets can be turned on/off separately
Voltage supply to sockets in a ring main?
Full 230V
Connection type for components in parallel circuits?
Separately connected
What happens if a component is disconnected in a parallel circuit?
Others are hardly affected
What is the voltage like in parallel circuits?
Same everywhere
What must be equal at the junctions of a parallel circuit?
Total current in = total current out
How is current divided between two identical components in parallel?
Equally divide
When do transistors automatically turn on?
When their input p.d. reaches a certain level
What are transistors used in?
Street lights, things that turn on automatically
Name two types of transistors.
Npn transistors and MOSFET transistors
What are npn transistors also known as?
Bipolar transistors
What are the parts of an npn transistor?
Base, collector, emitter
What is the input voltage in an npn transistor?
Voltage between base and emitter
What happens when the input voltage of a transistor reaches a certain value?
Transistor turns on
What are the three terminals of a MOSFET transistor?
Gate, drain, source
What is the input p.d in a MOSFET?
p.d between gate and source
What happens when the input p.d of a MOSFET reaches a certain value?
Transistor turns on and the current will start flowing between the source and the drain
What is electric current?
the electric charge transferred
How does electric current flow in A.C circuits?
Electrons can flow either way
Alternating Current
Direction changes constantly
How can A.C circuits be created?
can be produced when alternating p.d.s are used which means the positive and negative ends of the p.d keep alternating
Direct Current
Direction does not change
Frequency
Times current changes direction
What formula calculates the charge that has passed in a certain amount of time?
Q=It
What is measured in coulombs?
Charge
When drawing a circuit diagram in an exam, make sure to draw it with what?
wires have straight lines. needs to be fully closed (ignoring switches).
What are the places where the circuit splits represented by?
black dots
What is the driving force that pushes the charge round?
Voltage or potential difference
What is one volt?
One joule per coulomb
What is an LDR dependent on?
light intensity
What is an LDR?
resistor that is dependant on light intensity
What will the resistance fall in?
brightness
What can LDRs be used for?
automatic night lights, outdoor lighting and burglar detectors
What’s a thermistor? And what happens when it gets hot?
a temperature dependant resistor. Resistance lowers
Thermistors can be used as temperature detectors in car engines and what type of electronic devices?
thermostats
What is a diode and what is it typically made from?
A diode is a device which lets current flow freely but only in one direction as resistance is too high in the other direction. Semiconductors like silicon. They are often used in electronic circuits
What happens when the capacitor is charged?
current stops flowing
What is a capacitor?
a component which stores charge but they can only store small amounts
What can capacitors be used to store charge in for a few seconds before releasing it instantaneously?
camera flashes
How do you charge a capacitor?
by connecting them to a source of p.d. as current flows round the circuit, the charge gets stored in the capacitor.
What happens to the current the longer you charge the capacitor?
it gets smaller
What happens when the potential difference across the capacitor is equal to that of the battery?
current stops flowing and the capacitor is fully charged.
The more charge being stored in the capacitor, the higher the —————- across it
potential difference
What happens to the capacitor when you remove the battery?
it discharges and the current is the same for discharging as it is for charging but the current flows in the opposite direction.
What are loudspeakers?
devices which transfer electrical energy into the vibration of a cone inside the loudspeaker, which creates sound waves
What does a loudspeaker transfer into the vibration of a cone?
electrical energy
What is electrical energy converted to in a microphone?
sound energy
What do sound waves do in a microphone?
cause a diaphragm in the microphone to vibrate, then turning those vibrations into electrical signals.
What is like a loudspeaker but in reverse?
Microphones
What is the same for each resistor added in parallel?
the driving force that pushes current through the circuit
What does this increase by adding another loop?
increases the total current
What are potential dividers?
pairs of resistors
What are pairs of resistors?
Potential dividers
What do potential divider circuits do to get different outputs of voltage?
divide the p.d
What is the frequency of D.C flowing in the same direction?
0 hertz
What is an example of D.C?
batteries
What is the UK’s main supply?
230V, A.C
What is the frequency of the supply in the UK?
50 hertz per second
What can supply direct current?
Cells and batteries
What is a cathode ray oscilloscope (CRO)?
a type of voltmeter which can be used to get current traces on a screen
What does the vertical height of an a.c trace on an oscilloscope show?
input voltage
What type of trace does A.C show?
wave trace and the trace shows how the voltage of the supply changes with time
What type of trace does D.C show?
straight line trace
What is the p.d on a D.C trace on a CRO?
the distance from the straight line trace to the centre line
Oscilloscopes can be connected to what in order to record the a.c and d.c traces?
data logging systems and the traces would look the same as those on the CRO
What does energy transferred depend on?
power
The power of an appliance is defined as what?
the amount of electrical energy transferred per second
P=…
E/t
What are appliances usually given?
a power rating
What does a power rating do?
shows the maximum safe power that an appliance can work at. This can normally be assumed to be the appliance’s maximum operating power.
What does power depend on?
on the p.d across the appliance and the current
What do the p.d and current show in relation to power?
the voltage will tell you the amount of energy transferred by each unit of charge and the current will tell you how much charge passes over a unit time.
How do you calculate the power of an appliance if you know the voltage and current?
P=IV
What is electrical power measured in?
Watts
Why are fuses used in appliances?
when appliances go above their power ratings they can cause fires
As power gets higher, what happens to the current?
it gets higher too
How do fuses work?
a fuse contains a strip of metal that will melt if current gets too high, causing the circuit to break, turning off the appliance and ensuring safety
What are fuses always labelled with?
the current they melt at
What must you remember when putting a fuse in an appliance?
put a fuse with a higher current than the maximum safe current of the appliance
What two types of fuses are there usually?
3A and 13A
If you don’t know the maximum safe current, use what rule to work out which fuse to use?
power rating<720W, use 3A.
power rating>720W, use 13A
Describe an experiment in which Ohm’s Law can be verified.
A D.C supply is varied and readings are taken off of an ammeter and voltmeter before being graphed. When graphed in an I-V graph, the resistance of the component should equal the gradient of the graph if the component is Ohmic (follows Ohm’s Law)
