Electricity Flashcards
What is a fuse?
a safety device which melts to break the circuit if the electrical current flowing through it exceeds a specified value
what is a thermistor?
an electrical device whose resistance decreases as the temperature increases
what is current?
a flow of electric charge
the greater the rate of flow….
the higher the current
equation for charge , current and time
charge (Columbus) = current (amperes) x time (seconds) Q=IT
potential difference
the force that pushes the charge round
resistance
anything that slows flow down
the greater the resistance across a component ………….
the smaller the current that flows
The more difficult it is for the charge to flow
two factors that current depends on
1) potential difference 2) resistance
the bigger the potential difference across a component …
the greater the flow of charge through the component the bigger the current
Equation for potential difference
current x resistance V= IR
What’s an ohmic conductor?
a device that obeys ohms law
What’s Ohm’s Law?
V=IR
What are I-V characteristics?
Graphs which show how the Current changes with Voltage in a components
shallow gradient in a I-V graph means
high resistance -large potential difference needed to produce a small amount
what does it mean when a straight line cuts through the origin
that the current and voltage are directly proportional
steep gradient in a I-V graph means
low resistance - large current will flow though a small voltage (large voltage not needed)
list four components for which the resistance is not constant due to the current changing
1) lamps 2) diodes 3)thermistor 4) LDR (light dependant resistor)
filament lamps
As the current through a filament lamp increases, its temperature increase. This causes the resistance to increase as the current increases. It is indicated by a curved graph.
Diodes
describe its graph as well
current flows through only one direction high resistance if current is reversed
LDR
dependant on light intensity
bright = resistance falls
darkness = resistance increases
What is a variable resistor?
a resistor where the vaue of the resistance can be changed
what is a light emitting diode?
only allows current to flow in the forward direction and emits light when current flows through them
What is the rule for voltage, current and resistance in series circuits?
voltage is shared
Vtotal = V1 + V2 + ……….
Current is the same everywhere
I = V/R
Resistance adds up
Rtotal = R1 + R2 + ……….
how must voltmeters and ammeters be connected
ammeters should always be connected in series
voltmeters should always be connected in parallel
What is the rule for voltage, current and resistance in parallel circuits?
Voltage is the same across all components
current is shared between branches
Adding a resistor reduces the total resistance
why does adding resistors in parallel reduces the total resistance?
Because, in parallel, there are more paths for the current to take, allowing it to flow more easily
the equation for power in a circuit?
P = voltage x current
p = current 2 x resistance
What is a direct current supply?
current that flows in one direction
- has a potential difference that is always positive or negative
- the type of current that is supplied by batteries and cells
What is an alternating current supply?
- the current is always changing direction
- has a potential difference that alternates from positive to negative
- used in mains electricity
What is the UK mains electricity and how many times does it change directions per second?
230V
changes direction 50 times per second therefore it has 50Hz
the 3 different wires in a three-core cable are:
- live wire (brown)
- neutral wire (blue)
- earth wire (green and yellow stripes)
What does the live wire do?
- provides the alternating current (at about 230V) from the mains supply
-
What does the neutral (blue) wire?
- completes the circuit and carriers away current
- it is around 0V
What does Earth Wire do?
- for protecting the wiring and for safety
- stops the appliance casing from becoming live
- The earth wire is at 0 V, it only carries a current if there is a fault.
What happens inside a three-core cable during operation?
- the potential difference causes current to flow through the live and the neutral wires
- the live wire carries the alternating potential from the supply
- the neutral wire completes the circuit
- current will only flow in the earth wire if there is a fault connecting it to a non-zero potential
A live wire can be dangerous even if a switch in the circuit is open because…..
there’s still an alternating current and all it needs is a path for the electricity to flow through
Whenever charge flow, it has to overcome the resistance of the circuit. This requires energy, therefore:
- work is done when charge flows
- the amount of work done depends on the amount of charge that flows and the potential difference
What are the two equations for energy transfer?
E = power x time
E = charge flow x potential difference
how would you carry out the investigating resistance practical?
1) set up the circuit
2) attach a length of wire along a metre rule using pieces of tape
3) attach crocodile clip to one end of the ruler and make it align with 0 cm mark on the ruler. then attach another crocodile clip to the length you want to start with
4) record the voltage and current at a range of lengths, using the crocodile clips
5) calculate the resistance
6) repeat steps 4-5
