# current, p.d and resistance

• Created by: ellie_225
• Created on: 19-05-18 16:01
what is needed for electrical charge ti flow through a closed circuit
source of potential difference
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what is electric current
flow of electrical charge. The size of the electric current is the rate of flow of electrical charge
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charge flow =
current x time
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units for charge flow
charge flow, Q, in coulombs, C current, I, in amperes, A (amp is acceptable for ampere) time, t, in seconds, s
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current in a series circuit
is the same at any point
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protons
positive charge
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electrons
negative charge
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what does the current depend on
the resistance (R) of the component and the potential difference (V) across the component
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the greater the the resistance of the component
the smaller the current for a given potential difference (pd) across the component.
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potential difference =
current x resistance
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units for pd equation
potential difference, V, in volts, V current, I, in amperes, A (amp is acceptable for ampere) resistance, R, in ohms, Ω
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how is current measured
using an ammeter
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what is current
how many electrons flow through the ammeter each second (1A = 625000000 per second)
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what is voltage
measure of the energy carried by the electrons.
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where is the voltmeter in a circuit
wired in parallel with the component, so that it can measure the difference in energy between the electrons as they enter and leave the component.
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how is voltage measured
voltmeter
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what happens in some resistors
the value of r stays constant but in others it can change as the current changes
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the current through an ohmic conductor
(at a constant temperature) is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes.
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what is the resistance in other components
components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.
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resistance in a filament lamp
increases as the temperature of the filament increases.
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current through a diode
flows in one direction only. The diode has a very high resistance in the reverse direction.
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the resistance of a thermistor
resistance decreases and temp increases
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as the resistance of an LDR decreases
light intensity increases
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use of LDR
switching lights when it get dark
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resistance
• The electrons have to push past the atoms in the wire as they move round the circuit. The atoms resist their flow.
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resistance =
voltage (potential difference) / current
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resistance is measured
ohms
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what does it mean if a component has a high resistance
hard for the current to flow
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resistance in a cold wire
constant
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voltage and current graph
straight line, through the origin. This shows that current is proportional to voltage. Doubling the voltage will double the current.
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graphy for when the wire is heated
is curves
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ohms law
current is proportional to the voltage.
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what happens when the wire heats up
the metal atoms vibrate more. This makes it hard for the electrons to flow through the wire. The resistance increases.
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LDR
• When the light level is low, the LDR has a high resistance. • When the light level is high, the LDR has a low resistance.
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what components are made from semiconductor materials
resistor, variable resistor, LDR
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what can LDRs be used for
used in circuits to measure light levels (eg or photography) and in control circuits (eg security circuits that turn lights on when it gets dark).
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The resistance of a thermistor changes with temperature.
The hotter the thermistor, the lower the resistance. [Note: this is opposite to a piece of wire, where the resistance increases when it gets hot.]
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what is a thermistor used for
measure temperature, control temperature
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diodes
conduct current in one direction; they are like one-way streets.
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are diodes ohmic
no
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current in a series circuit
there is the same current through each component
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pd in a series circuit
total potential difference of the power supply is shared between the components
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resistance in a series
total resistance of two components is the sum of the resistance of each component.
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pd in parallel
same
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current in parrallel
sum of the currents through the separate components
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resistance in parallel
total resistance of two resistors is less than the resistance of the smallest individual resistor.
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mains electricity
ac supply. In the United Kingdom the domestic electricity supply has a frequency of 50 Hz and is about 230 V.
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insulation covering
live wire – brown neutral wire – blue earth wire – green and yellow stripes.
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live wire
carries the alternating potential difference from the supply
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neutral wire
completes circuit
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earth wire
safety wire to stop it becoming live - The earth wire is connected to the metal casing of a device. The other end of this wire is connected to a metal rod or pipe that goes into the ground below a building.
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potential difference between the live wire and earth
230V
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potential difference of neutral wire
0V
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potential difference of earth wire
0V, only carries if there is a fault
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live can be dangerous when
a switch in the mains circuit is open
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why is mains dangerous
• too many plugs in a socket • appliances with damaged cables • damaged plugs • over long cables, particularly near sinks • mains electricity near water, eg in bathrooms
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direct current
only flows in one direction. The electrons flow from the negative side of the power supply to the positive side.
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what provide dc
batteries
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alternating current
flows back and forward
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what provides ac
mains - changes direction 50 times each second - frequency of 50Hz
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mains has a voltage of
230V
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changing voltage can be recorded by
oscilloscope or datalogger and voltage sensor
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three pin plug
• The Live wire (brown) is connected to the fuse • The Neutral wire (blue) is connected to the second pin at the bottom • The Earth wire (yellow and green striped) is connected to the top pin.
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fuse
is a piece of fine wire which is designed to melt and break if the current gets too large. It is a deliberate weakest link.
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the thicker the fuse
the greater the current required to cause it to melt
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fuses are supplied in
3, 5, 13 Amps
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re-settable fuses
Modern fuses are often re-settable fuses. Instead of a wire melting and breaking an electromagnet is used to trip a switch which turns off the current. Once the fault has been corrected the fuse can be reset and used again.
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double insulation
Some appliances have plastic covers. They are designed so that it is impossible to touch any metal parts when using it. This is called double insulation. These appliances do not need an earth wire
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why does double insulation mean no earth wire
if there is a fault the current cannot flow through a person
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circuit breakers
A circuit breaker is an electromagnetic device that breaks a circuit when the current goes above a certain value.
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what do circut breakers prevent
equipment from catching fire, by switching off the current, but they do not prevent electrocution.
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residual circuit breakers
compare the current flowing in with the current flowing out of the device. If the two currents differ by more than a few milliamps, the circuit breaker switches off the current. They switch off very quickly, means small current throughs through you
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RCCBs used in
any situation where there is a danger of electrocution, such as an electric lawn mower or hedge trimmer.
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power =
pd x current
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power =
current2 x resistance
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units for power
power, P, in watts, W potential difference, V, in volts, V current, I, in amperes, A (amp is acceptable for ampere) resistance, R, in ohms, Ω
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everyday appliances
designed to use energy transfers
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the amount if energy an applience uses depends on
how long it is switched and the power of the appliance
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when is work done
when charge flows in a circuit
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energy transferred =
power x time
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energy transferred =
charge flow x pd
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units for energy transferred
energy transferred, E, in joules, J power, P, in watts, W time, t, in seconds, s charge flow, Q, in coulombs, C potential difference, V, in volts, V
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how is energy supplied
electrons have energy provided by the power supply
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when is the energy transferred into other forms
when the electrons travel through the component in the circuit
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energy transferred =
current x time x voltage
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power
how fast the energy is transferred
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power =
current x voltage
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what is the national grid
system of cables and transformers linking power stations to consumers.
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how is electrical power transferred
power stations to consumers using the National Grid. Step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value
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why don't we transfer electricity with high currents
there would be too much loos of energy from heating
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how is the electricity transferred
electricity is transferred with low currents, but high voltages, so that the same power can be transferred, but with less heating loss.
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what are step up and down transformers
These are used in the National Grid to change the voltage
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step up transformer
In a step-up transformer the voltage across the secondary coil is greater than the voltage across the primary coil. The secondary turns must be greater than the primary turns.
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step down transformer
In a step-down transformer the voltage across the secondary coil is smaller than the voltage across the primary coil. The secondary turns must be smaller than the primary turns.
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why do we use step up and down transformers
Using a higher voltage and therefore a lower current to transmit electricity means there is less heating in the cables and so less energy loss.
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transformers only work with
alternating current which is why mains electricity is ac
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how is static charge made
materials are rubbed against each other, become electrically charged. Negatively charged electrons are rubbed off one material to the other.The material that gains electrons becomes negative,The material that loses electrons becomes equally positive
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what happens when two electrically charged objects are brought close together
they exert a force on each other.. Two objects that carry the same type of charge repel. Two objects that carry different types of charge attract.
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what is attraction and repulsion an example of
non contact force
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if an atom is uncharged
the number of protons = number of electrons
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if two objects are rubbed together
• friction can make electrons move from one material to the other. Each object is left with an unbalanced charge.
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what materials do this
insulators. • Electrons cannot flow through them, so the unbalanced charge remains.
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what is electrostatic induction
negative charges (electrons) on the balloon repel electrons in the wall, leaving the surface of the wall positive.
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what does an electric field do
A charged object creates an electric field around itself. The electric field is strongest close to the charged object. The further away from the charged object, the weaker the field.
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what happens if a second charged particle is in the field
it experiences a force, The force gets stronger as the distance between the objects decreases.
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what is an electric field
An electric field is the area round a charged object where the electric force is felt.
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electric field due to a charge sphere
As the distance from the charged object increases, field lines get further away from each other. the field gets weaker. Another charged object placed in an electric field will experience a force; either repulsion/attraction. causes electrical sparkin
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## Other cards in this set

### Card 2

#### Front

what is electric current

#### Back

flow of electrical charge. The size of the electric current is the rate of flow of electrical charge

### Card 3

charge flow =

#### Back ### Card 4

#### Front

units for charge flow

#### Back ### Card 5

#### Front

current in a series circuit

#### Back ## Comments

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