AS Physics - Current Electricty

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  • Created by: Lizzy Day
  • Created on: 16-05-15 15:45
What is current?
the rate of flow of charge
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What is current measured in?
Amperes (A)
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How can you make an electric current pass?
It must have a complete circuit and there must be a p.d.
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What is current due to?
the passage of charged particles called charge carriers
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How can you measure the current in a circuit?
By using an ammeter attached in series
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What are the charge carriers in metals?
conduction electrons
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What is conventional current?
The old belief that current passed from positive to negative. Before electrons were discovered.
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What is charge?
Charge is what is carried through the wires by electrons
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What is the unit for charge?
The coulomb (C)
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What is one coulomb defined as?
the amount of charge that passes in 1 second when the current is 1 ampere
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What is the equation for charge?
Q = It
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What happens in an insulator?
electrons cannot move from the atom so no current passes
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What happens in a metallic conductor?
some electrons aren't attached, these conduction electrons are attracted to the positive terminal
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What happens in a semi conductor?
the number of charge carriers increases with temp. Resistance decreases as temperature is raised
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What is a pure semiconducting material?
a.k.a an intrinsic semi conductor. Conduction is due to the electrons breaking free from the atoms of the semi conductor
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What is potential difference?
the work done per unit charge
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What is the equation for potential difference?
V = W/Q
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What is a volt?
1V = 1JC^-1
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How can you measure voltage in a circuit?
By using a voltmeter attached in parallel
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What is emf?
the electrical energy produced per unit charge passing through the source
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What is work done in a device with resistance?
work done is transferred as thermal energy. charge carriers repeatedly collide with atoms in the device and transfer energy
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What is work done in an electric motor?
work done is transferred as kinetic energy of the motor. charge carriers need to be forced through the wires against the force of the magnetic field
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What is work done in a loudspeaker?
work done is transferred as sound energy. charge carriers are forced through the wires against the force of the loudspeaker magnet
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What is resistance?
The p.d. across any component divided by the current through it.
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What are the units for resistance?
Ohms
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What is ohms law?
the p.d. across a metallic conductor is proportional to the current through it, provided physical conditions do not change
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What is a variable resistor used for?
it is used to adjust the current and p.d. as necessary
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What are ohmic conductors?
the conductors that obey ohms law
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what is resistivity?
a measure of how much a particular material resists current flow. Depending on the structure of the material aswell as environmental factors. It is the resistance of a 1m length with a 1m^2 area
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What is a superconductor?
a wire or device made of a material that has zero resistivity at/and below a critical temperature that depends on the material
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What is the p.d. across a superconductor?
When a current passes through the superconductor, there is no p.d. across it because its resistance is zero.
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What are superconductors used for?
to make high-power electromagnets that generate very strong magnetic fields and power cables that transfer electrical energy without wasting it.
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What are I-V characteristics?
an I-V graph. The shallower the gradient, the grater the resistance of the component
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How can you find the I-V characteristics of a component?
1. use a variable resistor to change the resistance in small steps. 2. for each change, take a reading of I and V. 3. reverse the direction of electricity flow, repeat
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What is the I-V graph for a metallic conductor/wire?
a straight diagonal line through the origin.
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What is the I-V graph for a lamp?
an elongated 's' through the origin
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What is a filament lamp?
a thin coil of metal wire. Current flowing through it increases with temperature. This increases its resistance
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Why do filament lamps blow?
when you first switch a bulb on, the filament has lower resistance as it is cold. The initial current is larger than normal. filament can burn out as it heats up very quickly
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What are 3 examples of semiconductors?
Thermistors, Diodes and LDRs
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What is a diode?
it allows current in one direction only
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What is a thermistor?
the resistance decreases with increasing temperature
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What is an LDR?
resistance decreases with increasing light intensity
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How can you measure the variation of current with pd?
use either a potential divide or a variable resistor
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What happens to the current when the diode is reverse biased?
the current is very small and can only be measured using a milliameter
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What happens to a metal when its temp increases?
The resistance of a metal increases, as the ions vibrate more. The charge carriers cannot pass through as easily (positive temperature coefficient)
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What happens to an intrinsic semiconductor when its temp increases?
The resistance decreases with increase of temp. the number of charge carriers increases (negative temperature coefficient)
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What is Kirchhoff's first law?
The total current entering a junction = the total current leaving it
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What is Kircchoff's second law?
The total emf around a series circuit = the sum of the pd's across each component
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What is the distribution of current in series?
the current entering a component is the same is what's leaving. the current passing through multiple components is the same in each.
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What is the pd of components in series?
the total pd across all the components is equal to the sum of pds across each
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What is the pd of components in parallel?
pd in parallel components is the same
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What is power?
the rate of transfer of energy, measured in Watts.
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What are the equations to calculate power?
P = E/t , P = VI , P = V^2/R , P = I^2R
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What is energy?
the total energy transferred
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What are the equations to calculate energy?
E = VIt, E = V^2t/R , E=I^2Rt
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How does power and energy relate to the starter motor in cars?
Starter motor provides the energy needed to start the engine. Power produced needs to be high, to provide alot of energy in a very short time to "turn over" the engine. pd is usually small, so current must be very high.
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What is the total resistance in series?
sum of individual emfs
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What is the total resistance in parallel?
1/R = 1/R1 + 1/R2 + 1/R3
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What is the heating effect of an electric current?
The heating effect of an electric current in any component is due to the resistance of the component
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What is the rate of heat transfer?
at a constant temp, heat transfer takes place at the same rate I^2R
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What is internal resistance?
the loss of potential difference per unit current in the source when the current passes through the source
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What is the equation for EMF?
= E/Q
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What is EMF?
electomotive force - energy per unit charge
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What is the pd across terminals?
electrical energy per unit charge delivered by the source
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What is the equation joining emf and internal resistance?
= IR + Ir
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What is the power supplied by the cell?
IE = I^2R + I^2r
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When is maximum power delivered?
maximum power is delivered to the load when the load resistance is equal to the internal resistance of the source
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How can you work out the cell current?
cell emf / total circuit resistance
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How can you calculate the pd across each resistor in series?
current x resistance of each resistor
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How can you calculate the current through a resistor in parallel?
pd across it / resistance of resistor
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What is the net emf?
If there is more than one cell, and they are connected in the same direction, the net emf is the sum of individual emfs
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What is total internal resistance?
the sum of individual internal resistances if there is more than one cell
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What is the current for a circuit with n identical cells in parallel?
current through each cell = I/n
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What is the terminal pd across a cell?
V = emf = Ir/n
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What happens each time an electron passes through a cell?
It travels through one of the cells only
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What is a potential divider?
a potential divider consists of two or more resistors in series with each other and with a source of fixed pd
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How can you find the potential difference of the source?
the pd is divided between the components in the circuit as they are in series with eachother
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How can a potential divider be used?
to supply a fixed pd at any value from 0 to the source, to supply a variable pd or to supply a pd that varies with a physical condition
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How can you supply a variable pd
the source pd is connected to a fixed length of uniform resistance wire, and a sliding contact is added
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What is a sensor circuit?
a circuit that produces an output pd which changes as a result of a change of physical variable such as temperature or light intensity
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What is a temperature sensor?
it consists of a potential divider made using a thermistor ad a variable resistor
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What is a light sensor?
it consists of a potential divider made using a light dependent resistor (LDR) and a variable resistor
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What is alternating current?
a current that repeatedly reverses its direction
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What is frequency?
the number of cycles it passes through each second
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What is the frequency of the mains electricity?
50Hz
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What is 'peak value'?
the peak value of an ac (or pd) is the maximum current (or pd) which is the same in either direction
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What does the peak current depend on?
it depends on the peak pd of the alternating current source and of the components in the circuit
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How can you observe alternating current?
use an oscilloscope to display the waveform of the alternating pd from a signal generator
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What happens when you increase the output pd?
you make the oscilloscope trace taller (the peak is bigger)
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What happens when you increase frequency of the signal?
it increases the cycles on the screen
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What is another way to observe alternating current?
connect the signal generator to a torch lamp and make the frequency low - the brightness will vary
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How does the heating effect of an electric current vary?
the heating effect varies accoring to the square of the current P = IV = I^2R
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What is the root mean square value?
the value of direct current that would give the same power as the mean power
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How can you work out the rms?
1 / |2 x peak value
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How can you work out the mean power?
P= Irms^2R = Vrms^2/R = IrmsVrms
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What is an oscilloscope?
it consists of a specially made electron tube and associated control circuits
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What happens to electrons in an oscilloscope?
an electron gun emits electrons in a beam towards a fluorescent screen at the other end of the glass tube
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How is light emitted in an oscilloscope?
Light is emitted from the spot on the screen the beam hits
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How is the position affected?
it is affected by the pd across the plates, if there is no pd, it stays in the same position, if there is a pd across the x plates - it moves horizontally. y plates - vertically
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How can you display a waveform?
x plates connected to time base circuit. (constant speed left to right) pd displayed, connected to y plates (spot moves up and down) this traces out waveform.
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What is the 'calibration value'?
the y-sensitivity (gain) of the oscilloscope
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How can you measure the peak pd on an oscilloscope?
to scale: waveform height, peak pd = (y gain x amplitude)
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What can be used to trigger an ultrasonic transmitter?
time base circuit
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How can a waveform be seen?
if a receiver signal is applied to the y-input
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Card 2

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What is current measured in?

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Card 3

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How can you make an electric current pass?

Back

Preview of the front of card 3

Card 4

Front

What is current due to?

Back

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Card 5

Front

How can you measure the current in a circuit?

Back

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