AQA A physics electricity

various notes from the electrictiy topics from unit 1

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  • Created on: 08-03-11 09:16
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Indices:
X1012 =Tera (T)
X109 =Giga (G)
X106 =Mega (M)
3
X10 =Kilo (K)
X103 =mili (m)
X106 =micra (µ)
X109 =nano (n)
X1012 =pico (p)
X1015 =femto (fm)
1. Current, Charge, Potential Difference, Resistance and Power:
Current:
· current = charge/time
· current measured in amps (A)
· current is the rate of flow of charge- charge measured in Coulombs
· current is measured using an ammeter which must be attached in series
· electrons leave the battery at the negative terminal and reenter at the positive terminal. However
the conventional current flows in the opposite direction (positive to negative)
· electrons are negatively charged, and therefore are the charge carriers in a circuit.
The work done of the charge carriers
can be found using:
work done = currentXvoltageXtime
Voltage:
· voltage = work done/charge
· voltage measured in volts (V)
· the voltage in a parallel circuit is the same for all branches, which is the same as the total
voltage supplied.
· p.d. is defined as the energy converted per unit charge moved.
· Voltage pushes the electrons around the circuit. For electrons to move there must be a
potential difference. (voltage = pd)
Electricity revision notes

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It also has
very high resistance to stop current flowing through it.
· When there is no internal resistance the EMF is equal to the potential difference.
· current is proportional to voltage ( I V) as shown in the graph.
I/V graph for an ohmic conductor. (wire has the same graph)
The straight line shows resistance doesnt change.
· Resistance:the amount of voltage flowing through a component depends on the resistance.…read more

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Components and their characteristics:
I/V graphs:
· The shallower the gradient the greater the resistance
· A curve shows the resistance is changing.
Cells:
· a cell is a source of electrical energy and a battery is a
combination of cells.
Diodes:
· a diode allows the current to flow in one direction only. The light emitting diode/LED
emits light when it conducts.
If the diode conducts then the it is classed as being forward biased most diodes require
a threshold voltage of 0.…read more

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I/V graph for a diode:
To investigate the characteristics of a diode,
one set of measurements are taken with the
diode forward biased. Then another set taken
in reverse biased. This gives you the I/V
graph to the right.
Resistors:
·a resistor is a component designed to have certain resistance
·Thermistor:
the resistance of a thermistor
decreases with increasing
temperature, as more charge carriers are
available (electrons)
Thermistor
Light dependent resistor (LDR):the
resistance of an LDR decreases
with increasing light intensity.…read more

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Investigating the characteristics of different components (in order to draw the I/V graph):
· in order to measure the variation of current with pd for a component use a potential
divider (vary from 0 pd) or variable resisistor (vary current to a min.)
Potential divider circuit.
Variable resisitor circuit.
· the values are then taken and plotted with current on the yaxis and voltage on the xaxis.
The ammeter and voltemeter are used to capture the data.
4. Circuit Rules:
· Charge is always conserved.…read more

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Energy is conserved. Energy transferred to a charge is EMF and energy transferred from a charge is
potential difference. Total emf areound a series circuit equals the sum of the pds across each
component.
· In a series ciruit, the pd splits across each component so the voltages across each
component are added together to find the total voltage.
· In a parallel circuit the voltage is
the same as the output voltage down
each branch.…read more

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EMF and internal resisistance:
· r = internal resistance
· R = external resistor
· E = EMF the terminal PD is less that then EMF due to the internal resistance but if there
was no internal resistance then PD would equal the EMF
· terminal pd, IR, = pd across load resistance, R
· lost volts, Ir, = pd across the internal resistance, Ir = EIR
· EMF (electromotive force)
measured in volts
= IR+Ir
= E/Q
IR= ­ Ir
= I(R+r)
· Power:…read more

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Maximum power is delivered to the load when the load resistance is equal to the internal resistance of the
source.
Measuring the internal resistance:
y2y1 / x2x1 > V2V1 / I2I1 = r
· first measure the pd across the
terminals of a cell, by connecting a
high resistance voltmeter directly across
the terminals of the cell.You then take
measurements of the voltage, whilst
varying the current with a variable
resistor, also taking note of the
current.…read more

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Circuit Calculations including Emf and cells (batteries):
· cell current = cell emf / total circuit resistance
· working out pd for each resistor, start with resistors in series with the cell which therefore passes
the same current as the cell current:
Pd across each resistor
in series with cell = current X resistance of each resistor
· working out the pd through parallel resistors, work out combined resistance and multiply by the cell
current to give pd across each resistor.…read more

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