as level electricity - circuits
DESCRIPTION
AS Level Electricity - Circuits. Taking Measurements. The p.d. across a component in a circuit is measured in volts (V) using a voltmeter connected across (in parallel with) the component. Taking Measurements. - PowerPoint PPT PresentationTRANSCRIPT
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Taking Measurements
• The p.d. across a component in a circuit is measured in volts (V) using a voltmeter connected across (in parallel with) the component.
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Taking Measurements
• The current (I) flowing through a component in a circuit is measured in amperes (A) using an ammeter connected in series with the component.
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Current
• A current will flow through an electrical component (or device) only if there is a voltage or potential difference (p.d.) across its ends.
• The bigger the potential difference across a component, the bigger the current that flows through it.
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Model• You can think of electrical
potential as being the topography of the electrical environment.
• The flow of charged particles is affected by the steepness of the ‘slope’.
• The change in volts per metre is a measure of how steep the slope between two points is… the steeper the ‘potential gradient’ the faster the charge will flow.
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Current
• An electric current is a flow of charge (Q) measured in coulomb (C).
• The charges 'flowing' are usually electrons (in a wire) but can be ions (in a solution).
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Current
• It is the 'net' flow of charge that makes the current.
• Charges going in opposite directions cancel out each other's effect.
• Double-charged ions will make double the current that single-charged ones would.
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Resistance
• Components resist a current flowing through them.
• The bigger their resistance, the smaller the current produced by a particular voltage, or the bigger the voltage needed to produce a particular current.
• Resistance (R) is measured in ohms ()
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Resistance
• When electrical charge flows through a resistor, electrical energy is transferred as heat according to the equation P=IV
• This makes components get hotter as current goes through them.
• A change in temperature can change the resistance of the component. You need to appreciate this.
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Cells and Batteries
•An electric cell provides the potential difference for a battery powered circuit by changing chemical energy into electrical energy.
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Cells and Batteries
•If more than one electrical cell is connected together the term for the power source is ‘battery’ – a single cell is just called an electric cell.
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Cells and Batteries
•A cell’s potential difference between its terminals has a chemical source and that this can ‘run down’ with use or incorrect storage providing less of an electrical gradient for the current (i.e. the voltage stamped on a battery might not be correct).
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Electrical Energy Transfer
• As an electric current flows through a circuit, energy is transferred from the battery or power supply to the components in the electrical circuit.
• An electric current is a flow of charge.
• Charge (Q), measured in coulomb (C) is a property of the electrons that move in the wire. Each electron has a very tiny charge of 1.6 X 10-19C
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Equations you should already know from GCSE
When electrical charge flows through a resistor, electrical energy is transferred as heat.
The rate of energy transfer (power) is given by: P = IV
Where: P = power (in watts, W)
V = potential difference (in volts, V) I = current (in ampere, A)
1 watt is the transfer of 1J of energy in 1s.
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Equations you should KNOW
The higher the voltage of a supply, the greater the amount of energy transferred for a given amount of charge which flows.
E = VQ Where
E = energy transferred (in joule, J) V = potential difference (in volt, V)
Q = charge (coulomb, C)
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Equations you should KNOW:
Q = I tWhere:
Q = charge (coulomb, C) I = current (in ampere, A) t = time (in seconds, s)
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Equations you should KNOW
V = I RWhere:
V = potential difference (in volts, V) I = current (in ampere, A)R = resistance (in ohm, )
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Equations you should KNOW
E = PtWhere:
E = energy transferred (in joule, J) P = power (in watts, W) t = time (in seconds, s)
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For all equationsyou should be able to:
• recall the equation • manipulate it • know the symbols, values and units • use it in calculations • be able to use S.I. Prefixes with the
units
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Symbols
connecting wire
connection between two crossing wires
two crossing wires that are not connected to each other
switch (open)
switch (closed)
signal lamp
filament lamp
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Symbols (cont)
cell
battery
power supply
fuse
resistor
diode
variable resistor
thermistor
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Symbols (cont.)
ammeter
voltmeter
L.D.R. (light dependant
resistor)
You have to be able to draw these symbols and incorporate them into circuits.
They must be drawn carefully.
Never put a symbol in a ‘corner’.
Never leave a gap.
Use a sharp pencil to draw the circuits.
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Series Circuits
When components are connected in series:
• their total resistance is the sum of their separate resistances RTOTAL = R1 + R2 + ..........RN;
• the same current flows through each component;
• the potential difference from the supply is shared between them.
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Parallel Circuits
When components are connected in parallel: • there is the same potential difference
across each component; • the current through each component
depends on its resistance; the greater the resistance of the component, the smaller the current;
• the total current through the whole circuit is the sum of the currents through the separate components - this follows from Kirchhoff's First Law - see below.
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Characteristic Curves
• Current-voltage graphs are used to show how the current through a component varies with the voltage you put across it.
• They are called characteristic curves of the components.
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The current through an ohmic conductor (e.g. a wire) is proportional to the voltage across the resistor at constant temperature.
This is known as Ohm's Law.
The straight line shows proportionality – the fact it goes through the origin shows it is directly proportional – double the voltage and the current doubles!
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The resistance of a filament lamp increases as the temperature of the filament increases.
When the filament is very cool the graph is a straight line – it curves most as the temperature changes rapidly (when it goes through the red glow to white glow stage). When it is really hot it gets to a steady temperature and the line straightens out again.
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The current through a diode effectively only flows in one direction only. It acts like a closed switch when connected in forward bias and an open switch when in reverse bias.
When connected in forward bias its resistance is very low (provided it has a potential difference of more than 0.6 volts across it).
The diode has a very high resistance in the reverse bias therefore only a tiny current flows.
Zero p.d. gives zero current.
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You also need to KNOW
• The resistance of a light dependent resistor decreases as the light intensity increases.
• The resistance of a thermistor decreases as the temperature increases. (There are some thermistors which behave in the opposite way to this but all of your questions will be set on this version).