# Electric Current

## Charge, Current and Potential Difference

An electric current is defined as the rate at which charged particles pass through a point in a circuit.

• Current is measured in coulombs per second or amperes.
• In metals, the charged particles are electrons which move from the negative to the positive terminals in d.c supply.
• In circuit diagrams, the charged particles move from +ve to -ve, which is known as conventional current.
• Current = change in charge / change in time
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## Charge, Current and Potential Difference (cont)

• A potential difference is what makes a current flow.
•  A potential difference is the electrical energy transfered/converted per unit of charge passing between 2 points.
• P.D is measured in joules per coulomb, or volts. Potential difference = Energy / Charge.
• A charged particle gains energy when it passes through a cell, and it releases this gained energy when it passes through a component in the circuit.
• Thus both the cell and a component have a p.d across them when charge flows in a circuit.
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## Charge, Current and Potential Difference (cont)

• Resistance is the opposition charged particles face when they flow around a circuit.
• The potential difference needed to make a current flow in a circuit depends on the resistance of circuit.
• The bigger the resistance, the more potential difference (energy per coulomb) is required to make a certain current flow.
• Resistance = potential difference / current
• It is measured in ohms
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## Current/Voltage Characteristics

• The effect of varied potential difference on the current through a component can be investigated using a variable p.d cell connected to an ammeter, switch and component in series, with a voltmeter in parallel to the component.
• By varying and recording the supply p.d, a range of current values can be recored for the component.
• The battery is reversed and the supply p.d is varied over the same range.
• An I/V characteristics curve for that component can be plotted from the results.
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## Current/Voltage Characteristics (cont)

• In a resistor or wire, a proportional straight line is plotted.
• The proportionality between current and p.d means that the conductor follows Ohm's law.

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## Current/Voltage Characteristics (cont)

• For a semiconductor diode, the shape of the graph depends on the direction the current is flowing.
• When the diode is forward biased (facing direction of convectional current), between 0-0.7V the diode offers a large resistance to the current.
• From 0.7V onwards the resistance of the diode falls rapidly and a large current flows, shown by a steep rise in the graph.
• When the diode is reversed biased, the diode offers high resistance until the breakdown voltage, where the diode is destroyed and a large current flows.
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## Current/Voltage Characteristics (cont)

• When the p.d across a filament lamp is steadily increased, the graph becomes less and less steep.
• The p.d and the current are not proportional because the increasing current heats the filament lamp.
• An increase in temperature increases the resistance of the filament and so decreases the rate of increase of current with p.d.
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## Current/Voltage Characteristics (cont)

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## Current/Voltage Characteristics (cont)

• Ohm's law states that the current in a conductor is directly proportional to the p.d across it, provided that the temperature and other physical conditions remain the same.
• A proportional I/V characteristics graph shows that a component obeys Ohm's law (wires and resistors).
• These are called Ohmic conductors, while components that do not obey Ohm's law are called non-Ohmic conductors.
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## Resistivity

• The 2 factors that affect the resistance of a conductor are its length and its cross-sectional area.
• Resistance is proportional to length.
• Resistance is inversely proportional to cross-sectional area.
• The resistivity of a conductor=
(cross sectional area x resistance of conductor) / length of conductor.
• The resistivity is a constant of the material from which the conductor is made.
• Its units are ohm metres
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## Resistivity (cont)

• Resistivity of a wire can be measured using a battery, a switch, an ammeter, and a 100cm piece of wire under test taped to a ruler, all in series. Add a voltmeter in parallel to the wire on the ruler.
• Record the p.d and current for the full 100cm of wire.
• Vary the length of the wire that it across the voltmeter from 100-30cm, recording the current and votlage throughout.
• Calculate the resistance for each recorded length.
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## Resistivity (cont)

• Measure the diameter of the wire using a micrometer, and use this value to calculate the cross-sectional area of the wire.
• Plot resistance against length (y=mx+c).
• The gradient is the resistivity/cross-sectional area, so the resistivity can be calculated.
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## Resistivity (cont)

• Temperature always affects conduction, no matter wha the material (conductor, semiconductor ect)
• In conductors, as the temperature increases the resistance increases.
• Metal conductors (wires and resistors) contain +ve ions as well as free electrons. The electrons collide with the ions as they try to carry charge through, causing a resistance
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## Resistivity (cont)

• As temperature increases, the +ve ioons and electrons have more kinetic energy, so the +ve ions vibrate more (greater amplitude), and electrons move faster.
• Both of these increase the number of collisions of the charge carriers with the +ve ions (frequency), so resistance increases.
• The resistance does not change greatly, so in small circuits we consider wires and restitors ohmic conductors.
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## Resistivity (cont)

• In a thermistor, the resistance decreases significantly as temperature increases.
• A thermistor is made from semiconductor material and so there are few free electrons to produce a current.
• As temperature increases, extra electrons are released from the semiconductor ions due to increased thermal energy.
• This makes the thermistor far more conducting and far less resistive
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## Resistivity (cont)

• When the temperature of a conductor approaches absolute zero, the electrical resistance disappears completely.
• This occurs at a specific temperature for a material known as the critical temperature
• At and below this temperature, no energy is transferred to the conductor as a current passes through it.
• This is called a superconductor.
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