# physics 5

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Conventional current
flows around a circuit from the positive (+) side of the cell to the negative (-). However the electrons are flowing around the circuit in the opposite direction from the negative (-) side of the cell to the positive (+).
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A resistor at constant temperature (ohmic conductor)
Current is directly proportional to potential difference. Doubling the potential difference doubles the current in the circuit. The resistance remains the same.
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Ohm’s Law
“The electrical current in a conductor is proportional to the potential difference applied to it provided the temperature remains the same.”
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Measuring current and potential difference
Current is measure with an ammeter, ammeters are always connected in series with the component of interest. Potential differences are measured using a voltmeter, voltmeters are connected in parallel with the component of interest.
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A diode
A diode only allows current to flow in one direction through it (forward biased), when the current tries to flow the other way (reverse biased) no current is allowed to flow through the diode.
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A diode
When the diode is reversed biased if we keep increasing the potential difference the diode will eventually begin to conduct in the reverse direction, this is called the break down voltage.
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Thermistor
The resistance of a thermistor decreases as it’s temperature increases. Thermistors can be used as thermostats, the thermistor is used in circuits which monitor and control the temperature of rooms, freezers & fridges etc.
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Thermistor
Thermistors can have a positive or a negative temperature coefficient. A negative temperature coefficient means that its resistance decreases with an increase in temperature, this is caused by the release of extra charge carriers in the thermistor.
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LDR – Light Dependant Resistor
The resistance of an LDR decreases as the light intensity falling on it increases. LDR’s are used in circuits which automatically switch on lights when it gets dark, for example street lighting.
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Resistivity
The resistance of a piece of wire at a constant temperature depends on both the length of the wire and the cross-sectional area of the wire.
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Resistivity
the longer the wire the greater the resistance the greater the cross-sectional area the smaller the resistance
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Resistivity
Resistivity (r) is a property of materials which takes account of their resistance (R), length (L) and cross-sectional area (A).
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Resistivity and temperature
In metals increases in temperature make the atoms in the structure of the metal vibrate more and this makes it more difficult for the electrons to move through the material, so the resistance of the material goes up. A filament lamp
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Cells in Series
When cells are connected in series with each other and they are all connected in the same direction the total potential difference supplied to the circuit is the individual potential differences added together.
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Identical cells in parallel with each other
When identical cells are in parallel with each other the total potential difference supplied to the circuit is equal to the potential difference of just one of the cells.
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Resistors in Series
When resistors are in series with each other there total resistance is just there individual resistance added together.
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Resistors in parallel
When resistors are in parallel with each other there total resistance is found using the equation below.
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Current in parallel circuits.
The total current flowing from the cell towards the branches in the circuit must always equal the current flowing through each component in the branches of the circuit when they are added together.
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Potential difference in a series circuit.
The total potential difference supplied by the cell is divided up between the components. If the components all have the same resistance they will have equal amounts of potential difference across them.
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Potential difference in parallel circuits.
The potential difference supplied by the cell is the same potential difference as that across each component in the parallel circuit.
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Potential Divider
A potential divider is a simple circuit that uses resisters(or thermistors / LDR’s) to supply a variable potential difference.
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Electromotive Force and Internal Resistance
The electromotive force (e) or e.m.f. is the energy provided by a cell or battery per coulomb of charge passing through it, it is measured in volts (V). It is equal to the potential difference across the terminals of the cell when no current is flowi
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internal resistance
Batteries and cells have an internal resistance (r) which is measures in ohm’s (W). When electricity flows round a circuit the internal resistance of the cell itself resists the flow of current and so thermal (heat) energy is wasted in the cell
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## Other cards in this set

### Card 2

#### Front

A resistor at constant temperature (ohmic conductor)

#### Back

Current is directly proportional to potential difference. Doubling the potential difference doubles the current in the circuit. The resistance remains the same.

Ohm’s Law

### Card 4

#### Front

Measuring current and potential difference

A diode