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Chapter 13 DC Circuits (13.3 (Measurement of internal resistance (Measure…
Chapter 13 DC Circuits
13.1
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PD Rules
Series
Total pd across all components is equal to the sum of the potential difference across each component
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Complete loop- Sum of emfs round the loop is equal to the sum of the potential drops around the loop (Kirchhoff's laws)
13.2
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Resistance heating
Charge carriers repeatedly collide with the positive ions of the conducting material- transfers energy from charge carriers to positive metal ions
If component is constant temp, heat transfer to the surroundings takes place at the same rate so 
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13.3
Internal resistance
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Internal resistance- loss of pd per unit current in the source when current passes through the source
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Power
Maximum power is delivered to the load when the load resistance is equal to the internal resistance of the source
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13.5
Potential divider
Potential divider- work done or energy transfer per unit charge between two points when charge moves from one point to the other
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Can be used to - 1) Supply a fixed value pd (0-source pd) 2) Supply a variable pd 3) Supply a pd that varies with a physical condition
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2) Variable
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Uses- simple audio volume control, vary brightness of a lightbulb
3) Sensor
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Temperature- Potential divide made using a thermistor and variable resistor- temp constant source pd is divided between the thermistor and the variable resistor. if temp raises, resistance of thermistor falls so pd falls
Light sensor- Uses LDR and variable resistor- if light intensity increases resistance of LDR falls so pd across it falls
13.4
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Cells in series
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If cells are connected in opposite direction in the circuit- net emf= difference between emfs in each direction
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Total internal resistance= sum of individual internal resistances because cells and therefore resistances are in series
Cells in parallel
If a circuit has n identical cells in parallel, the current through each cell= I/n where I is the current supplied to each cell
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Every time an electron passes through the cells is passes through on cell only so the cells act as a source of emf and internal resistance