5.2 electric currents
Definitions
Emf (electromotive force) is the amount of chemical energy converted to electrical energy, per unit charge. The unit is the volt (v).
Potential difference (p.d. or v) is the amount of electrical energy converted to heat, per unit charge. The unit is the volt (v).
emf specifically applies to cells, batteries, generators, and any other device that gives the charges potential energy.
Internal resistance - resistance of the cell. As current flows through internal resistance, some energy is converted from electrical to heat inside the cell.
Power delivered - amount of chemical energy converted to electrical energy per unit time.
Power dissipated - amount of electrical energy converted to heat per unit time
Potential difference (p.d. or v) is the amount of electrical energy converted to heat, per unit charge. The unit is the volt (v).
emf specifically applies to cells, batteries, generators, and any other device that gives the charges potential energy.
Internal resistance - resistance of the cell. As current flows through internal resistance, some energy is converted from electrical to heat inside the cell.
Power delivered - amount of chemical energy converted to electrical energy per unit time.
Power dissipated - amount of electrical energy converted to heat per unit time
Alternative ways of writing P = VI
P = I²R
P = V²/R
P = I²R
P = V²/R
There are two simple arrangements of connecting a number of components - series and parallel.
"Resistors." Resistors. DIT: School of Electronic and Communications Engineering, n.d. Web. 03 Feb. 2014.
resistors in series
- Same current flows through each resistor
- The p.d. across R1 plus the p.d. across R2 must be equal to the p.d. across the combination
- The p.d. across R1 plus the p.d. across R2 must be equal to the p.d. across the combination
Resistors in parallel
- The current splits in two
- current going into a junction must equal the current coming out
- current going into a junction must equal the current coming out
Multiple combinations
- The total is found by splitting the circuit into small units of parallel and series resistors
Cells in series
The p.d. of cells in series simply add (V = V1 + V2)
Electrical measurement
Voltmeters measure potential difference.
Two main types - digital and analogue
To measure the p.d. between A and B, one lead of the voltmeter must be connected to A and the other to B.
An ideal voltmeter has infinitely high resistance so it does not take away any current.
Ammeters measure the current flowing through a resistor.
Must be connected so that the same current will flow through the ammeter as flows through the resistors
The circuit must be broken so the ammeter can be connected.
An ideal ammeter has zero resistance so it doesn't change the current in the circuit.
Two main types - digital and analogue
To measure the p.d. between A and B, one lead of the voltmeter must be connected to A and the other to B.
An ideal voltmeter has infinitely high resistance so it does not take away any current.
Ammeters measure the current flowing through a resistor.
Must be connected so that the same current will flow through the ammeter as flows through the resistors
The circuit must be broken so the ammeter can be connected.
An ideal ammeter has zero resistance so it doesn't change the current in the circuit.
Electrical sensors
Electrical sensor is a device whose electrical properties change with changing physical conditions.
Thermistor - made from semiconducting material whose resistance decreases as temperature increases.
Light sensor (LDR) - light that releases more charge carriers, resulting in lower resistance
Strain gauge - thin metal wire if stretched, its length increases and its cross-sectional area gets smaller. This results in an an increased resistance
Thermistor - made from semiconducting material whose resistance decreases as temperature increases.
Light sensor (LDR) - light that releases more charge carriers, resulting in lower resistance
Strain gauge - thin metal wire if stretched, its length increases and its cross-sectional area gets smaller. This results in an an increased resistance
Use of sensors
- More useful if the devices gave a changing p.d. rather than a changing resistance
- Potential divider - to convert changing resistance to changing potential difference
Potential divider formula:
- Potential divider - to convert changing resistance to changing potential difference
Potential divider formula:
Automatic light switch needs p.d. to active it.
doesn't switch on lights until Vout is big enough
The fire alarm - increase in Vout activates an electronic switch that rings a bell
Strain gauge - if connected in a potential divider circuit, Vout can be used to measure how much bridge stretches
doesn't switch on lights until Vout is big enough
The fire alarm - increase in Vout activates an electronic switch that rings a bell
Strain gauge - if connected in a potential divider circuit, Vout can be used to measure how much bridge stretches