physics: electricity

• Current electricity is the flow of charge per unit time, or the rate of flow of charge, represented by the symbol I.
• Potential difference (V) is the energy transferred per unit charge between two points in a circuit, represented by the symbol W.
• Resistance (R) is a measure of how difficult it is for charge carriers to pass through a component, and is measured by dividing the potential difference across a component by the current flowing through it.
• Ohm’s law states that for an ohmic conductor, current is directly proportional to the potential difference across it, given that physical conditions (e.g temperature) are kept constant.
• An ohmic conductor’s current-voltage graph will look like a straight line through the origin, provided physical conditions are kept constant.
• A semiconductor diode’s current-voltage graph must be considered in both its forward and reverse bias directions.
• A filament lamp contains a length of metal wire, which heats up as current increases, therefore the resistance of this component increases as current increases.
• Unless a question states otherwise, ammeters can be assumed to have zero resistance, meaning they will not affect the measurement of current in a circuit at all, and voltmeters can be assumed to have infinite resistance, meaning no current can flow through them, meaning their measurement of potential difference across a component is exact.
• Resistivity (ρ) is a measure of how easily a material conducts electricity, it is defined as the product of resistance and cross-sectional area, divided by the length of the material.
• When the temperature of a metal conductor increases, its resistance will increase because the atoms of the metal gain kinetic energy and move more, causing the charge carriers (electrons) to collide with the atoms more frequently, slowing them down, therefore current decreases and so resistance increases.
• The opposite is true for thermistors: as the temperature of a thermistor increases, its resistance decreases because increasing the temperature of a thermistor causes electrons to be emitted from atoms, increasing the number of charge carriers and causing current to increase, causing resistance to decrease.
• In a parallel circuit, the sum of the currents in each parallel set of branches is equal to the total current, and the potential difference across each branch is the same.
• These could be used in maglev trains, where there would be no friction between the train and rail, and in certain medical applications.
• When joining together battery cells, you can use either a series or parallel configuration.
• When joined in series, the total voltage across the cells is equal to the sum of the individual voltages of the cells: V T = V 1 + V 2 + V 3.
• A superconductor is a material which, below a certain temperature, known as the critical temperature, has zero resistivity.
• There are two rules for adding the resistances of resistors in circuits, depending on whether the resistors are in series or in parallel: in series - (where is total resistance and is the resistance), and in parallel - (where is total resistance and is the resistance).
• With a resistivity of zero, resistance also drops to zero therefore applications of superconductors include: power cables, which would reduce energy loss through heating to zero during transmission, and strong magnetic fields, which would not require a constant power source.
• In DC circuits, charge and energy are always conserved, as described by Kirchoff’s two laws: Kirchoff’s first law - the total current flowing into a junction is equal to the current flowing out of that junction.
• The critical temperature of a superconductor depends on the material it is made out of, and most known superconductors have an extremely low critical temperature which lie close to 0 K (-273 °C).
• When identical cells are joined in parallel, the total voltage is equal to the voltage of one cell, because the current is split equally between branches, therefore the overall potential difference is the same as if the total current was flowing through a single cell: V T = V 1 = V 2 = V 3.
• In a series circuit, the current is the same everywhere in the circuit, and the battery p.d is shared across all elements in the circuit, therefore the total sum of the voltages across all elements is equal to the supply p.d.
• One application of a thermistor in circuits is a temperature sensor, which can trigger an event to occur once the temperature drops or reaches a certain value.
• If the resistance across R1 increases, the output p.d will decrease as circuit current has decreased and V=IR.
• The sum of the internal resistance (r), and load resistance (R) is the total resistance (R T ) in the circuit.
• A light dependent resistor’s resistance decreases as light intensity increases.
• A potential divider is a circuit with several resistors in series connected across a voltage source, used to produce a required fraction of the source potential difference, which remains constant.
• The emf of a battery can be measured by measuring the voltage across a cell using a voltmeter when there is no current running through the cell, which means it is in an open circuit.
• Electromotive force (emf / ) is the energy transferred by a cell per coulomb of charge that ε passes through it: ε = E/Q.
• The output potential difference of a potential divider can be varied by using a variable resistor as one of the resistors in series.
• Kirchoff’s first law states that the current entering any node in a circuit is equal to the current leaving that node.
• The p.d across the resistor R, is known as the terminal p.d (V), whereas the p.d across the resistor r, is known as lost volts (v) because this value is equal to the energy wasted by the cell per coulomb of charge.
• Kirchoff’s second law states that the sum of all the voltages in a series circuit is equal to the battery voltage.