Circuits Flashcards
Describe why there is potential energy. What’s the formula? How does this relate to the voltage formula?
Work is done on the charges, causing them to gain electrical potential energy, so work is done.
W(J) = ~E (~ = change in). Potential difference is the potential difference in energy between two points. V=~E/Q=W/Q. This can be positive, such as a positive charge moving uphill from 0 to 1.5V, or negative, such as a negative charge moving uphill.
What is joule heating and how does this work? What formula connects the charge formula to the work formula?
Joule heating is where power is dissipated in a wire as charges have collisions with positive ion cores of metal atoms. This obstructs movement so charges don’t gain kinetic energy but they do work on the wire through losing potential energy. This wasted energy is dissipation by heat - joule heating.
V = W/Q W= VQ Q=It W= VIt
What is power, what are it’s units, and what’s the formula?
Power (Js ^-1), in watts, is the rate of energy transfer. P=VI, but can also be written as P=1^2R
How is electrical potential energy created and how does it work?
In a cell, chemical reactions produce an electrical potential energy difference, resulting in a positive and negative terminal. Charges move as a result of attraction and repulsion. Negative charges move to the positive terminal and positive charges to the negative terminal, and this gives metals their properties.
What is current? Why is current the same in the circuit, and what are the formulae for total charge regarding the number of particles and current?
Current is the rate if flow of charged particles. They each have the same charge and as they travel at the same speed in the same time, current is therefore constant in the circuit. If the number of particles is N and the charge on each Q, the total charge = NQ. Current = charge / current.
What is Kirchoff’s first law?
When there is a junction, charge has to split as charge must be conserved.
How does resistance build up in non-ohmic conductors? What metal makes a good sensor as a result?
As the voltage is increased, this causes the resistance to increase. As there is a greater voltage, there are more collisions with ions in the wire and work is done on these through electrical potential energy which causes power dissipation. As the heat builds up, this causes resistance to rise. In platinum, knowledge of the effect of temperature on resistance makes this a good temperature sensor. In semi-conductors when temperature rises, resistance falls.
What is emf? How do electrons move around in the circuit, and what is emf equal to? How is internal resistance created in the cell?
This is the electromotive force. This is energy transferred to each unit charge and this is measured in volts. Electrons are pushed from the negative to positive terminal, (this is the anode), and the electrons are taken in. EMF gives energy to the charges and resistance in the circuit dissipates this energy. P=I^2R. The EMF is equal to the sum of all PDs across the circuit. Chemicals provide resistance in the cell and this is the internal resistance.
What are lost volts? What is the formula for electromotive force?
Lost volts are created when the load resistance in the circuit falls, causing current to increase and the voltage to fall. This is referred to as Vr. E= V+Vr (V = terminal voltage). Vr can equal IR.
How can inconvenience with joule heating be resolved in computers?
Fans are used in computers to prevent them from overheating due to the wires heating.
What is power?
The rate at which energy is dissipated into the resistance.
What are IV characteristic graphs? What is expected for ohmic conductors?
This is a graph which describes how current changes when the PD is increased. For ohmic conductors, the resistance is constant for constant temperature, so as voltage is increased, current is proportional to this.
What is the formula for conductance and what are its units?
G (siemens) Ω^-1 = I / V
What is resistance?
How difficult it is for current to flow. Conductance is how EASY it is.
How do lamps work, and what is the graph for these non-ohmic conductors?
For a filament lamp, as current rises this causes the coil to heat up, creating increasing resistance, and so an S shaped graph for V (x), I, (y), makes an ‘s’ shape.
