Sensing Flashcards
Current
Rate of flow of charged particles. Conserved across the circuit –> current flowing into a junction = current flowing out of a junction: I(total)=I1+I2+I3
Current=
Q/t (Q is the charge in coulombs)
1 coulomb
the amount of charge that passes in 1 second when the current is 1 ampere
Potential difference
Energy per unit charge. To make a charge flow, work must be done. Potential difference is the energy converted per unit charge moved
Potential difference, V=
E/Q
1 volt
Potential difference is 1 volt when 1 joule of energy is used to move 1 coulomb of charge: 1V = 1JC^-1
Resistance
How much current flows across a component when a potential difference is applied to it. Comes from electrons colliding with atoms and losing energy
1 ohm
A component has resistance 1 ohm when a potential difference of 1 volt results in a current of 1 amp
Resistance, R=
V/I
I/V graphs
The gradient of current/p.d. graphs shows the resistance - be careful!!! if current is on the y axis, and voltage on the x, the gradient is I/V which is the conductance –> shallow gradient is a high resistance
Ohmic conductors
Resistance is constant for all currents and voltages - the conductors obey Ohm’s law
Ohm’s law
Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it.
I/V graph of a filament lamp
Has an s-shaped curve - starts steep, then gets shallower. This is because as the filament gets hotter, the resistance increases.
Resistance of a thermistor
Depends on temperature. For most thermistors, the resistance decreases as the temperature increases - I/V graph starts shallow and gets steeper. As thermistor gets warmer, more electrons are freed to become charge carriers.
Sensitivity
Change in dependent variable/Change in independent variable –> change in voltage/change in environment
Series circuits
- current is constant across all points (no junctions)
- e.m.f. split between components: E=V1+V2+V3+etc => IR(total)=IR1+IR2+IR3+etc (as V=IR) => R(total)=R1+R2+R3+etc
Parallel circuits
- current is split at each juction: I(total)=I1+I2+I3+etc
- same p.d. across each loop => V/R(total)=V/R1+V/R2+V/R3+etc => 1/R(total)-1/R1+1/R2+1/R3+etc => G(total)=G1+G2+G3+etc
Internal resistance
Chemical energy is used to make electrons move in batteries. When they move, they collide with atoms, so the battery has resistance.
E.M.F
Electromotive force - NOT A FORCE!!! The amount of electrical energy produced for each coulomb of charge, measured in volts
Terminal potential difference
The p.d. across the load resistance (R). The energy transferred when one coulomb of charge flows through the load resistance.
E.M.F and internal resistance equations
e=V+v ; e=I(R+r) ; V=e-v ; V=e-Ir (where e is e.m.f, V is terminal p.d, v is lost volts across battery, I is current, R is load resistance and r is internal resistance)
Why low internal resistances are important (and exception)
In a car battery, a high current is delivered, so a low internal resistance is required.
High voltage power supplies are the exception. HT and EH (high tension and extremely HT) have very high internal resistances, so if they’re short-circuited, only a small current can flow - much safer.
Power and power dissipation
Rate that a component converts electrical energy into other types of energy, e.g. heat - power dissipation. Useful in light bulbs - makes them shine. Unhelpful in computers - have to keep them cold
Power=
IV=I^2R=V^2/R
Potential divider
A circuit with a voltage source and some resistors in series. The potential of the voltage source (e.g. power supply) is divided between the resistors
Potential divider equations
V(out)=(R1/(R1+R2)).V(total) ; V1/V2 = R1/R2
Transistor
A kind of switch. Off when voltage across is low, on when voltage across is high
LDR
High resistance in dark, low resistance in light
NTC thermistor
High resistance at low temperatures, low resistance at high temperatures - OPPOSITE to a normal resistor
Conductance, G=
1/R=I/V
Energy, E=
Pt=VIt
