Sensing Flashcards
Define Current, and give its equation
The rate of flow of charged particles.
Current = change in charge / change in time.
What is potential difference and give its equation from the definition.
The energy converted per unit charge.
p.d. = W (Energy) / (Q) Charge
Define power and give its equation.
The rate of energy transfer.
Power = energy / time
What is the simple equation for power?
Power = current * p.d.
How do you calculate energy if you know power, then give the equation when you know p.d. and current
Energy = power * time
Energy = p.d. * current * time
Define resistance and give its equation
A measure of how difficult it is for current to flow across a component.
R = V / I
Define conductance and give its equation, complete with units
A measure of how good of an electrical conductor a component is.
G ( Conductance / Siemens ) = I / V
What is power within the context of electrical circuits and what is it also known as?
The rate at which a component converts electrical energy into other types of energy.
Power dissipation.
Describe how to gather the I-V characteristics of a component
1.) Use a variable resistor to change the p.d. across a component, and measure the p.d. and current across said component
2.) Repeat the measurements and take an average
3.) Plot a graph of current against p.d. The resistance is the gradient.
State Ohm’s law.
If external factors are constant, the current through an ohmic conductor is directly proportional to the p.d.
Describe the 4 types of graphs for conductors and the components they correspond to.
1.) Directly proportional, straight line graph. When any conductor obeys Ohm’s law.
2.) filament lamp. Starts steep but gets shallower.
3.) (Negative Temperature Coefficient)* Thermistor. Starts shallower, gets steeper.
4.) Diode. Incredibly flat, then very steep.
Why does the resistance of a bulb increase with time?
As time passes, the filament in the bulb heats up, which causes the electrons to have a harder time passing through, therefore increasing resistance
What 3 things does resistance depend on?
1.) Length. The longer the wire, the more particles the electrons can collide with.
2.) Area. The wider the wire, the more space the electrons have to pass through the wire
3.) Resistivity. This depends on the material of the wire as well as external factors.
What is the equation for resistivity and what are its units?
Resistance = (resistivity * Length of wire) / Cross-sectional area.
Resistance - Ohms
Resistivity - Ohm metres
Length - metres
Cross-sectional area - metres squared
What is conductivity, and give its equation and units.
The conductance of a 1m length wire with a cross-sectional area of 1m^2.
Conductivity = (Conductance * Length) / Cross-sectional area
Measured in Siemens per metre (S m^-1)
Describe the process to find the Resistivity and conductivity of a wire
1.) Use a micrometer to find the diameter of the wire, then calculate the CSA of the wire
2.) Fix the wire to a ruler, with the ruler attached to the rest of the circuit at 0
3.) Create a circuit with a voltmeter around the test wire and an ammeter at the end of the circuit
4.) attach a flying lead to the wire
5.) Record the the length of the test wire when connected, the voltmeter reading and the ammeter reading.
6.) Calculate the resistance of the wire
7.) Repeat for different wire lengths and make sure to take an average for each length
8.) plot a graph of Resistance against Length
9.) the gradient of the line of best fit is equal to the resistivity over the CSA, so multiply the gradient by the CSA to get resistivity.
10.) to find conductivity, take the inverse of resistivity
The resistivity of the wire depends on its temperature, so you need to keep the temperature constant
What determines how conductive a material is?
The number density of mobile charge carriers. I.e. the number of free electrons per cubic metre
Describe the electrical properties of metals
The charge carriers are free electrons, and the number density of mobile charge carriers is high. If you increase the temperature of a metal, the lattice vibrates more, so the electrons are less free to move and therefore the conductivity will slightly decrease
Describe the electrical properties of a semiconductor
The charge carriers are free electrons, but they have a lower number of mobile charge carriers. As you increase the temperature of the semiconductor, the number of mobile charge carriers increases, so the conductivity also increases rapidly. The temperature increase affects the semiconductor’s conductivity less. Most resistors are made of semiconductors.
Describe the electrical properties of an insulator
A perfect insulator has no mobile charge carriers, so no current can flow through it
Explain what internal resistance is and how it occurs
The resistance within a battery
Batteries use chemical energy to move electrons. As the electrons move, they collide with the atoms in the battery, so the battery must have a resistance.
Explain what e.m.f. is and give its units
The amount of electrical energy per coloumb. It’s measured in volts.
What is the terminal p.d.?
The p.d. across the total resistance of all components in a circuit per coloumb.
How do you calculate e.m.f?
Terminal p.d. (V) = e.m.f. (ε) - Current (I) * Internal resistance (r)
