Chapter 9 - Energy, Power and Resistance Flashcards
Volt
The potential difference across a component when 1J of energy is transferred per Coulomb of charge passing through the component
Variable resistor vs thermistor symbols
Variable resistor is a resistor with a north-east facing arrow
Thermistor is a resistor with a flat line below and then a line off the right side of that in the same direction
Capacitor symbol
Two parallel lines of the same length going up and down with horizontal lines coming in
Potential Difference
The energy transferred from electrical energy to other forms per unit charge
Throughout the circuit
New p.d. equation
V = W/Q
Energy Transferred/Charge
Voltmeter vs ammeter placement
An ammeter involves replacing a component with it
Place a voltmeter around an existing component
Ideal voltmeter
Has infinite resistance so no current passes through the voltmeter itself
Electromotive Force (emf)
ε. The energy transferred from chemical energy to electrical energy per unit charge
Happens in the cell/battery
Thermionic emission
Where electrons gain kinetic energy with some gaining enough to ‘escape’ from the surface of the metal
How does an electron gun work
Place a heated filament in a vacuum with an anode in front
Apply a huge p.d. to accelerate the escaping electrons to the anode
A small hole in the anode creates a beam of electrons with a specific kinetic energy
Electron gun equation
eV = 1/2mv^2
where e is the elementary charge
1/2 mv^2 is the kinetic energy transferred to the electrons
Resistance
A measure of a component’s opposition to the flow of charge
Ohm
The resistance of a component when a potential difference of 1 volt is produced per ampere of current
Ohm’s Law
The ratio of the potential difference between the ends of a conductor to the current flowing through it is constant at a constant temperature
Ohm’s Law in a metallic conductor
In a metallic conductor at a constant temperature, the current is directly proportional to the potential difference across its ends
Effects of an increase in temperature on the resistance
In a metallic conductor it increases the resistance
In a semi-conductor it decreases the resistance
Diode
Only allows current to flow in one direction
Has a forward bias and reverse bias
The anode is on the side with the straight edge of the triangle
Forward bias
A diode has a threshold potential difference at which stage the number density increases so the resistance decreases and current increases
Reverse bias
The diode has infinite resistance, the potential difference is equal to that of the power supply
LEDs
Made of a material that emits light when they conduct, emits one specific wavelength
Electrical energy is transferred directly to light so they don’t heat up, more efficient
Axes for IV characteristics
V on the x-axis
I on the y-axis
IV characteristics practical
Change p.d. and current with a variable resistor or move a pentiometer along a resister
Take readings of I and V including negatives
IV characteristics resistor
Straight line through the origin
- Constant resistance
- Polarity doesn’t matter
- Ohmic component
IV characteristics filament lamp
Straight line through the origin, curves horizontally at high and low current
- Varying resistance
- Polarity doesn’t matter
- Non-ohmic component
