4.2 Energy, Power and Resitance Flashcards
What is potential difference (V) and state its unit
Energy transferred from electrical energy (in the circuit) to other forms (in the components) per unit charge
Volts (V)
What does p.d. measure
Work done by charge carriers
They lose energy as they pass through components
What are the equations for p.d. / emf
Potential difference (V) = work done (J) / charge (C)
V=W/Q
Potential difference (V) = current (A) x resistance (Ω)
What is electromotive force (emf) and state it units
Energy transferred form chemical energy (in the cell) to electrical energy (in the circuit)
Volts (V)
What does emf measure
The work done to charge carries when they gain energy as they pass through a cell
What is resistance (R) and state its unit
Resistance is the opposition to current in a circuit
Ohms (Ω)
How to find resistance from IV graph
Resistance = 1 / gradient
Ohms law
For a metallic conductor at a constant temperature, the current in the wire is directly proportional to the p.d. across it
This is true for ohmic components
How does an ohmic component present on an IV graph
Linear relationship through the origin
Why is ohms law not always true
- In non-ohmic components
- ions are heated and gain kinetic energy when current increases
- they vibrate more which increases frequency of collisions with electrons
- more work is done on the charge carriers
- resistance increases
IV characteristics of a fixed resistor
Linear relationship - ohmic component
Resistance is constant
IV characteristics of a filament lamp
Shape: curve/straightens out/curve
- non-ohmic component
- at the y axis (small currents) it acts like an ohmic component
- as current increases, temperature increases so resistance is higher and current increases at a slower rate
IV characteristics of a diode
Shape: positive axis, exponential
- made from semiconductors so current flows in only one direction
- non ohmic component
- when in reverse bias (arrow is against direction of current) current is zero
- when in forward bias the diode does conduct
- after threshold value for p.d. is reached current increases rapidly, but resistance decreases
IV characteristics of a thermistor
Shape: cubic, repeated root at zero
- non-ohmic component used to measure temperature
- semi conductor material
- resistance decreases as current increases
General characteristics of IV graphs
Current on y
Pd on x
Resistance is V/I and highest at the x axis
Resistance / temperature graph for NCT (negative temperature coefficient) thermistors
Resistance on y
Temperature on x
Downward curve
As temp increases, resistance decreases
Made of semi conductor materials
Resistance / Light intensity graph for light dependent resistor (LDR)
Resistance on y / light intensity on x
Downward curve
- As light intensity increases, resistance decreases
What is resistivity (ρ) and state it’s unit
A physical property of the material that acts as a constant to link resistance with length and area. It varies with temperature
Ohm metre (Ωm)
Equation for resistance, considering resistivity
Resistance (Ω) = (resistivity (Ωm) x length (m) ) / cross sectional area (m2)
R = (ρL) / A
How does temperature affect resistivity in conductors
- When temperature increases ions vibrate more
- more collisions occur between electrons and ions
- resistance increases
- resistivity increases
How does temperature affect resistivity in semiconductors
- when temperature increases number density of charge carriers (n) increases
- current increases
- resistance decreases
- resistivity decreases
What is electrical power (P) and state its unit
Rate of electrical energy transferred to other forms in the circuit per unit time
Watts (W)
What are the equations for power
Power (W) = Work done (J) / time (s)
P = W/t
Power (W) = (potential difference (V) x charge (C) ) / time (s)
P = VQ/t
Power (W) = potential difference (V) x current (A)
P = VI
Power (W) = current^2 (A) x resistance (Ω)
P = I2R
Power (W) = potential difference^2 / resistance (Ω)
P = V2/R
How to measure energy transfer for domestic or industrial use
Joules are a very small measure of energy
Use kWh instead
Cost per kWh can be multiplied by the number of kWh to calculate the total running cost of a device
