4.2 Electricity:Energy, power and resistance Flashcards
Do you know the circuit symbols?
Probably not
Potential difference
The energy transferred from electrical energy to other forms of energy (heat, light, etc) per unit charge
can be defined by the equation V=W/Q
Electromotive force
the energy transferred from chemical, mechanical or other forms of energy into electrical energy per unit charge.
can be defined by the equation ε=W/Q
sources of e.m.f
power supplies and cells
Volt
One volt is the p.d. across a component when 1J of energy is transferred per unit charge passing through the component
1V = 1 JC-1
What’s a voltmeter?
A Voltmeter is used for measuring p.d. and e.m.f.
- Always connected in parallel
- They measure the amount of energy transferred in Joules per coulomb of charge across a component.
- An ideal voltmeter should have infinite resistance, so that no current passes through the voltmeter itself. High resistance means that negligible current flows through the voltmeter.
Derive eV=1/2mv^2
Since W = VQ
And for an electron W = Ve
And W = work done = kinetic energy = ½ mv2
Therefore, 𝒆𝑽 = ½ 𝒎𝒗^𝟐
When an electron and a proton are accelerated by the same p.d. what will the velocity of the electron
be compared to a proton?
the velocity of the electron will be greater than the proton.
The kinetic energy of each will be the same, but since the mass of the proton is greater, it travels more slowly at the same kinetic energy.
Resistance
resistance is the potential difference per unit current
R=V/I
Ohm (Ω)
A component has a resistance of 1 Ω if a potential difference of 1V makes a current of 1A flow through it.
Ohms Law
At a constant temperature, current through an Ohmic conductor is directly proportional to the potential difference across it
Why resistance is effected by temperature?
Increase in temperature causes an increase in internal kinetic energy, which means ions in the metallic lattice will vibrate more vigorously.
Therefore, delocalised electrons will be more likely to collide with the ions, losing their energy as they travel through.
This causes an increase in resistance.
I-V Characteristics of a Resistor at constant temperature.
The current flowing through the conductor is proportional to the potential difference across it, meaning that resistance is constant.
Resistance = 1/𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡 if ‘V’ is on x axis and ‘I’ is on the y axis.
Passes through the origin, proving that the constant of proportionality is resistance.
This graph is OHMIC.
I-V characteristics of a filament lamp
Initially, with positive potential differences, the current is directly proportional to the p.d.
However, as the current through the filament
increases, the temperature of the filament lamp also
increases.
This causes an increase in the resistance of the
filament.
As a result the rate of increase of the current decreases (decrease in gradient) and a greater change in the potential difference is required to cause a change in the current.
This same pattern is repeated when a negative potential difference is applied across the filament.
A filament lamp is non-Ohmic component because there is a change in temperature, so current is not proportional to p.d.
S shape
Whats a Thermistor?
A thermistor is a resistor with a resistance that depends on its temperature.
The resistance of a negative temperature coefficient (NTC) thermistor decreases as the temperature increases.
I-V characteristics of a Thermistor (NTC)
Increasing the current through an NTC thermistor increases its temperature.
Warming the thermistor releases more electrons. More charge carriers means a lower resistance.
A Thermistor does not obey Ohm’s law.
An increase in temperature causes a decrease in the resistance of the thermistor.
As a result the rate of increase of the current increases or in other words the gradient increases.
Diodes
Diodes (including LEDS) are designed to let current flow in one direction only.
Diodes have a forward and reverse bias.
Forward Bias
This is the direction in which the current is allowed to flow in a diode/LED
Reverse Bias
This is the direction in which the current isn’t allowed to flow in a diode/LED.
The resistance of a diode is very high and the current that flows is very tiny
Threshold Voltage
The minimum forward voltage value across the terminals of a diode at which the diode will start to conduct current. Most diodes require a threshold voltage of about 0.6V
LED
Components that are diodes but emit light when they conduct. They emit light of a single wavelength.
I-V characteristics of a Diode/LED
A diode/LED only allows current to flow in one direction.
When connected in the forward bias direction, they give a low resistance.
When connected in the reverse bias direction, they give a high resistance.
A diode is a non-Ohmic component.
How to Determine the electrical characteristics of a component?
Equipment: Power pack/cells, Wires, chosen component, variable resister, ammeter
and voltmeter.
- Produce the setup as shown.
- Take readings from voltmeter and ammeter.
- Use the variable resistor to alter the potential difference across the component.
- Take more readings and repeat results. Take averages to reduce the effect of random errors.
- Plot graph of current against p.d. Resistance is 1/gradient
Light Dependent Resistor: LDR and its variation with light intensity.
The greater the intensity of light shining on an LDR, the lower its resistance.
The greater the intensity of light shining on an LDR, the more the number of electrons released. More charge carriers means a lower resistance.
The resistivity of a material
the resistance of a piece of material having a length of one metre and a cross sectional area of one square metre
Resistivity of Metals
Increasing temperature increases the amount of energy that metals have.
Since its volume stays the same, the increase in energy comes in the form of kinetic energy caused by the vibration of atoms. Therefore, the delocalised electrons must progress through a more turbulent mass of atoms.
This increases the resistance and in turn increases the resistivity.
Resistivity of Semiconductors
The resistivity of a semiconductor decreases with temperature.
As the temperature increases, more electrons can break free of their atoms to become conduction electrons.
At the same time, there are more collision, but this number is small in comparison.
This increases the current as there are more charge carriers, which decreases the
resistance, and in turn decreases the resistivity.
How do you Determine Resistivity of a metal/wire
- Find the cross sectional area of the test material. If you are using a wire then use a micrometer to measure the diameter of the wire.
- Clamp the test material/wire to a ruler where the ruler reads zero.
- Attach a flying lead to the wire.
- Record the length of the test wire connected in the circuit, the voltmeter reading and the ammeter reading.
- Use your readings and the formula ‘V=IR’ to calculate the resistance of the length of wire.
- Repeat for several lengths.
- Plot a graph of resistance against length.
- The gradient of the line of best fit is equal to R/L. Multiply it by A to find the resistivity of the material.
Power
the rate at which energy is transferred (to a component) in Watts (W)
1 watt
1 Joule of work done per second
Kilowatt-hour
1 kW h is the energy transformed by a 1 kW device in a time of 1 hour. The kilowatt-hour is a unit of energy used for domestic energy consumption.
