4.2 Electricity:Energy, power and resistance

Question 1

Do you know the circuit symbols?

Answer 1

Probably not

Question 2

Potential difference

Answer 2

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

Question 3

Electromotive force

Answer 3

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

Question 4

sources of e.m.f

Answer 4

power supplies and cells

Question 5

Volt

Answer 5

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

Question 6

What’s a voltmeter?

Answer 6

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.

Question 7

Derive eV=1/2mv^2

Answer 7

Since W = VQ
And for an electron W = Ve
And W = work done = kinetic energy = ½ mv2
Therefore, 𝒆𝑽 = ½ 𝒎𝒗^𝟐

Question 8

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?

Answer 8

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.

Question 9

Resistance

Answer 9

resistance is the potential difference per unit current

R=V/I

Question 10

Ohm (Ω)

Answer 10

A component has a resistance of 1 Ω if a potential difference of 1V makes a current of 1A flow through it.

Question 11

Ohms Law

Answer 11

At a constant temperature, current through an Ohmic conductor is directly proportional to the potential difference across it

Question 12

Why resistance is effected by temperature?

Answer 12

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.

Question 13

I-V Characteristics of a Resistor at constant temperature.

Answer 13

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.

Question 14

I-V characteristics of a filament lamp

Answer 14

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

Question 15

Whats a Thermistor?

Answer 15

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.

Question 16

I-V characteristics of a Thermistor (NTC)

Answer 16

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.

Question 17

Diodes

Answer 17

Diodes (including LEDS) are designed to let current flow in one direction only.
Diodes have a forward and reverse bias.

Question 18

Forward Bias

Answer 18

This is the direction in which the current is allowed to flow in a diode/LED

Question 19

Reverse Bias

Answer 19

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

Question 20

Threshold Voltage

Answer 20

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

Question 21

LED

Answer 21

Components that are diodes but emit light when they conduct. They emit light of a single wavelength.

Question 22

I-V characteristics of a Diode/LED

Answer 22

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.

Question 23

How to Determine the electrical characteristics of a component?

Answer 23

Equipment: Power pack/cells, Wires, chosen component, variable resister, ammeter
and voltmeter.

1. Produce the setup as shown.
2. Take readings from voltmeter and ammeter.
3. Use the variable resistor to alter the potential difference across the component.
4. Take more readings and repeat results. Take averages to reduce the effect of random errors.
5. Plot graph of current against p.d. Resistance is 1/gradient

Question 24

Light Dependent Resistor: LDR and its variation with light intensity.

Answer 24

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.

Question 25

The resistivity of a material

Answer 25

the resistance of a piece of material having a length of one metre and a cross sectional area of one square metre

Question 26

Resistivity of Metals

Answer 26

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.

Question 27

Resistivity of Semiconductors

Answer 27

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.

Question 28

How do you Determine Resistivity of a metal/wire

Answer 28

1. 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.
2. Clamp the test material/wire to a ruler where the ruler reads zero.
3. Attach a flying lead to the wire.
4. Record the length of the test wire connected in the circuit, the voltmeter reading and the ammeter reading.
5. Use your readings and the formula ‘V=IR’ to calculate the resistance of the length of wire.
6. Repeat for several lengths.
7. Plot a graph of resistance against length.
8. The gradient of the line of best fit is equal to R/L. Multiply it by A to find the resistivity of the material.

Question 29

Power

Answer 29

the rate at which energy is transferred (to a component) in Watts (W)

Question 30

1 watt

Answer 30

1 Joule of work done per second

Question 31

Kilowatt-hour

Answer 31

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.