Module 4 - electricity

Question 1

What is mean drift velocity?

Answer 1

The free electrons make collisions with the metal ions and this gives them a random velocity.
The free electrons drift towards the positive end of the supply along the length of the wire.
The distance travelled per unit time along the length of the wire is called the mean drift velocity of the electrons

Question 2

What is Ohm’s law?

Answer 2

For ohmic conductor at a constant temperature, the current is directly proportional to the p.d across it.

Question 3

Explain the I-V characteristics for a filament lamp?

Answer 3

As the current increase the resistance increases.
Resistivity increases with temperature: heating metal causes ions to vibrate move, electrons collide with them more losing energy.

Question 4

What is the conservation of charge?

Answer 4

An electrical charge cannot be created or destroyed - total amount of charge is constant.

Question 5

What is Kirchhoff 1st law?

Answer 5

For any point in a circuit, sum of the current entering the point is equal to the sum of current leaving the point.

Question 6

What does number density mean?

Answer 6

Number of electrons per m^3.

Question 7

What affects the mean drift velocity?

Answer 7

The narrower the wire —> HIGHER mean drift velocity .

Question 8

What is potential difference?

Answer 8

Work done by charge carriers

Question 9

What is e.m.f?

Answer 9

Work done on charge carriers per unit charge.

Question 10

What are 3 things that determine the resistance of a component?

Answer 10

1. The length - the longer the wire the more difficult it is to make current flow.
2. Area - the wider the wire the easier it will be for the electrons to pass along it
3. Resistivity - this depends on the material the wires made from, as the structure of the material may make it easy or difficult for charge to flow.

Question 11

What are the I-V characteristics for a metallic conductor?

Answer 11

1. At constant temperature, the current through a metallic conductor is directly proportional to the potential difference.
2. They are ohmic - they have constant resistance provided the temperature doesn’t change.

Question 12

What is internal resistance in a battery?

Answer 12

Resistance comes from electrons colliding with atoms and losing energy. In a battery, chemical energy is used to make electrons move. As they move, they collide with atoms inside the battery - this is internal resistance.

Question 13

What is Kirchhoff second law?

Answer 13

The total e.m.f = the sum of the p.d around a closed loop

Question 14

Explain the properties of an LDR?

Answer 14

It has a very high resistance in the dark, but a lower resistance in the light.

Question 15

Explain the properties of a NTC thermistor?

Answer 15

It has a high resistance at low temperatures, but a much lower resistance at high temperatures

Question 16

What is thermionic emission

Answer 16

The emission of electrons from the surface of a hot metal wire.

Question 17

Describe an experiment to obtain IV characteristics?

Answer 17

• measure current through and voltage across
• adjust the potential divider (rheostat) to give a range from 0-6V.
• plot graphs I against V - gradient =R

Question 18

Explain how you would carry out a resistivity experiment?

Answer 18

• measure the diameter using a micrometer.
• clamp the wire into circuit
• obtain values for p.d. across different lengths of wire.
• if the current in each wire is measured too, we can use R=V/I to calculate the resistance for each length.
• as resistance ∝ length. plot a graph for resistance and length and the gradient = ρ/A.

Question 19

What three factors affect the resistance of a wire?

Answer 19

• the material of the wire
• the length of the wire
• the cross sectional area of the wire.

Question 20

Examples of where thermistors are used?

Answer 20

• in simple thermometers
• in thermostats to control A/C and heating
• to monitor the temperarure of components inside electrical devices.
• to monitor engine temperarures to ensure engines don’t overheat

Question 21

Explain how you could carry out a simple investigation inro how the resistance of a thermistor changes with temperature?

Answer 21

• use a water bath to control the temperature of a thermistor and an ohmmeter for a quick and simple recording of the resistance .

Question 22

Explain the I-V characteristics of Thermistors?

Answer 22

• with a thermistor as the current increases the temperature increase. however unlike a lamp this temperature increase leads to a drop in resistance because the number density of charge carriers increases.

Question 23

what is electrical power?

Answer 23

the rate of energy transfer by each electrical component is called the electrical power

Question 24

What are the 4 equations for power in electricity?

Answer 24

P=VI
P=I²R
P = VQ/t

Question 25

What two factors does the energy transferred to each individual electrical device depend on?

Answer 25

• the power of the device

- how long the device is used for

Question 26

Definition of kilowatt-hour?

Answer 26

the energy transferred by a device with a power of 1kW operating for a time of 1 hour.

Question 27

What is conventional current?

Answer 27

It’s a model used to describe electric currents.in a circuit.
It travels from the positive to negatives it is the direction in which positive charges would travel.