Energy, power and resistance

Question 1

What is the volt?

Answer 1

1V=1JC^-1. One volt is the p.d. across a component when one joule of energy is transferred per unit charge across the component.

Question 2

What is potential difference?

Answer 2

The transfer of energy done by charge carriers from electrical energy to other forms like heat or light per unit charge.

Question 3

What is the equation for p.d.?

Answer 3

V=W/Q

Question 4

What is e.m.f.?

Answer 4

Electromotive force is when work is done on the charge carriers. This is the transfer of energy from chemical energy from the cell or battery to electrical energy.

Question 5

What is the equation for emf?

Answer 5

Epsilon=W/Q

Question 6

What is resistance?

Answer 6

Defined as the ratio between V and I of the component.

Question 7

What is the ohm?

Answer 7

The ohm is defined as the resistance of a component when a pd of 1V is produced per ampere of current.

Question 8

What is Ohm’s law?

Answer 8

For a metallic conductor kept at constant temperature, the current in the wire is directly proportional to the pd across its ends.

Question 9

What happens to the resistance when temperature increases?

Answer 9

The resistance increases because the positive ions have more energy to vibrate at greater amplitudes about their mean positions. The frequency of the collisions between the free electrons and the positive ions increases, the charge carriers do more work and transfer more energy. The energy transferred from the electrons causes the positive ions to vibrate more.

Question 10

What is the IV characteristic for a resistor?

Answer 10

Directly proportional and linear
Obeys Ohms law, so ohmic conductor
Resistance is constant
Behaves the same way regardless of the polarity

Question 11

What is the IV characteristic for a filament lamp?

Answer 11

Not directly proportional
Non ohmic as it doesnt obey ohms law
Resistance is not constant
Behaves in the same way regardless of the polarity

Question 12

What is a diode?

Answer 12

A component that only allows current to flow in one direction.

Question 13

What is the IV characteristic for a diode?

Answer 13

Not directly proportional
Resistance is not constant
Non ohmic
Behaviour depends on polarity

Question 14

What is the threshold pd?

Answer 14

The pd of a diode where the resistance gradually starts to drop. After this value, the resistance drops rapidly.

Question 15

What is resistance affected by?

Answer 15

The material of the wire
The length of the wire
The cross sectional area

Question 16

What is resistivity?

Answer 16

The electrical property of a material.

Question 17

Is R directly proportional to L?

Answer 17

Yes

Question 18

Is R inversely propertional to A?

Answer 18

Yes

Question 19

What is the equation for resisitivity?

Answer 19

rho=RA/L

Question 20

What happens to the resistivity as the material gets hotter?

Answer 20

Increases

Question 21

How is resistivity determined from a graph of R against L?

Answer 21

gradient*A

Question 22

What is a negative temperature coefficient?

Answer 22

When the resistance drops as the temperature increases.

Question 23

Do thermistors have a negative temperature coefficient?

Answer 23

Yes

Question 24

What is the IV characteristic for a thermistor?

Answer 24

Opposite to the filament lamp

Question 25

What is an LDR?

Answer 25

Made from a semiconductor in which the number density of charge carriers changes depending on the intensity of the incident light. In the dark, R is very high and vice versa.

Question 26

Power equations?

Answer 26

P=VI
P=I^2R
P=V^2/R
P=W/t
P=VIt

Question 27

What does the energy transferred to an electrical device and the running cost depend on?

Answer 27

The power of the device
How long the device is used for

Question 28

Energy transferred equation?

Answer 28

E=Wt