4.2 Energy, Power And Resistance

Question 1

What is the name of the component whose circuit symbol is a rectangle with two arrows pointing at it?

Answer 1

LDR light dependent resistor

Question 2

Should voltmeters be connected in series or parallel?

Answer 2

Parallel

Question 3

What is potential difference? State the units

Answer 3

The work done per unit charge; the difference in potential between two points in a circuit
Measured in volts

Question 4

What is the difference between voltage and emf?

Answer 4

Voltage is the energy transferred per coulomb across a load resistance
Emf is the total amount of work done by the battery per coulomb

Question 5

Give the equation relating energy transferred to voltage

Answer 5

W=VQ
W=work done (energy transferred)
V= potential difference
Q= charge

Question 6

True or false: the kinetic energy of an electron is multiplied by its charge multiplied by the voltage

Answer 6

True
Energy transferred=eV=1/2mv^2

Question 7

What is resistance?

Answer 7

How difficult it is for current to flow through an appliance.
The potential difference required for each unit of current that flows through a resistor
R=V/I

Question 8

What is the definition of 1 Ohm?

Answer 8

When a resistor is subject to a voltage of 1V and allows a currents of 1A through, it’s resistance is 1 Ohm

Question 9

What is meant by an object conductor?

Answer 9

A conductor that obeys Ohm’s law

Question 10

What does Ohm’s law state in words?

Answer 10

The current through an ohmic conductors is directly proportional to the potential difference across it. (Ie. Resistance doesn’t vary with voltage or current)
(Only true if temperature is constant)

Question 11

What does the gradient of a current potential difference graph represent?

Answer 11

1/R
The gradient=rise in current/change in potential difference
The gradient= I/V
So resistance is 1/R

Question 12

Why does the current increasing on a filament lamp cause an increase in the resistance?

Answer 12

-the flow of current causes collisions between the electrons and the metal lattice
-these collisions increase the temperature (vibration) of the lattice
-as the temperature increases, more collisions occur, causing the resistance to increase

Question 13

What is a diode?

Answer 13

An appliance that only allows current to flow in one direction

Question 14

What is a Light dependent resistor (LDR)?

Answer 14

A semiconductor that is sensitive to light: as the light intensity increases, the resistance decreases

Question 15

How does a thermistor work?

Answer 15

Similar to an LDR, but as the temperature increases, the resistance decreases (NTC thermistors)

Question 16

What is resistivity?

Answer 16

A property of a metal which shows how easy or difficult it is for current to flow in the material (at a specific temperature)
p=RA/L

Question 17

Describe an experiment to determine resistivity of a metal

Answer 17

-measure diameter of wire using micrometer
-halve and calculate area
-description of circuit set up
- ray length record voltage and current for each length
-R=V/I to work out resistance
-gradient= resistivity/area
Rearrange for resistivity

Question 18

True or false: the resistivity of both metals and semiconductors increases with temperature

Answer 18

False
The resistivity of a metal increases with temperature because as the metal ions heat up, they vibrate more and the electrons bang into the, and slow them down
Some semiconductors get less resistive as temperature increases, because supplying energy causes more charge carriers to be released, so current can flow more easily

Question 19

What is power?

Answer 19

The rate of energy transfer
J/s or W

Question 20

Give an equation for power in terms of current and voltage

Answer 20

P=IV

Question 21

What is a kilowatt-hour(kWh) a measure for?

Answer 21

Energy

Question 22

Why do electricity companies use units (kWh) rather than joules or watts?

Answer 22

Joules and Watts are both so small that everyone would use tens or hundreds of millions of them monthly- it’s impractical to have such large numbers used