Energy, Power And Resistance

Question 1

What is a transformer?

Answer 1

Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC
voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection
between the coils.

Question 2

What is an oscilloscope?

Answer 2

An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.

Question 3

What is an LDR?

Answer 3

A transducer whose resistance decreases as light intensity increases
LDR = Light Dependent Resistor

Question 4

What is a thermistor?

Answer 4

A transducer whose resistance decreases as temperature increases

Question 5

What is an inductor (coil,solenoid)?

Answer 5

A coil of wire which creates a magnetic field when current passes through it. It may have an iron core inside the coil. It can be used as a transducer converting electrical energy to mechanical energy by pulling on something

Question 6

What is a diode?

Answer 6

A device which only allows current to flow in one direction

Question 7

Define potential difference

Answer 7

The potential difference between two points in a circuit is equal to the amount of electrical energy converted to other forms per coulomb of charge flowing between them

Question 8

Define the volt

Answer 8

The volt is equal to the potential difference between two points in a circuit, when 1 joule of electrical energy is converted to other forms when 1 coulomb of charge passes between them

1 volt = 1 joule per coulomb

Question 9

Define electromotive force

Answer 9

The e.m.f of a cell or other device is the energy converted to electrical form from other forms for each unit of charge passed through the device

Question 10

What is the difference between potential difference and electromotive force?

Answer 10

P.d. is energy changed from electrical to other forms per unit charge, whereas e.m.f. is energy changed from other forms into electrical per unit charge

Question 11

Derive an equation for energy transferred (W)

Answer 11

V = W/Q or W = VQ (1)

And Q = It (2)

Substitute (2) into (1) to give

W = VIt

Question 12

What is the equation for energy transfer for electrons and other charge particles?

Answer 12

eV = 1/2mv^2

Question 13

Define resistance

Answer 13

The resistance of the component is defined as the ratio of the potential difference V, across it to the current I, flowing through it

Question 14

Define the ohm

Answer 14

The ohm is the resistance in which a current of 1A produces a p.d. of 1V

Question 15

State Ohm’s law

Answer 15

The current through a metallic conductor is directly proportional to the p.d. across its ends, providing that its temperature and other physical conditions remain constant

Question 16

How does resistance change with temperature of metals, insulators and semiconductors?

Answer 16

The resistance of metals increases with temperature
The resistance of insulators does not change as the temperature increases
The resistance of semiconductors decreases exponentially with temperature

Question 17

Define Resistivity

Answer 17

Resistivity can be defined as numerically equal to the resistance of a sample of the material of unit length and unit cross-sectional area at a particular temperature

Question 18

State an equation for resistance containing Resistivity (ρ)

Answer 18

R = ρl/A, where l is the length of wire, and A is the cross sectional area of the wire

Question 19

What does a resistance/temperature graph look like for metals?

Answer 19

A straight line through origin

Question 20

What does a resistance/temperature graph look like for insulators?

Answer 20

A straight line parallel to the x-axis (temperature)

Question 21

What does a resistance/temperature graph look like for semiconductors?

Answer 21

A decreasing curve of decreasing gradient

Question 22

Define power

Answer 22

Power is the rate at which energy is given out or converted in a component

Question 23

What are the different ways power (P) could be calculated?

Answer 23

P = W/t
P = IV
P = I^2R
P = V^2/R

Question 24

State an equation for energy transferred (W)

Answer 24

W = Pt

or since P = IV

W = VIt

Question 25

How many joules in a kilowatt hour (kWh)

Answer 25

Energy = power x time
1 kWh = 1000 x 60 x 60
= 3600000J
= 3.6MJ

Question 26

How would you calculate the cost of running an appliance?

Answer 26

Cost = no. of units x cost/unit
(p) (kWh) (p)