CGP AS Section 6 - Electricity

Question 1

What is current?

Answer 1

Current is the rate of flow of charge

Question 2

Define a coulomb:

Answer 2

One coulomb (C) is defined as the amount of charge that passes in 1 second if the current is 1 ampere (A)

Question 3

How can you measure current?

Answer 3

Using an ammeter

-needs to be connected in series

Question 4

What is voltage?

Answer 4

Potential difference or voltage, is defined as the work done (energy converted) per unit charge moved.

Question 5

How do you measure voltage?

Answer 5

Using a voltmeter

-needs to be connected in parallel to the component

Question 6

Define a volt:

Answer 6

The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component

Question 7

What is resistance?

Answer 7

A components resistance is a measure of how difficult it is o get current to flow through it (measured in ohms)

Question 8

What does resistance in a circuit mean to the energy transfer going on?

Answer 8

-when electricity flows through them, they heat up and some of the electrical energy is wasted as thermal energy

Question 9

How can you lower the resistivity of a material?

Answer 9

by cooling a material down

Question 10

What if you cool some materials down below a critical transition temperature?

Answer 10

their resistivity disappears entirely and they become a superconductor

Question 11

What can a material being a superconductor mean to the energy transfers that occur?

Answer 11

-without any resistance, none of the electrical energy is turned into heat, so none wasted

Question 12

What would happen if you started a current flowing in a superconductor circuit?

Answer 12

if you started a current flowing with a magnetic field and then take away the magnet the current would carry on flowing forever

Question 13

What are the challenges of using superconductors?

Answer 13

-the critical temperatures are really low (below 10 kelvins) and getting things that cold is hard and really expensive

Question 14

What do solid state physicists do to do with superconductors?

Answer 14

they are trying to develop room-temperature superconductors (managed to get a metal oxide things to superconduct at about 140K)

Question 15

What are the uses of superconductor wires?

Answer 15

• power cables that transmit energy without loss of power
• strong electromagnets that don’t need a constant power source (for medical use and for Maglev trains)
• electronic circuits that work really fast, as there’s no resistance to slow them down

Question 16

Define power:

Answer 16

is the rate of transfer of energy (measured in watts (W) where 1 watt = 1 joule per second)

Question 17

Where does resistance in a circuit come from?

Answer 17

from electrons colliding with atoms and losing energy to other forms

Question 18

How does a battery work in a circuit?

Answer 18

chemical energy is used to make electrons move
-as the electrons move they collide with atoms inside of the battery so batteries must have resistance (this is called internal resistance)

Question 19

What makes batteries and cells warm up in a circuit?

Answer 19

internal resistance

Question 20

What is e.m.f.?

Answer 20

Electromotive force (not an actual force,measured in volts)
-it's the amount of electrical energy the battery produces for each coulomb of charge

Question 21

What is the terminal p.d.?

Answer 21

-the potential difference across the load resistance (R) is the energy transferred when one coulomb of charge flows through the load resistance

Question 22

What would be true in a circuit if there was no internal resistance?

Answer 22

-the terminal p.d. would be the same as the e.m.f.

(in real power supplies there is always some energy lost overcoming internal resistance

Question 23

What are lost volts?

Answer 23

the energy wasted per coulomb overcoming the internal resistance

Question 24

What does the conservation of energy tell us about internal and external resistance?

Answer 24

energy per coulomb supplied by the source = energy per coulomb transferred in the load resistance + energy per coulomb in internal resistance

Question 25

What are the equations for e.m.f.?

Answer 25

e.m.f. = V + v
e.m.f. = I(R + r)
V = e.m.f. - Ir

V = terminal p.d.
v = lost volts
R = load resistance
r = internal resistance