DC electricity Flashcards Preview

Electricity and Magnetism > DC electricity > Flashcards

Flashcards in DC electricity Deck (40):

What are converters?

Components that change electrical energy into useful energy


What is an electrical current

a movement of charged particles. In electrical circuits, the charged particles are usually electrons and they flow along conducting wires.


What is current?

The flow of coulombs

Coulombs are packets of electrons. One coulomb per second is called one Amp.

The current in a circuit is the energy transporter.


What is voltage of an energy source?

The number of joules of energy it supplies to each coulomb of the current that is flowing through it.


What happens when a current goes through a converter?

Energy is taken from each coulomb. The converter always takes the same amount of energy from each coulomb.


What is voltage of a converter?

The amount of energy taken per coulomb


What happens to each coulomb when it arrives at the energy source again?

When current flows around a closed loop, each coulomb gives up ALL the energy it took from the energy source to the converters in the loop. Each coulomb arrives back at the energy source "empty handed". Each coulomb shares the energy it picked up from the energy source between all the converters it flows through.


What is power?

The rate (amount per second) at which energy is delivered.


Power equation with J and s

P = E/t

P = W
E = J
t = seconds


What is the relationship between power, current and voltage?

Coulombs deliver energy to the converters in a circuit in "packets". The rate at which energy is delivered depends on:
-the number of coulombs passing through the converter per second (A)
-the size of the packets (V)


Power equation with I and V

P = IV

P = W
I = A
V = V


Which line is the positive and which line is the negative terminal?

The positive terminal is the longer vertical line.


What happens when a converter fails in a series circuit?

A series circuit has all the components connected in a single loop, so that whatever current goes through the energy source, the same current must go through all the energy converters. If one fails, the circuit loop will be broken and the current will stop flowing, so the other converters also stop operating.


What happens when a converter fails in a parallel circuit?

In a parallel circuit, after the current has gone through the energy source, it has a choice of two or more pathways (usually called branches) it can go round to get back to the source for more energy.

In a parallel circuit, each branch operates as if the other branches were not there. The battery supplies current to both lamps but the two current supplies are independent of each other. If one circuit was broken, the other lamp would just carry on as normal.


Advantage of connecting converters in parallel

If one fails and breaks the circuit branch, the other will keep operating as if nothing had happened.


Which direction does current enter the battery?

It enters the negative terminal and leaves the positive terminal.


How are two batteries connected in series?

The positive terminal of one is connected to the negative terminal of the other - meaning that the current has to go through both of them. So each coulomb picks up one lot of energy from battery A and another lot from battery B


Advantage of having several batteries in series

It provides a source that has a greater voltage than any of the individual batteries could give.


What happens when batteries are connected in parallel? and how are they connected

Half the current goes through one battery, the other half goes through the other. The pos terminal of one is connected to the pos terminal of the other, and vice versa. So the current always goes through the battery entering the neg and leaving the pos terminal.

So coulombs only pick up energy from one battery.


The advantage of having battery in parallel?

Each battery is drained of its energy store a lot more slowly than if there had been only one battery supplying the energy, so the batteries do not have to be charged often.


How is ammeter connected

In series with the component


What does the voltmeter show

the difference between the energy carried by each coulomb at the two points in the circuit that the voltmeter sockets are connected to.


How must an ammeter be connected?

The ammeter has a positive red connector and a negative black connector. The positive connector must be connected to the positive of the source. And vice versa.


How must a voltmeter be connected?

The positive connector must be connected to the higher energy side of the component whose voltage is being measured. This means the positive of the voltmeter must be connected to the positive of the source.


What is resistance?

The resistance of a component is an indicator of how difficult it is for current to flow through the component. Easy = low resistance


Do conductors have high or low resistance?

Good conductors have very little resistance. Metals are good conductors. Carbon is also a good conductor (in the form of graphite). Copper and Al are particularly good conductors and are used for electrical wiring.

Poor conductors have high resistance but are still able to allow current to flow.

An insulator has such a high resistance than under normal conditions, current is completely stopped. Such as paper, plastics, wood, cotton, wool and glass.


What is resistance measured in?

Ohms, symbol Ω

Symbol R


Resistance in series

If two components are connected in series, the current goes through all components so experiences the resistance of all components.

so R(T) = R1 + R2 + ...


Does a converter of high resistance take up more or less energy?

The resistance of a converter is also an indicator of what share of the energy carried by each coulomb the converter will get from the current that is flowing through it. The greater the resistance, the harder it is for the current to flow and so the more energy is taken by the converter.

The converters that have a greater resistance are harder for the current to flow through, so take a bigger share of the energy. So, the energy that is picked up by each coulomb at the source is divided amongst the other converters in the ratio of their resistances.

If a lamp has twice the resistance, it will take up twice the energy as the other lamp.


Does the current divide in parallel circuits?

Yes. The total current is the sum of the current in each branch.


What's the relationship between current and resistance?

It is easier for current to flow through a lower resistance, so, when the current divides, more goes through the low-resistance branch than goes through the high resistance branch. The current divides in the INVERSE ratio of the resistances.


Equation for voltage, current and resistance

V = IR

This relationship is known as Ohm's Law.


Relationship between brightness and voltage

As the bulb gets brighter it converts electrical energy into light & heat so more electrical E is used. Brighter bulb = bigger p.d.


Relationship between brightness and current

Current is the rate of flow of charge so the larger the current the more charge flowing each second. As the charge carries energy, there is more energy supplied per second.


Kirchoff's first law

At any junction in a circuit, the sum of the currents arriving at the junction = the sum of the currents leaving the junction.


Which principle applies?

The principle of conversation of energy.


What is an Ohmic resistor?

A resistor that obey's Ohm's law at all currents.


How to calculate resistance in parallel?

1/Rt = 1/R1 + 1/R2 +...


Equation using power, current and resistance

P = I^2R

because P = I (IR)


Equation for power, volts and resistance

P = V^2/R

because V/R = I
and P = IV