Wednesday, 11 April 2012

Energy and potential difference in circuits

Energy and potential difference in circuits
This would be Part C of Section 2: Electricity of the IGCSE Physics specification. 

-          The circuit symbols for a cell and a battery (2 or more cells connected)
-          An ammeter – you connect it in series to measure the current
-          A voltmeter – you connect it in parallel to the circuit component in question (e.g. a bulb) to measure the potential difference across it

2.9 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting

Parallel circuits are more appropriate for domestic lighting:
a) Bulbs are connected in parallel glow brighter because 2 bulbs in series have a higher resistance than a single bulb. (Current doesn't have to flow through combined resistance.)
b) If one light bulb blows, the others will continue to glow as there will still be a complete circuit through the other parallel branch. This also means that you can control each bulb independently, why would you want a circuit where if someone in your family switches off their light yours goes off too?
c) In a series circuit, the potential difference (p.d.) across each bulb in smaller. Each charge only gives up some of its energy in each bulb, relates to (a), so the bulbs are dimmer.

2.10 understand that the current in a series circuit depends on the applied voltage and the number and nature of other components

P= I x V

If there are resistors then the current will be smaller, and having similar resistors connected in series in a circuit would mean the combined resistance is larger, hence the current will be even smaller.

2.11 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how this can be investigated experimentally

  1. A wire would have the graph of the resistor as it has a fixed resistance too-as long as the temperature doesn't change, the resistance will be constant as well. 
  2. For a filament bulb, as the p.d. (potential difference/voltage) across the lamp increases, the current does not increase proportionally. Its resistance increases at higher temperatures. 
  3. For a semi-conductor diode, as voltage increases the resistance decreases (resistance decreases at higher temperatures). A semiconductor diode is a device which allows current to flow in one direction only, called the forward direction. So the graph basically shows a relatively larger current flowing through when a p.d. is applied in the forward direction. Almost no current will be observed if the p.d. is in the reverse direction. 

2.12 describe the qualitative effect of changing resistance on the current in a circuit

The resistance R of a component is defined as the ratio of the potential difference across it to the current flowing through it. 
The unit for resistance is ohm Ω. 
[One ohm is the resistance of a material through which a current of one ampere flows when a potential difference of one volt is maintained across it. 1 ohm Ω = 1 volt (V) / 1 ampere (A) ]

From this definition of resistance, we can see that for a particular potential difference, the higher the resistance, the smaller the current passing through. You can see by putting numbers into the equation. 

2.13 describe the qualitative variation of resistance of LDRs with illumination and of thermistors with temperature

LDR-as light intensity increases, resistance decreases hence current increases.
Thermistor-as temperature increases, resistance decreases hence current increases. This is commonly used in air conditioners to control the temperature.

You may be wondering why their resistance decreases, well, as there is more light energy and heat energy, more electrons are shaken free so it can conduct better and let more current through. I think that's the simplest explanation I can think of.

2.14 know that lamps and LEDs can be used to indicate the presence of a current in a circuit
If they light up there has to be a current in the circuit...

2.15 recall and use the relationship between voltage, current and resistance:

voltage= current x resistance
V= I x R

2.16 understand that current is the rate of flow of charge

  • Current is the rate of flow of charge.
  • Current is not used up, what flows into a component must flow out.
  • Current is measured in amps (amperes)A.
  • Current is measured with an ammeter, connected in series

Potential difference (voltage)
A potential difference/voltage across an electrical component is needed to make a current flow through it. Cells or batteries often provide the potential difference needed.

Measuring potential difference
  • Potential difference is measured in volts, V
  • Potential difference across a component in a circuit is measured using a voltmeter
  • The voltmeter must be connected in parallel with the component.

Some of you may be confused about voltage and current, I am too sometimes. Well:  It is possible to have voltage without current, but current cannot flow without voltage.

Voltage but not current:
The circuit is broken and current cannot flow.

Voltage and current:
The circuit is complete so current can flow.

2.17 recall and use the relationship between charge, current and time: 

charge = current x time
Q= I x t

2.18 recall that electric current in solid metallic conductors is a flow of negatively charged 

Remember that the electrons carry a negative charge, and that they are repelled from the negative terminal and are attracted to the positive terminal in a circuit. So they flow from negative to positive.

But the conventional current goes the other way… (see Conventional current vs Electron flow notes)

SS 2.19 recall that:

  • voltage is the energy transferred per unit charge passed
  • the volt is a joule per coulomb
Sometimes voltage is called potential difference. So what is potential difference? E.g. when a dry cell is connected to a light bulb, the electrical energy provided by the dry cell is converted into light and thermal energy by the bulb. This amount of energy converted across the light bulb for each unit of charge is called the potential difference. So the potential difference (p.d.) between two points is one volt if one joule of electrical energy is converted into other forms of energy when one coulomb of positive charge flows through it. 

In symbols,
V=W/Q where V is the p.d., W is the electrical energy converted to other forms, and Q is the amount of charge. 
As stated above under 'Measuring potential difference', you must connect a voltmeter in parallel with the component, e.g a lamp, to record the potential difference across it. 

d) Electric Charge

2.20 identify common materials which are electrical conductors or insulators, including metals and plastics

Any metal is a conductor of electricity. Any non-metal is an insulator, apart from graphite due to its unusual  structure.
Copper wires are common, as copper is a very good electrical conductor. PVC is now often used to insulate wires-Polyvinyl chloride. In IGCSE Chemistry, you might know this as poly(chloroethene), a polymer. PVC is cheap to make and flexible, so it now replaces rubber in insulating wires. Because as rubber grew old, it would crack and it would be dangerous if you touched the live wire. Insulation can become unsafe if it is damaged or wet because water can conduct electricity.


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      And, does that matter? :P

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Note: This blog will no longer be updated as I finished IGCSEs in 2012. Sorry! :( If you are interested in buying IB notes though, please contact me. :)