AQA Physics Triple Higher - Electricity

• If the switch is closed, then electrons flow out of the battery and move around the circuit. This is called the current - the flow of electrons in a circuit. 
• The unit for current is Amps (A). 
• We can measure the current using an ammeter. 
• An ammeter is placed in series with the component being measured. 
• The current flows from the negative end of the cell to the positive end of the cell. 
• When the electrons return to the cell again, they have less energy than when they left.

Current in a series circuit

• In a series circuit, there are no branches, so current can only flow in one path. 
• Current is never used up in a circuit - so in a series circuit, there current is the same all the way around at any point in the circuit. 

Current in a parallel circuit

• A parallel circuit contains branches, this means that the current splits at the junctions. 
• The current in the branches add up to the total current leaving the cell. 

• Energy is transferred in electrical circuits. 
• Potential difference, or voltage, is the push or the force that causes the electrons to move through the circuit. 
• A p.d. of 1 volt tells us that 1 joule of energy is transferred per coulomb of charge that is moving through the circuit. 
• It is measured in Volts (V). 
• We use a voltmeter to measure the potential difference in a circuit, which is placed in parallel with the component being measured. 

Potential difference in a series circuit

• The potential difference in a series circuit is shared between the components, they add to give the total voltage. 

Potential difference in a parallel circuit

• The potential difference is the same across each component when they are connected in parallel. 

• Electrical charge is measured in Coulombs (C)
• The current of 1 Amp is equal to 1 coulomb of charge flowing per second. 
• The size of the electrical current is the rate of the flow of charge. 

To calculate charge, we use this equation:

• Charge (C) = Current (A) x Time (s)
  Q = I x T

• An electrical current is the flow of electrons through a conductor such as a metal wire. As the electrons flow, they collide with the atoms in the metal. This is resistance, or the obstacles that the current needs to overcome. 
• The greater the resistance is across the component, the smaller the current that flows through it. 
• The resistance tells us the potential difference required to drive a current through a component.

We can calculate resistance using this equation:

·         Resistance (Ω) = Potential difference (V)
                                      Current (A)

·         R = V ÷ I

• The current moving through a resistor is directly proportional to the potential difference, because a straight line is drawn through the origin which shows a linear relationship.
• This means that resistance is constant. The resistance does not change if we increase the current.
• This kind of a resistor is an ohmic conductor
• The resistance will only stay cosntant if the TEMPERATURE is constant.

• Resistors in series add together, as the current has to pass through each resistor in turn. 
• Resistors in parallel have the total resistance less than the smallest individual resistor. 

Method - RESISTOR

• Attach an ammeter in series to the resistor.
• Attach a voltmeter in parallel to the resistor. 
• Attach a resistor in the circuit. 
• Attach a variable resistor.
  • First, use the voltmeter to read the voltage across the resistor. 
  • Secondly, use the ammeter to read the current across the resistor. 
  • Record this data. 
  • Adjust the variable resistor and record the new readings on the voltmeter and the ammeter. 
  • Repeat this several times to get a range of readings.
  • Now, switch the battery, so that the charge flows in the opposite direction. Both of the meters should have negative values. 

Results: We should get a straight line graph that shows direct proportionality between the current and the potential difference - so resistance is constant through the resistor - therefore it is an ohmic conductor, only if the temperature is constant. 

Method - FILAMENT LAMP

• Attach an ammeter in series to the filament lamp.
• Attach a voltmeter in parallel to the filament lamp. 
• Attach a filament lamp in the circuit, and attach a variable resistor.
  • First, use the voltmeter to read the voltage across the filament lamp. Secondly, use the ammeter to read the current across the filament lamp. Record this data. 
  • Adjust the variable resistor and record the new readings on the voltmeter and the ammeter. Repeat this several times to get a range of readings.
  • Now, switch the battery, so that the charge flows in the opposite direction. Both of the meters should have negative values. 

Results: We should get a curve shape showing that current is not directly proportional to the voltage, this is because the filament gets hot, which causes the resistance to increase. At high temperatures, the atoms in the filament vibrate more. The electrons collide more with the atoms, so more energy is needed to push the current through the filament. As the voltage increases, the current does not increase as much which tells us resistance is increasing - therefore it is not an ohmic conductor - as temperature is changing, which skews the resistance. 

The current through a diode only flows in one direction. 

This is because diodes have a very high resistance in the reverse direction. 

When current flows through the way that the diode allows it, the current increases as the potential difference increases, like normal. However, when it flows in opposing way, the current cannot pass, as the diodes very high resistance blocks it. 

• Diodes are very useful for controlling the flow of circuits. 
• An LED is a diode. 
  • An LED works the same way as a diode, but when current passes through it, it emits light. 
  • LEDs are an extremely energy-efficient source of light. 

• A light dependent resistor is a resistor. 
• In dark conditions, the LDR has a high resistance. 
• In the light, the LDR has a low resistance. 

In the Light

• In the light, the resistance of the LDR is very low. It takes very little energy for the current to pass through the LDR. Because of that, the voltage across the LDR is very low. 
• Because the resistor has a very low voltage, the lamp has a very high voltage, as the potential difference in a series circuit adds up. This means that the lamp will now shine brightly.

In the Dark 

• In the dark, the resistance of the LDR is very high. It takes a lot of energy for the current to pass through the LDR. Because of that, the voltage across the LDR is very high. 
• Because the resistor has a very high voltage, the lamp has a very low voltage, as the potential difference in a series circuit adds up. This means that the lamp will now become very dim. 

• A thermistor is a resistor. 
• Under cool conditions, the resistance of the thermistor is high. 
• Under hot conditions, the resistance of the thermistory is low. 

Cool conditions

• Under cool conditions, the resistance of the thermistor is high, so it takes a lot of energy for the current to pass through the thermistor. Because of that, the potential difference across the thermistor is high, which means that the potential difference across the cooling fan is low, as componenets in series must share the voltage. This means the fan operates at a very small speed, or doesn’t at all.

Warm conditions

• Under warm conditions, the resistance of the thermistory is low, so it doesn’t take a lot of energy for the current to pass through thermistor. Because of that, the potential difference across the thermistor is low, which means that the potential difference across the cooling fan is very high, as components in series must share the voltage. This means the fan operates at its maximum speed.

Method - DIODE

• Attach an miliammeter in series to the diode, to measure the sensitive data.
• Attach a voltmeter in parallel to the diode. 
• Attach a diode. in the circuit, and attach a variable resistor.
• Attach a resistor, in order to bring down the current, as diodes are sensitive.
  • First, use the voltmeter to read the voltage across the diode.. Secondly, use the ammeter to read the current across the diode. Record this data. 
  • Adjust the variable resistor and record the new readings on the voltmeter and the ammeter. Repeat this several times to get a range of readings.
  • Now, switch the battery, so that the charge flows in the opposite direction. Both of the meters should have negative values.

Result: You should get a graph that has no values for the reverse direction, as there is high resistance in that direction, so you should get a flat line. However, in the reverse direction, the current peaks at 0.6 V sharply. This is because the diode can let current flow through this direction, as there is low resistance. 

Power is the rate at which energy is transferred.

1 watt (W) is an energy transfer of 1 J per second.

Appliances, such as an iron or a hairdryer, which have a purpose of creating thermal energy usually have a much high power rating than any other appliance.

Appliances convert electrical energy to a useful energy store.

·         A kettle has a power rating of 2200 watts, and is used for 80 seconds. Calculate the total energy transferred.

·         We can solve this by using this equation:

·         Energy (J) = Power (W) x Time (s)
E = P x t

2200 W x 80 s = 176 000 J

Power of Components

Power is the rate at which energy is transferred.

1 watt (W) is an energy transfer of 1 J per second.

We can calculate the power of a component using this equation:

·         Power (W) = Voltage (V) x Current (A)
P = V x I

·         Power (W) = Resistance (Ω) x Current squared (A)

P = R x I²

DC and AC supply

The current from a cell is a direct current – the electrons only travel in one direction. (DC)

Mains electricity is an alternating current – the current is constantly changing direction (AC)

• The benefit of the alternating current, is that it is very easy to use a transformer to increse or decrease the voltage.
• In the UK, alternating current has a frequency of 50 Hertz, i.e. it changes direction 50 times a second.
• In the UK, alternating current has a potential difference of around 230 Volts.
• We can see this pattern by using an oscilloscope.

For the alternating current, we measure peak to peak to see one cycle. As we can see, if we look at the alternating current diagram, there is a 0.02 second gap between the peaks.

The frequency is the number of cycles in one second, so we divide 1 by 0.02 s to get 50 Hertz.
1 ÷ 0.02 s = 50 Hz

Electrical appliances in the UK are usually connected to the mains supply, using a three core cable.

• The three core cables are made of copper, which is a good conductor of electricity, however, the coatings are made of plastic, which does not conduct electricity.
  
  
• Double insulated means that the object's case is made from an insulator such as plastic, so that it does not conduct electricity. In this case, the object does not need a earth wire. 

The Brown Wire

·         The brown wire is also known as the live wire. ·         It carriers the alternating voltage from the supply (230 V). ·         The live wire is connected to a fuse in the plug. ·         It is extremely dangerous and could easily be fatal if touched.

The Blue Wire

·         The blue wire is also known as the neutral wire. ·         It completes the circuit. ·         The voltage it carriers is 0 V compared to the live wire.

The Green-Yellow Wire

·      The green-yellow wire is also known as the Earth wire. ·         It is a safety wire to stop to appliance from becoming live. ·         If the live wire came loose, then it could touch the metal case which is a good conductor for electricity, therefore it will carry 230 V.  The earth wire is attached to the metal case, and the earth wire is connected into the ground with a metal rod. If the case becomes live, a huge current flows to the Earth. The fuse melts and shuts off the current. This prevents anyone from gettng an electric shock.

Static Electricity

·         Metals are good conductors of electricity, as electrons can flow through them easily.

·         Insulators do not conduct electricity, such as plastic and cotton.

·         Everything is covered with electrons.

·         If we take a piece of cloth (insulator) which is covered with electrons, and then we take a plastic rod that is covered with electrons (insulator), and rub them together, the electrons move from the rod onto the cloth, making the plastic rod positively charged, and the cloth negatively charged. The overall charges on both of the items are equal.

·         So, when two insulators are rubbed together, electrons can pass from one to the other.

·         When an insulator gains electrons, it becomes negatively charged, and when it loses electrons, it becomes positively charged.

Electric Field

·         Opposite charges attract – this is a non contact force.

·         Same charges repel – this is a non contact force.

·         Scientists represent these attractions and repulsions using field lines.

·         Field lines are always perpendicular to the surface of the charge.

·         The closer the charge to the charge, the stronger the force of repulsion or attraction, which can be portrayed by the length of size of the lines.