10

• 10.1 Describe the structure of the atom, limited to the position, mass

  and charge of protons, neutrons and electrons

  The nucleus contains:

  Protons - positively charged particles with a relative atomic mass of one unit

  Neutrons - no charge, and also with a relative atomic mass of one unit

  Almost all of the atom is empty space, but moving around the nucleus there are:

  Electrons - negative charge with almost no mass (1/2000 the mass of a proton or neutron)
• 10.3 Describe the differences between series and parallel circuits

  In series circuits, the different components are connected in a line, end to end, between the positive and negative of the power supply. In parallel circuits, each component is separately connected to the positive and negative of the supply.

  -In a series circuit, the current is the same at all points

  In a parallel circuit, the current splits at junctions - some of it going one way and the rest going the other

  -In a series circuit, the voltage of the power supply is shared between the components

  In a parallel circuit, the voltage across each component is the same

  -In a series circuit, the total resistance is the sum of the resistance in each component

  In a parallel circuit, the total resistance = 1/resistor 1 + 1/resistor 2 and so on (in the fraction 1 over the resistance)
• 10.4 Recall that a voltmeter is connected in _________ with a

  component to measure the...

  parallel

  the potential difference (voltage), in volt, across it
• 10.5 What is potential difference?

  Explain that potential difference (voltage) is the energy

  transferred per unit charge passed and hence that the volt is a joule per coulomb
• 10.7 Recall that an ammeter is connected in _________ with a component

  to measure...

  series

  to measure the current, in amp, in the component
• 10.8 Describe an electric current as...

  the rate of flow of charge
• 10.8 the current in metals is...

  a flow of electrons
• 10.10 Describe that when a closed circuit includes a source of

  potential difference there will be...

  a current in the circuit
• 10.11 What happens to current at the junction of a circuit?

  current is conserved at a junction in a circuit
• 10.12 Explain how changing the resistance in a circuit changes the...

  current - When resistance is increased in a circuit , for example by adding more electrical components , the current decreases as a result
• Describe electrical currents relationship with potential difference and resistance in a circuit

  The relationship between current, voltage and resistance is expressed by Ohm's Law. This states that the current flowing in a circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit, provided the temperature remains constant
• 10.12 Changing the resistance in a circuit changes the

  current how can this be achieved using a variable resistor?

  The variable resistor is a resistor that controls (increase or decrease) the flow of electric current when we vary or change its resistance. In other words, when we vary the resistance of the variable resistor, the electric current flowing through it will increase or decrease.
• 10.14 Explain why, if two resistors are in series, the net resistance is

  __________, whereas with two in parallel the net resistance is________

  increased

  decreased

  because in a parallel circuit, the net resistance decreases as more components are added, because there are more paths for the current to pass through. The two resistors have the same potential difference across them. The current through them will be different if they have different resistances. Whereas when more resistors are added in series, the total resistance of the circuit increases. As the total resistance increases, the current strength decreases.
• 10.16 Explain the design and construction of series circuits for testing

  and measuring

  A series circuit is one loop; all electrons in that loop form one current. An ammeter will measure the same current wherever it is placed in the circuit
• 10.17 Core Practical: Construct electrical circuits to:

  a) investigate the relationship between potential difference,

  current and resistance for a resistor and a filament lamp

  1)Set up the circuit as shown with the fixed resistor

  2)Vary the voltage across the component by changing the resistance of the variable resistor, using a wide range of voltages (between 8-10 readings). Check the appropriate voltage reading on the voltmeter

  3)For each voltage, record the value of the current from the ammeter 3 times and calculate the average current

  4)Increase the voltage further in steps of 0.5 V and repeat steps 2 and 3

  5)Make sure to switch off the circuit in between readings to prevent heating of the component and wires

  6)Reverse the terminals of the power supply and take readings for the negative voltage (and therefore negative current)

  7)Replace the fixed resistor with the filament lamp and repeat the experiment from step 1
• 10.17 Core Practical: Construct electrical circuits to:

  b) test series and parallel circuits using resistors and

  filament lamps

  1)Set up the circuit as shown with the single fixed resistor

  2)Record the voltage using the voltmeter and the current using the ammeter

  3)For each pair of voltage and current, calculate the resistance and record this

  4)Change the resistor and repeat step 2 and 3

  5)Arrange the two resistors in series as shown in the image, then repeat step 2

  6)Arrange the two resistors in parallel as shown in the image, then repeat step 2

  7)Replace the fixed resistor with a filament lamp and repeat the experiment from step 1
• 10.18 Explain how current varies with potential difference for the

  following devices and how this relates to resistance

  a) filament lamps

  The increasing current increases the temperature of the filament, which makes the resistance increase so their I-V graphs are curved.

  This is because:

  .As the current increases, the temperature of the filament in the lamp increases

  .The higher temperature causes the atoms in the metal lattice of the filament to vibrate more

  .This causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through

  .Resistance opposes the current, causing the current to increase at a slower rate
• 10.18 Explain how current varies with potential difference for the

  following devices and how this relates to resistance

  b) diodes

  Current will only flow through a diode in one direction. The diode has very high resistance in the opposite direction.

  .A diode allows current to flow in one direction only

  - this is called forward bias

  .In the reverse direction, the diode has very high resistance, and therefore no current flows

  -this is called reverse bias
• 10.18 Explain how current varies with potential difference for the

  following devices and how this relates to resistance

  c) fixed resistors

  The current through a fixed resistor increases as the potential difference across it increases

  In other words, current is directly proportional to the potential difference for a fixed resistor (if the temperature stays the same)

  An I-V graph shows that the line is straight and goes through the origin.
• 10.19 Describe how the resistance of a light-dependent resistor (LDR)

  varies with light intensity


  The resistance of an LDR changes depending on the light intensity on it - as the light intensity increases the resistance of an LDR decreases and vice versa because LDRs are a type of sensory resistor
• 10.20 Describe how the resistance of a thermistor varies with change

  of temperature (negative temperature coefficient thermistors

  only)

  Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers

  As the thermistor gets hotter, its resistance decreases

  As the thermistor gets cooler, its resistance increases
• Go over the symbols for different circuit components


  don't be lazy do it now
• 10.21 Explain how the design and use of circuits can be used to

  explore the variation of resistance in the following devices

  a filament lamps

  Connected to a increasing d.c of 2-12V (in increments of 2)

  - Connect the filament lamp to ammeter in series/voltmeter in parallel.

  - Measure the current for each voltage.

  - Plot a graph to show relationship between the p.d and current.

  - Non-linear relationship shows the variation of resistance.
• 10.21 Explain how the design and use of circuits can be used to explore the variation of resistance in the following devices

  b. diodes

  Connected to an increasing d.c of 1 to 1.5, 2, 4, 6, 8, 10, 12V

  - Connect to an ammeter in series and voltmeter in parallel.

  - Measure the current for each voltage.

  - Switch the diode the other way round to record current for -1, -1.5, -2, -4V.

  - Plot graph for the positive and negative potential differences to show the

  non-linear relationship and the variation of resistance.
• 10.21 Explain how the design and use of circuits can be used to explore the variation of resistance in the following devices

  c. thermistors

  Constant voltage of 12V

  - Connect to an ammeter

  - Place in ice water with thermometer

  - Measure current at 0 degrees.

  - Add hot water and stir, measuring current at 10 to 60 degrees.

  - Calculate the resistance.

  - Plot a graph of resistance against temperature to show non-linear relationship.
• 10.21 Explain how the design and use of circuits can be used to explore the variation of resistance in the following devices

  d. LDRs

  Constant voltage of 12V

  - Connect to ammeter.

  - Shine lamp immediately onto thermistor and measure current.

  - Move the lamp ~10cm away and measure current

  - Keep doing this until 50cm

  - Calculate resistance at each light intensity

  - Plot graph of resistance against light intensity
• 10.22 Recall that, when there is an electric current in a resistor, there

  is...

  an energy transfer which heats the resistor
• 10.23 Explain that electrical energy is dissipated as thermal energy in

  the surroundings when...

  an electrical current does work against electrical resistance
• 10.24 Explain the energy transfer when energy is dissipated as thermal energy in the surroundings as the result of...

  collisions between electrons and the ions in the lattice
• 10.25 Explain ways of reducing unwanted energy transfer

  through low resistance wires

  Low resistance wires made from metals such as copper, reduce unwanted energy transfer to thermal stores as the current flows between components.

  Thicker wires also can be used as they have a lower resistance.

  Also cooling metals reduce the resistance so the lattice ions are not vibrating as much.
• 10.26 Describe the advantages and disadvantages of the heating

  effect of an electric current - advantages

  - Fuses use the effect to protect circuits - they melt and break the circuit if the current gets too high.

  -The heating effect of the electric current is good if you want to heat something. There is a coil of high resistance wire which gives off infrared radiation, which transfers energy to the item needed to be heated. (food)
• 10.26 Describe the advantages and disadvantages of the heating effect of an electric current - disadvantages

  - Heating up a component generally reduces its efficiency - less is transferred to useful energy stores as more is being transferred to the thermal store of the component.

  - If the temperature gets too high, this can cause components in the circuit to melt. This will stop the circuit from working or work properly.
• 10.28 Describe power as...

  measured in...

  the energy transferred per second and recall

  that it is measured in watt
• 10.30 Explain how the power transfer in any circuit device is related to

  the potential difference across it and the current in it

  The power of a device depends on:

  The voltage (potential difference) of the device

  The current of the device

  The potential difference indicates how much energy each unit of charge transfer.

  The current indicates how much charge passes per unit time.

  Both affect the rate that energy is transferred and the rate it transfers energy to other stores.