P3.2 - Simple Circuits

• Circuit symbols:

  A) cell

  B) battery

  C) open switch

  D) closed switch

  E) filament lamp

  F) LED

  G) power supply

  H) resistor

  I) variable resistor

  J) ammeter

  K) voltmeter

  L) diode

  M) LDR

  N) thermistor

  
• The Standard Test Circuit

  You can use this circuit to investigate components e.g. how their resistance changes with current and potential difference.

  A) resistor

  B) component

  C) A

  D) V

  
• Ammeter:

  The ammeter measures the current (in amps) flowing through the component. It must be placed in series (in line with) the component so can be put anywhere in series in the main circuit, but never in parallel like the voltmeter.
• Voltmeter:

  The voltmeter measures the potential difference across the component. It must be placed in parallel with the component under test, NOT the variable resistor or battery (so it can compare the energy the charge has before and after passing through the component).
• You can use a standard test circuit to produce I-V graphs for different components:

  1.Connect the circuit. The component, the ammeter and the variable resistor are all in series, meaning they can be put in any order in the main circuit. (voltmeter must be in parallel around the component under test.) Vary resistance of variable resistor. This alters current flowing through circuit and the potential difference across component. Take several pairs of readings from the ammeter and voltmeter to see how the potential difference across the component varies as the current changes.
• standard test circuit experiment results:

  Plot the current against the potential difference to get I-V graphs. You can use this data to work out the resistance for each measurement of I and V so you can see if the resistance of the component changes as I and V change.
• safety in standard test circuit experiment:

  Make sure the circuit doesn't get too hot over the course of your experiment, as this will mess up your results. If the circuit starts to warm up, disconnect it for a while between readings so it can cool down. And, like any experiment, you should do repeats and calculate averages.
• Potential difference:

  For potential difference (V) in volts, V, current (I) in amps, A, and resistance (R) in ohms, Ω: potential difference = current × resistance

  V = I x R

  If you rearrange this equation, you can use it to calculate the resistance of a component from measurements of potential difference and current.
• I-V graphs show how the current varies as you change the potential difference (p.d). Here are three examples, plotted from experiments:

  A) resistors and wires

  B) directly proportional

  C) p.d.

  D) temp

  E) resistances

  F) slopes

  G) filament lamp

  H) temp

  I) resistance increase

  J) curved

  K) diode

  L) one direction

  M) high resistance

  N) opposite

  O) linear

  P) non-linear

  
• I-V graph:

  You can find the resistance for any point on any I-V graph by reading the p.d. and current at that point and sticking them in the formula above. A resistor or wire has a constant resistance (it doesn't change with current or p.d.), so its I-V graph is linear. If the line goes through the origin, the resistance of the component equals the inverse of the gradient of the line, or "1/gradient". The steeper the graph, the lower the resistance. For some components, the resistance changes as the current and p.d. change - the I-V graph curves.
• Resistance Increases with Temperature (Usually)

  When an electron flows through a resistor, some of its energy is transferred to the thermal energy store of the resistor, heating it up. The thermal energy store of a substance is the kinetic energy store of its particles. So as the resistor heats up its particles start to vibrate more. Meaning it's more difficult for the charge-carrying electrons to get through the resistor - the current can't flow as easily and the resistance increases.
• Temp and resistors:

  For most resistors there's a limit to the current that can flow. More current means an increase in temperature, which means an increase in resistance, which means the current decreases again. This is why the graph for the filament lamp levels off at high currents. Thermistors are different their resistance decreases with increasing temperature.
• Diode:

  A diode is a special device made from semiconductor material such as silicon. It lets current flow freely through it in one direction, but not in the other (i.e. there's a very high resistance in the reverse direction). This is really useful in various electronic circuits, e.g. in radio receivers. Diodes can also be used to get direct current from an alternating supply.
• Light-Dependent Resistor (LDR):


  An LDR is a resistor that's dependent on the intensity of light. In darkness, the resistance is highest. As light levels increase the resistance falls so (for a given p.d.) the current through the LDR increases. They have lots of applications including automatic night lights, outdoor lighting and burglar detectors.
• Temperature-Dependent Resistor:


  In hot conditions, the resistance of a thermistor drops. In cool conditions, the resistance goes up. This is known as an NTC (Negative Temperature Coefficient) thermistor. In constant conditions, their I-V graphs are curved - as the current increases, the thermistor warms up, so the resistance decreases. They're used as temperature detectors, in e.g. thermostats, irons and car engines.

  
• Sensing circuits:

  The fixed resistor and the fan will always have the same potential difference across them (because they're connected in parallel). The p.d. of the power supply is shared out between the thermistor and the loop made up of the fixed resistor and the fan according to their resistances - the bigger a component's resistance, the more of the p.d. it takes. As the room gets hotter, the resistance of the thermistor decreases and it takes a smaller share of the p.d. from the power supply. So the p.d. across the fixed resistor and the fan rises, making the fan go faster.
• Sensing circuits:

  Sensing circuits can be used to turn on or increase the power to components depending on the conditions that they are in.

  A) thermistor

  B) fixed resistor

  C) fan

  
• Sensing circuits:

  You can also connect the component across the variable resistor instead. For example, if you connect a bulb in parallel to an LDR, the p.d. across both the LDR and the bulb will be high when it's dark and the LDR's resistance is high. The greater the p.d. across a component, the more energy it gets. So a bulb connected across an LDR would get brighter as the room got darker.

  A) fixed resistor

  B) LDR

  C) bulb

  
• Series circuits:

  In series circuits, the different components are connected in a line, end to end, between the +ve and -ve terminals of the power supply. Current has to flow through all of the components to get round the circuit, so if you remove one of them it can have a big effect on the others.
• Parallel circuits:

  In parallel circuits each component is separately connected to the +ve and -ve terminals of the supply. This means if you remove or disconnect one of them, it will hardly affect the others at all.

  Parallel circuits are usually the most sensible way to connect things, for example in cars and in household electrics, where you have to be able to switch everything on and off separately.
• If you add more cells to any circuit, connect them in series, not parallel. Connecting several cells in series, all the same way (+ to-) gives a bigger total p.d. because each charge in the circuit passes through each cell and gets a 'push' from each one. So two 1.5 V cells in series would supply 3 V in total.
• Potential difference in series circuits:

  In series circuits, the total potential difference (p.d.) of the supply is shared between the various components. So the p.d.s round a series circuit always add up to equal the p.d. across the power supply :V =V1+V2. This is because the total energy transferred to the charges in the circuit by the power supply equals the total energy transferred from the charges to the components.

  A) v1

  B) v2

  C) series

  D) parallel

  
• Current in series circuits:

  In series circuits the same current flows through all parts of the circuit: I1= I2 = I3

  The size of the current is determined by the total p.d. of the power supply and the total resistance of the circuit: i.e. I = V/R.

  
• Resistance in series circuits:

  In series circuits, the total resistance is just the sum of the individual resistances: R = R1 + R2 + R3

  You can treat multiple resistors connected in series like this as a single resistor with equivalent resistance R.

  The resistance of two (or more) resistors in series is bigger than the resistance of just one of the resistors on its own because the battery has to push each charge through all of them.
• Resistance in series circuits:


  The bigger the resistance of a component, the bigger its share of the total p.d. because more energy is transferred from the charge when moving through a large resistance than a small one).

  If the resistance of one component changes (e.g. if it's a variable resistor, light-dependent resistor or thermistor) then the potential difference across all the components will change too.

  
• p.d. in parallel circuits:

  In parallel circuits all branches get the full source p.d., so the p.d. is the same across all branches: V1 = V2= V3

  This is because each charge can only pass down one branch of the circuit, so it must transfer all the energy supplied to it by the source p.d. to whatever's on that branch.

  A) branch

  
• Current in parallel circuits:

  In parallel circuits the total current flowing round the circuit equals the total of all the currents through the separate branches.

  I = I1 + I2

  Current in a branch = I = V/R, where V is the p.d. across the branch (which is equal to the source p.d.) and R is the resistance of the component on the branch (or the equivalent resistance of the branch if there's more than one component on it).

  In a parallel circuit, there are junctions where the current either splits or rejoins. The total current going into a junction has to equal the total current leaving.

  
• Resistance in parallel circuits:

  The total resistance of a parallel circuit is tricky to work out, but it's

  always less than that of the branch with the smallest resistance.

  The resistance is lower because the charge has more than one branch to take - only some of the charge will flow along each branch.

  A circuit with two resistors in parallel will have a lower resistance than a circuit with either of the resistors by themselves which means the parallel circuit will have a higher total current.

  A) <

  B) <

  
• Set up of investigation series and Parallel Circuits using bulbs:

  Set up circuit consisting of a power supply and bulb. Use voltmeter to measure the p.d. across bulb, and ammeter to measure the current in circuit.

  You connect voltmeters in parallel, and ammeters in series.

  Add second bulb in series with the first. Measure current flowing through circuit and p.d. across each bulb. The bulbs should both look dimmer. Add third bulb in series with the first two. All three will look even dimmer. Again, measure the p.d. across each bulb, and current through circuit.

  
• Investigating series and Parallel Circuits using bulbs:


  You'll find that each time you add a bulb, the p.d. across each bulb falls - this is because the p.d.s across the bulbs in the circuit need to add up to the source p.d. The current also falls each time you add a bulb, because you're increasing the resistance of the circuit. Less current and less p.d. means the bulbs get dimmer (i.e. the power of each bulb is decreasing).
• Investigating series and Parallel Circuits using bulbs:


  Repeat the experiment, this time adding each bulb in parallel on a new branch. Measure the current on each branch each time you add a bulb. The bulbs don't get dimmer as you add more to the circuit. The p.d. across each bulb is equal to the source p.d.. no matter how many bulbs there are. The current on each branch is the same, and doesn't change when you add more bulbs, because the resistance of each branch stays the same.

  
• Potential Difference in Terms of Energy and Charges:

  Anything that supplies electricity is supplying energy - an electrical current transfers energy from the power supply (e.g. cells, batteries, generators etc) to the components of the circuit. The potential difference between two points is the energy transferred by one coulomb of charge between these points - energy = charge x potential difference
• how electric circuits actually work:

  Energy is supplied to the charge at the power source to 'raise' it through a potential.

  The charge gives up this energy when it 'falls' through a potential drop in any components elsewhere in the circuit.

  A) charges

  B) resistors

  C) battery

  D) charges

  
• Potential Difference in Terms of Energy and Charges:


  A battery with a bigger p.d. will supply more energy to the circuit for every coulomb of charge which flows round it, because the charge is raised up "higher" at the start - and more energy will be transferred in the circuit too. The greater the resistance of a component, the more energy the charge has to transfer to it to pass through, so the bigger the drop in p.d
• Power:

  The power of a component tells you how much energy it transfers per second. Energy (E) and power (P) are related by the formula: energy transferred joules (J) = power watts (W)X time seconds (s)

  Energy is given in joules, but you may also see it in kilowatt-hours.

  A kilowatt-hour (kWh) is the amount of energy a device with a power of 1 kW (1000 W) transfers in 1 hour of operation. It's much bigger than a joule, so it's useful for when you're dealing with large amounts of energy.

  To calculate the energy transferred in kWh, you need power in kilowatts, kW, and the time in hours, h.
• Power from Current and Potential Difference:

  You can calculate electrical power of a component in watts, W, from the potential difference across it in volts, V, and the current through it in amperes, A, using the formula: power = potential difference X current or P = V x I

  Another way to calculate power: power = current2 × resistance or, in symbols: P = I2 x R

  Resistance is measured in ohms, Ω.