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Edexcel Physics Topic 10 ((The difference between a series circuit and a…
Edexcel Physics Topic 10
The difference between a series circuit and a parallel circuit is that in a series circuit all the components are in series to each other, or all connected in a line to the power source. Whereas in the parallel circuit, all the components are connected separately to the power source and if you disconnect one it will mark almost no difference to the rest of the circuit.
Voltmeters are connected, always, in parallel across a component in order to measure the potential difference across it.
Potential difference (voltage) is the energy transferred per unit charge passed, hence one J per C.
Ammeters are always connected in series to the component in order to measure the current around it.
Current is the rate of flow of charge.
Current is conserved at the junction of a circuit, this means that current across branches in a parallel circuit is the same when added together as what went in and comes out of each component.
Potential difference, or p.d or voltage across branches in a parallel circuit is always the same across all components.
Current can only flow through a component if there is a source of p.d across it and if the circuit is complete/closed.
Adding resistors in series:
-The total resistance of the two components is just the sum of their resistances. This is because, adding a resistor in series, the two resistors have to share the total p.d. The p.d across each component is lower so the current through each resistor is also lower. In series, the current is the same everywhere in the circuit so the total current is reduced when a resistor is added. This means the total resistance in the circuit increases.
The amount of current flowing through a circuit depends on the amount of potential difference and resistance in the circuit. Generally speaking, the higher the p.d in the circuit, the higher the current but the higher the resistance, the smaller the current will be. The effect of resistance can be observed/ measured with a variable resistor.
Adding resistors in parallel:
-If you have two resistors in parallel, their total resistance is less than the resistance of the smaller of the two resistors.
-In parallel, the two components have the same p.d across them as the source. But by adding another loop/branch, the current has another place to go which increases the total current that can flow around the circuit which results in a decrease in the total resistance of the circuit. We can see this using V=IR
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Circuits
In series circuits, the current is the same everywhere as there are no branches for it to be split across.
The total p.d of the supply is shared between the components in the circuits; the voltage for each component depends on its resistance.
In order to measure current, p.d and resistance, set up a series circuit normally and connect an ammeter and the component which you're investigating in series while you connect a voltmeter in parallel. Take multiple readings from both while changing the p.d output which would change the current flowing through the component. Plot a I-V graph and use V=IR in order to calculate the resistance. You have to make sure that your circuit doesn't heat up as this would affect your results, disconnect the circuit and wait for a while before continuing.
Different components effects on current:
-Diodes- after taking measurements for a range of currents, you van remove the diode and swap its direction. The current cannot flow through the diode anymore.
-Thermistor- keeping the supply of p.d constant and heating the thermistor gradually, you should find that the temperature will gradually increase and as this happens, the current will increase through the component and the resistance will decrease
-LDRs- conduct the experiment in a dim room. Keep the p.d constant and slowly adjust the light level near the LDR. You should find that as the level of light increases, the current through the LDR increases, the resistance decreases..
I-V Graphs explained:
Filament lamp- 
- The increasing current increases the temperature of the filament which makes the resistance increase. This causes the gradient to be curved.
Resistors and wires- - The current is directly proportional to p.d when the temperature stays constant.
Diode- - Current will only flow through the diode in one direction . The diode has very high resistance in the opposite direction.
LDR is short for Light Dependent Resistor. In bright light, the resistance falls and in dark, the resistance is highest.
A thermistor is a resistor that is dependent on temperature. In hot conditions, the resistance falls but in cool conditions, the resistance goes up.
When an electrical charge flows through a component, it has to do work against resistance. This causes an electrical transfer of energy; some of this energy is transferred usefully but some of it is dissipated to the thermal energy stores of the component and the surroundings. Due to this, when a current flows through a resistor, the resistor heats up. This happens because the electrons collide with the ions in the lattice that make up the resistor as they flow through it; this gives the ions energy so they vibrate and heat up.
The more ions vibrate, the harder it is for the electrons to get to the resistor since there is more collisions. This means that more a given p.d, the current decreases as the resistor heats up.
Low resistance wires reduce the energy dissipated to thermal stores as the current flows between components.
Advantages and disadvantages of the heating effect in a component:
-Heating up a component generally reduces its efficiency- less energy is transferred to useful energy stores because more of it is being transferred to the thermal energy store of the component.
-If the temperature gets too high, this can cause components in the circuit to melt- which meant the circuit will stop working or not work properly.
-Fuses use this effect to protect circuits- they melt and break the circuit if the current gets too high.
+The heatign effect can be good if you want to heat something. Toasters have a coil of wire with a really high resistance. When a current passes through the coil, its temperature increases so much that it flows and gives off infrared radiation. This radiation transfers energy to the bread and cooks it.
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