# PHYSICS - Circuits

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#1
Could someone please lay out the basic rules for series and parallel circuits for me? And how charge, current, resistance and voltage are related? I get confused with how they all link together!
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1 year ago
#2
(Original post by SnowDragonFired)
Could someone please lay out the basic rules for series and parallel circuits for me? And how charge, current, resistance and voltage are related? I get confused with how they all link together!
This is from an answer I wrote a few weeks ago but you may find it useful:

Current is the rate of flow of charge through a circuit. Electrons are negatively charged (with a relative charge of -1 or actual charge of (-)1.60 x 10^-19C), and they can flow (unlike protons, which are bound to a nucleus). This flow of electrons is known as a charge flow. Current describes how much charge flows in a given time, and can be found with the equatiion I = Q/t. This is where I = current in Amps, Q = charge in Coloumbs, where one Coloumb is 6.25 x 10^18 electrons, and t = time in seconds). This means 1A is equal to 1C of charge flowing past a point in 1 second.

The next idea of electricity is potential difference, or voltage. What potential difference is, is the work done per unit charge across a component. To move an electron, work must be done (or energy must be transferred to it) in order to get it to move. When a voltmeter is connected between the 2 terminals of a component, it measures the difference in energy in the ciruit between the first point and the second. This difference is the work that has been done in order to allow the charge to flow there. Potential difference can be found from the equation V = W/Q. In this equation, V = Potential difference (or voltage) measured in volts (V), W is work done (or energy transferred), measured in Joules (J), and Q is charge in C. This means if one Joule of work is done on 1 Coloumb of charge to move it across a component, the potential difference of that component is 1V. Always remember to talk about potential difference across a component, not through it.

The final idea to consider is resistance. Resistance is a measure of the difficulty faced by current while flowing through a circuit. Resistance is caused by the molecules of the component the current is flowing through. The electrons that are flowing can collide with ions in the component, increasing the difficulty faced by the electrons to flow, increasing current. For certain components, such as a filament bulb, resistance increases with temperature. This is because of with an increase in temperature, the molecules and ions of the component gain energy, and therefore vibrate faster. This makes the collisions between electrons and ions more frequent, hence there is an increase in current. In other components, like a thermistor, the resistance decreases when temperature increases. These can be used in sensor circuits, like a thermostat.

An idea that connects these together is Ohm's Law. This states that the current dlowing through a component, at fixed resistance, is directly proportional to potential difference across the component. Another way to say this is that current is directly proportional to PD but inversely proportional to resistance, with fixed potential difference. What this means is that if resistance stays the same for a component, then as the current through it increases, the potential difference will increase. This is because with an increase is current, the power of the component increases, as P = IV = I^R. So, when power increases, energy transferred across the component will increase (since power is the rate of energy transfer), so the potential difference increases. Similarly to this, if PD stays the same, and current increases, then resistance has to decrease, meaning the are inversely proportinal. The equation for Ohm's Law is V = IR (V = Potential Difference (V), I = Current (A), R = Resistance (Ohms, Ω)). This can be rearranged to give I = V/R, or R = V/I.

The last concept is what the behaviour of current and potential difference is in series and parallel circuits. I'd recommend answering some questions based on this, because practice will make it easier, but essentially:

In a series circuit, which is a circuit made up of one loop only:
--> Current is the same at any point in the circuit
--> The potential difference is shared between components, meaning the potential difference of the cell is the total of each individual component's potential difference. This relies on the assumption that any wires and voltmeters, ammeters etc. have zero or infinite resistance, meaning no energy is transferred so there is zero potential difference. This means you only have to consider other components.
--> The total resistance of the circuit is each individual component's resistance added to make a total. So Rt = R1 + R2...

In a parallel circuit, which is a circuit that has multiple loops and branches:
--> The current entering a branch (or junction) must be the same as the current leaving (so current cannot be lost). This means, when the circuit splits, the current entering must be split, so the total current is the sum of the currents through each branch.
--> Potential difference between each component (in parallel) is the same. This is because in a parallel circuit, where there are loops, the charge flows as loops. So in one loop, all the potential will be lost, then in another loop, the same potential is lost, but the charge will take the other branch. This means the PD of a component in parallel is the same. If there is a component in the main branch, so in series, this is not the same as the components in parallel.
--> The total resistance of the circuit is less than the resistance of the smallest individual component. This creates the equation 1/Rt = 1/R1 + 1/R2. So, if there is a branch, with resistors of 2Ω and 6Ω, 1/Rt = 1/2 + 1/6, which is 3/2. To find Rt, you would then find the reciprocal (essentially flip the fraction) to get a total resistance of 1.5Ω (which is smaller than both the 2Ω and the 6Ω resistor).

I know this is a long answer but hopefully it helps clear up some of the problems you had with the topic.
0
#3
Thank you so much for sharing this with me! This genuinely helped so so much, I’m definitely a lot more confident going into my exam!
1
#4
(Original post by Dan_N_2002)
This is from an answer I wrote a few weeks ago but you may find it useful:

Current is the rate of flow of charge through a circuit. Electrons are negatively charged (with a relative charge of -1 or actual charge of (-)1.60 x 10^-19C), and they can flow (unlike protons, which are bound to a nucleus). This flow of electrons is known as a charge flow. Current describes how much charge flows in a given time, and can be found with the equatiion I = Q/t. This is where I = current in Amps, Q = charge in Coloumbs, where one Coloumb is 6.25 x 10^18 electrons, and t = time in seconds). This means 1A is equal to 1C of charge flowing past a point in 1 second.

The next idea of electricity is potential difference, or voltage. What potential difference is, is the work done per unit charge across a component. To move an electron, work must be done (or energy must be transferred to it) in order to get it to move. When a voltmeter is connected between the 2 terminals of a component, it measures the difference in energy in the ciruit between the first point and the second. This difference is the work that has been done in order to allow the charge to flow there. Potential difference can be found from the equation V = W/Q. In this equation, V = Potential difference (or voltage) measured in volts (V), W is work done (or energy transferred), measured in Joules (J), and Q is charge in C. This means if one Joule of work is done on 1 Coloumb of charge to move it across a component, the potential difference of that component is 1V. Always remember to talk about potential difference across a component, not through it.

The final idea to consider is resistance. Resistance is a measure of the difficulty faced by current while flowing through a circuit. Resistance is caused by the molecules of the component the current is flowing through. The electrons that are flowing can collide with ions in the component, increasing the difficulty faced by the electrons to flow, increasing current. For certain components, such as a filament bulb, resistance increases with temperature. This is because of with an increase in temperature, the molecules and ions of the component gain energy, and therefore vibrate faster. This makes the collisions between electrons and ions more frequent, hence there is an increase in current. In other components, like a thermistor, the resistance decreases when temperature increases. These can be used in sensor circuits, like a thermostat.

An idea that connects these together is Ohm's Law. This states that the current dlowing through a component, at fixed resistance, is directly proportional to potential difference across the component. Another way to say this is that current is directly proportional to PD but inversely proportional to resistance, with fixed potential difference. What this means is that if resistance stays the same for a component, then as the current through it increases, the potential difference will increase. This is because with an increase is current, the power of the component increases, as P = IV = I^R. So, when power increases, energy transferred across the component will increase (since power is the rate of energy transfer), so the potential difference increases. Similarly to this, if PD stays the same, and current increases, then resistance has to decrease, meaning the are inversely proportinal. The equation for Ohm's Law is V = IR (V = Potential Difference (V), I = Current (A), R = Resistance (Ohms, Ω)). This can be rearranged to give I = V/R, or R = V/I.

The last concept is what the behaviour of current and potential difference is in series and parallel circuits. I'd recommend answering some questions based on this, because practice will make it easier, but essentially:

In a series circuit, which is a circuit made up of one loop only:
--> Current is the same at any point in the circuit
--> The potential difference is shared between components, meaning the potential difference of the cell is the total of each individual component's potential difference. This relies on the assumption that any wires and voltmeters, ammeters etc. have zero or infinite resistance, meaning no energy is transferred so there is zero potential difference. This means you only have to consider other components.
--> The total resistance of the circuit is each individual component's resistance added to make a total. So Rt = R1 + R2...

In a parallel circuit, which is a circuit that has multiple loops and branches:
--> The current entering a branch (or junction) must be the same as the current leaving (so current cannot be lost). This means, when the circuit splits, the current entering must be split, so the total current is the sum of the currents through each branch.
--> Potential difference between each component (in parallel) is the same. This is because in a parallel circuit, where there are loops, the charge flows as loops. So in one loop, all the potential will be lost, then in another loop, the same potential is lost, but the charge will take the other branch. This means the PD of a component in parallel is the same. If there is a component in the main branch, so in series, this is not the same as the components in parallel.
--> The total resistance of the circuit is less than the resistance of the smallest individual component. This creates the equation 1/Rt = 1/R1 + 1/R2. So, if there is a branch, with resistors of 2Ω and 6Ω, 1/Rt = 1/2 + 1/6, which is 3/2. To find Rt, you would then find the reciprocal (essentially flip the fraction) to get a total resistance of 1.5Ω (which is smaller than both the 2Ω and the 6Ω resistor).

I know this is a long answer but hopefully it helps clear up some of the problems you had with the topic.
Thank you so much for sharing this with me! This genuinely helped so so much, I’m definitely a lot more confident going into my exam!
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