Kirchoff's Laws
Kirchoff's first law states that the sum of currents into a point always equals the sum of currents out of a point. This is conservation of charge.
Kirchoff's second law states that the sum of the emfs is equal to the sum of the pds around a closed loop. This is conservation of energy.
During GCSE you will have come across series and parallel circuits (I hope!). Here is a reminder...
A series circuit has only one path for current. From K1 we can deduce that, since charge is not used up, the rate of flow of charge is the same at all points in the circuit. Since current is the rate of flow of charge this means current is the same at all points in the circuit. From K2 we know that the emf is shared between each component as there is only one closed loop. Adding additional components in series reduces the emf that is shared between the original components. We know that V=IR and I is constant (in series circuits). The total V = V1+V2... this means that the total resistance (R)= R1+R2...
A parallel circuit has more than one possible path for current. The resistance of each path determines how much current will flow down it. The greater the resistance, the lower the current that passes down it (V=IR and V is constant). Each branch of a parallel circuit forms it's own loop. from K2 we can deduce that the pd around each individual loop is equal to the emf from the power supply. In parallel circuits, V is constant. This means that I/V=I1/V+I2/V..... I/V = 1/R. This means that 1/R = 1/R1+1/R2...
These rules can be used to create a circuit with a required resistance.
When using more than one emf source, we must look at which polarity the emfs are in. We need to think about the internal resistance when combining emf sources in series and parallel.
In series: the emf will change but resistance will be constant
In parallel: the emf will be constant but resistance will change
Image source: Kerboodle OCR A Physics textbook p183
Kirchoff's first law states that the sum of currents into a point always equals the sum of currents out of a point. This is conservation of charge.
Kirchoff's second law states that the sum of the emfs is equal to the sum of the pds around a closed loop. This is conservation of energy.
During GCSE you will have come across series and parallel circuits (I hope!). Here is a reminder...
A series circuit has only one path for current. From K1 we can deduce that, since charge is not used up, the rate of flow of charge is the same at all points in the circuit. Since current is the rate of flow of charge this means current is the same at all points in the circuit. From K2 we know that the emf is shared between each component as there is only one closed loop. Adding additional components in series reduces the emf that is shared between the original components. We know that V=IR and I is constant (in series circuits). The total V = V1+V2... this means that the total resistance (R)= R1+R2...
A parallel circuit has more than one possible path for current. The resistance of each path determines how much current will flow down it. The greater the resistance, the lower the current that passes down it (V=IR and V is constant). Each branch of a parallel circuit forms it's own loop. from K2 we can deduce that the pd around each individual loop is equal to the emf from the power supply. In parallel circuits, V is constant. This means that I/V=I1/V+I2/V..... I/V = 1/R. This means that 1/R = 1/R1+1/R2...
These rules can be used to create a circuit with a required resistance.
When using more than one emf source, we must look at which polarity the emfs are in. We need to think about the internal resistance when combining emf sources in series and parallel.
In series: the emf will change but resistance will be constant
In parallel: the emf will be constant but resistance will change
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