Physics A level - Electricity
Electric Current (I) is the amount of charge flowing through a certain point of a circuit every second. In other words, it is the amount of coulombs passing a point every second and it is measured in Amps (A). The instrument that measures charge in a circuit is called an ammeter and it is represented by the letter A in a circuit diagram.
There are 150 coulombs of charge passing a point in a circuit in 10 seconds. What is the current through the circuit?
I = 150/10 = 15 A
Emf is the amount of energy gained by each coulomb of charge when it passes through a battery or an emf source in a circuit. In other words it is the amount of joules gained by unit of charge. The unit of emf is volts and is represented by a V.
Resistance (R) is a measure of how difficult it is to push charge along a conductor. It is measured by an ohmmeter and it has units of ohms.
The following formula is used to measure the resistance of a conductor:
R = pL/A
The following equation represents the relationship between voltage, resistance and current:
V = I R
The equation to measure the energy delivered to the circuit every second is the following:
Measure the current of the following circuit if the emf is 10V.
I = V/R = 10/5 = 2 Amps
CIRCUIT IN SERIES
In a circuit in series we take the sum of each individual resistance to calculate the total resistance of the circuit and also the current passing through each resistor is the same. The following diagram shows a circuit with resistances placed in series and we will show how to calculate the total resistance, current and potential difference (across different points in the circuit).
Total Resistance = 2 + 5 + 7 = 14 Ω
Current (I) = V/R = 50/14 = 3.57 A
Potential difference through the 2 Ω resistor (V) = IR = 3.57 x 2 = 7.14V
Potential difference through the 5 Ω resistor (V) = IR = 3.57 x 5 = 17.85V
Potential difference through the 7 Ω resistor (V) = IR = 3.57 x 7 = 25V
CIRCUITS IN PARALLEL
In a circuit with resistors placed in parallel the total resistance () in the circuit is calculated using the following equation:
Another additional consideration is that the potential difference is the same in each resistor placed in parallel however the current across each of these resistors is not the same. Also the current going into the junction must be equal to the current coming out of the junction.
Total Resistance = Ω
Total current =
Current across the 5Ω resistor =
Current across the 8Ω resistor =
Current across the 4Ω resistor =
Up to this point we have considering batteries which have 0 resistance but real batteries actually poses resistance and this resistance is called internal resistance. Terminal potential difference is defined as the potential difference across the terminals of the source. To calculate the terminal p.d you simply subtract the emf from the voltage drop across the internal resistance. In simple words it is the output voltage left taking into account the energy dissipated due to the internal resistance of the cell. In a circuit diagram the internal resistance is represented by the symbol r.
Calculate the following:
The current in the circuit.
Because the resistors are placed in series, we can add the resistors to calculate the total resistance in the circuit and hence the current in the circuit.
The terminal potential difference.
This means that 35.7 V would be the output voltage of the source and 14.3 V would be the potential difference across the internal resistance.
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About The Author
I am a graduate in Manufacturing Engineering and Management with 3 years of experience in tutoring students for GCSE Spanish and Physics. For my A levels I scored A in Physics, A in Spanish and B in Mathematics.