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Current
Current is the rate of flow of charge through the cross-section of a point of a conductor.
I = Q / t , I is current (A), Q is charge (C) and t is time (s).
V = I R , V is voltage (V), I is current (A) and R is resistance (Ω).
Conventional current flows in the direction from positive terminal to negative terminal of a battery (this is the commonly accepted view by people of the direction of current flow but is not actually correct in reality).
Electron flows in the direction from negative terminal to positive terminal of a battery (this is the actual correct view of current as the rate of flow of electrons through a circuit).
See following video which examines the formula I = Q / t , V = I R , conventional current flow and flow of electrons.
I = Q / t , I is current (A), Q is charge (C) and t is time (s).
V = I R , V is voltage (V), I is current (A) and R is resistance (Ω).
Conventional current flows in the direction from positive terminal to negative terminal of a battery (this is the commonly accepted view by people of the direction of current flow but is not actually correct in reality).
Electron flows in the direction from negative terminal to positive terminal of a battery (this is the actual correct view of current as the rate of flow of electrons through a circuit).
See following video which examines the formula I = Q / t , V = I R , conventional current flow and flow of electrons.
Electromotive force
Electromotive force (e.m.f.) of an electrical source is the total work done by the electrical source in driving a unit charge round a complete circuit.
e.m.f = W / Q , W is work done by the electrical source, Q is charge.
**Many students are confused over the definition of e.m.f. because it sounds abstract. E.m.f. describes an electrical source (e.g. a battery). E.m.f also describes that there is certain total amount of work done by the electrical source (a battery) to move a unit charge (meaning 1 Coulomb of charge) round through the circuit.
By using common sense here, we can appreciate that charges do not magically move on their own round the circuit without something to do work to move them. There must be work done by the battery to move them round the circuit. Thus, the definition talks about the total work done by an electrical source (a battery) in driving (moving) a unit charge round a complete circuit.
As a result, the formula speaks of the definition as well. e.m.f = W / Q . W is divided by Q since we are talking about work done for every unit charge.
For example, suppose the total work done (W) to drive 2 C of charges is 20 J. Surely, to find total work done to drive every 1 C of charge, we will take e.m.f = W / Q = 20 J ÷ 2 C = 10 J/C and thus this means it needs 10 J of work done to drive every 1 C of charge round a circuit. This is a simple primary school math ratio problem. Do not think too complicated. Thus e.m.f = W / Q.
e.m.f = W / Q , W is work done by the electrical source, Q is charge.
**Many students are confused over the definition of e.m.f. because it sounds abstract. E.m.f. describes an electrical source (e.g. a battery). E.m.f also describes that there is certain total amount of work done by the electrical source (a battery) to move a unit charge (meaning 1 Coulomb of charge) round through the circuit.
By using common sense here, we can appreciate that charges do not magically move on their own round the circuit without something to do work to move them. There must be work done by the battery to move them round the circuit. Thus, the definition talks about the total work done by an electrical source (a battery) in driving (moving) a unit charge round a complete circuit.
As a result, the formula speaks of the definition as well. e.m.f = W / Q . W is divided by Q since we are talking about work done for every unit charge.
For example, suppose the total work done (W) to drive 2 C of charges is 20 J. Surely, to find total work done to drive every 1 C of charge, we will take e.m.f = W / Q = 20 J ÷ 2 C = 10 J/C and thus this means it needs 10 J of work done to drive every 1 C of charge round a circuit. This is a simple primary school math ratio problem. Do not think too complicated. Thus e.m.f = W / Q.
Potential difference
The potential difference (V) across two points of a circuit component is the amount of electrical energy converted to other forms of energy for every one unit charge that passes between the two points.
V = W / Q or V = E / Q ,
V is potential difference (V), W is work done (J), E is amount of electrical energy converted to other forms (J), Q is charge (C).
See following video which shows the explanation of this definition.
V = W / Q or V = E / Q ,
V is potential difference (V), W is work done (J), E is amount of electrical energy converted to other forms (J), Q is charge (C).
See following video which shows the explanation of this definition.
Resistance
The resistance of a conductor is the ratio of the potential difference across it to the current that flows through it.
R = V / I or V = I R , R is resistance (Ω), V is potential difference (V), I is current (A).
See following video which explains the definition of resistance. We understand that certain materials like nichrome has higher resistance than other materials such as copper. This simple idea of resistance is also a measure of the ability of a material to resist the flow of current through it (or more accurately the flow of electrons through it). For O level answers, do use the proper definition which expresses resistance as a ratio of the potential difference to current.
R = V / I or V = I R , R is resistance (Ω), V is potential difference (V), I is current (A).
See following video which explains the definition of resistance. We understand that certain materials like nichrome has higher resistance than other materials such as copper. This simple idea of resistance is also a measure of the ability of a material to resist the flow of current through it (or more accurately the flow of electrons through it). For O level answers, do use the proper definition which expresses resistance as a ratio of the potential difference to current.
Resistance of wire affected by the length and cross-sectional area
R = pl / A ,
R is resistance (Ω), p is resistivity of wire, l is length of wire (m), A is cross-sectional area of wire (m²)
Based on the above formula, we can see resistance as directly proportional to length while resistance is inversely proportional to cross-sectional area:
1. The longer the wire, the larger the resistance. E.g. doubling the length doubles the resistance.
2. The thinner the wire, the larger the resistance. E.g. halving the radius of the wire increases the resistance by four times.
R is resistance (Ω), p is resistivity of wire, l is length of wire (m), A is cross-sectional area of wire (m²)
Based on the above formula, we can see resistance as directly proportional to length while resistance is inversely proportional to cross-sectional area:
1. The longer the wire, the larger the resistance. E.g. doubling the length doubles the resistance.
2. The thinner the wire, the larger the resistance. E.g. halving the radius of the wire increases the resistance by four times.
Resistors in series circuit
Assuming there are three resistors in a series circuit, Total resistance R = R1 + R2 + R3 , R1, R2 and R3 refers to the three different resistors.
Also, Total voltage (e.m.f) = V1 + V2 + V3 , V1, V2 and V3 refers to the different potential differences across the three resistors.
See following video which explains how to look at resistance, current and also potential differences in series circuit.
Also, Total voltage (e.m.f) = V1 + V2 + V3 , V1, V2 and V3 refers to the different potential differences across the three resistors.
See following video which explains how to look at resistance, current and also potential differences in series circuit.
Resistors in parallel circuit
Assuming there are two resistors connected in parallel in a circuit, to find total resistance, we use the following formula,
1 / R = 1 / R1 + 1 / R2 , R is total resistance, R1 and R2 are the resistance of the two different resistors.
**Note that when using the above formula, be careful that the left side of equation is 1 / R , so remember to invert the answer to get R. Some students tend to be always careless and forget to invert their answer to get R and so lost unnecessary marks.
Also, Total V = V1 = V2 , in this case V1 and V2 are the potential differences across the two different resistors and the potential difference is exactly the same for the electrical source (battery) and each of the resistors when they are connected in parallel.
See following video on finding total resistance in parallel arrangement and also finding current in each branch of circuit in the parallel arrangement. Notice that the two different currents flowing through the two branches in this parallel arrangement adds up to be equal to the current flowing through the main circuit.
Total current I in main circuit = I1 + I2 , I1 is current in branch 1 and I2 is current in branch 2.
4 A = 0.8 A + 3.2 A
1 / R = 1 / R1 + 1 / R2 , R is total resistance, R1 and R2 are the resistance of the two different resistors.
**Note that when using the above formula, be careful that the left side of equation is 1 / R , so remember to invert the answer to get R. Some students tend to be always careless and forget to invert their answer to get R and so lost unnecessary marks.
Also, Total V = V1 = V2 , in this case V1 and V2 are the potential differences across the two different resistors and the potential difference is exactly the same for the electrical source (battery) and each of the resistors when they are connected in parallel.
See following video on finding total resistance in parallel arrangement and also finding current in each branch of circuit in the parallel arrangement. Notice that the two different currents flowing through the two branches in this parallel arrangement adds up to be equal to the current flowing through the main circuit.
Total current I in main circuit = I1 + I2 , I1 is current in branch 1 and I2 is current in branch 2.
4 A = 0.8 A + 3.2 A