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# Easy physics questions? What is current/voltage/emf watch

1. Despite studying Physics A2, I seriously do not understand these terms or often get the definitions muddled up! Which is embarrassing.

Please correct me if I am wrong. Or help me in a way to understand in any way such as non scientific way

Charge is basically a bag of electrons.

Current is the rate of flow of charge, so basically the rate of these electrons bags.

Therefore electron flows from positive to negative then why do we use the term conventional current where it flows from positive to negative although it's disproved the theory is wrong!

Voltage is the thing that pushes current around the circuit also it's defined as the energy transferred per unit charge when the electrical energy is converted to another e.g. heat/friction/sound.

Emf is basically voltage but it's the energy transferred per unit charge when converted from one form to electrical energy

Electrical energy is the potential difference

Oh gosh...

WHAT WHAT WHAT
2. Electric charge is a property of matter (it's slightly tricky to define without reference to fields and forces, so just take this for granted). The smallest possible unit of charge which can be transferred is the elementary charge, i.e. the magnitude of the charge possessed by an electron, 1.6×10−19 C.

Current is the rate at which charge is transferred. A current of 1 A means that 1 C is being transferred every second (current = charge/time).

Conventional current was introduced when scientists randomly assigned positive and negative, and dictated the flow to be from positive to negative. The entirety of the field of electronics was developed around this; it was only later that the electron and its negative charge was discovered. So, yes, electrons physically move from the negative terminal to the positive, but the original standard was too ingrained in terms of components and the way we make everything work to change it, so it's more useful to stick with that (it's confusing, I know)

Voltage is more correctly referred to as potential difference, which literally means that it is the difference in electric potential between two points (hence why voltmeters have two ends and measure over a component, unlike ammeters which simply measure the amount of charge flowing through it). To understand what electric potential actually is, again, you'll have to look into electric fields, so don't worry too much about it. For a circuit following Ohm's law, voltage can be measured by current x resistance (this is the all-important V=IR equation).

Electromotive force (EMF) is a specific kind of voltage, measured in volts just like as above. It specifically refers to the voltage of a power source, i.e. the driving force behind a circuit (such as a battery or dynamo). 'Voltage' refers to the potential difference across any old component, but 'electromotive force' specifically refers to the voltage induced by an electrical power source. Literally, look at that name: it's a force which is moving electrons. Dimensionally, it's identical to any other kind of voltage. In terms of formulae, whilst generic voltages are more applicable by considering the current and resistance of a circuit, EMF is more referring to the electrical energy transferred per charge ( = E/Q). Considering the 'charge' in a regular circuit is the electrons physically flowing, this can be interpreted as the power source, the EMF, giving kinetic energy to the electrons: the energy transferred per (to an) electron. Again, this is a force which is literally moving electrons!

Electrical energy is simply the form of energy, in joules, which is relevant here. Electrical energy is related to electric charge as explained in the above section about EMF. Electric power, in watts, is simply the rate at which this energy is transferred (P = dE/dt). Power can be determed via Ohm's law with the formulae (P = IV , P = I2R , P = V2/R), that's just simple algebra to rearrange that, but those equations will come in handy.

I've never liked electricity as a topic. I found the terminology and concepts relatively hard to grasp earlier on and - like the conventional current fiasco - whole chunks just seem illogical to me. But, I hoped I managed to help out even a little bit!

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Updated: January 27, 2015
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