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# Brief explanation of EMF? watch

1. Hi,

I'm trying to get my head around emf, internal resistance and electrical energy.

Am I right in thinking that: emf is the difference in charge across a battery. So when charge, carried by current, is lost to appliances, the battery restores current to the charge across the cell and this is emf?

2. (Original post by mydearthing)
Hi,

I'm trying to get my head around emf, internal resistance and electrical energy.

Am I right in thinking that: emf is the difference in charge across a battery. So when charge, carried by current, is lost to appliances, the battery restores current to the charge across the cell and this is emf?

emf is the energy transfered per unit charge (J per C = the volt) when one form of energy in converted into electrical energy. so that means the chemical energy of the battery being turned to electrical to pass through the circuit.

the potential difference is the energy transferred per unit chharge when that electrical energy is converted into another form of energy (as done by the electrical components in the cicuit e.g. the light bulb converts the electrical energy to heat and light energy)

edit: internal resistance
E = V + Ir
The internal resistance is the energy used for the chemical energy of the battery being turned into electrical energy. (you can think of this as no battery really having 100% efficency so some energy is used up passing through the battery) the internal resistance is also from the wire; thats why it heats up after a while and R is porportional to temperature.
3. you can check edexcel physics unit 2 jan 2009 (new syllabus) for the perfect definition
4. (Original post by mydearthing)
Hi,

I'm trying to get my head around emf, internal resistance and electrical energy.

Am I right in thinking that: emf is the difference in charge across a battery. So when charge, carried by current, is lost to appliances, the battery restores current to the charge across the cell and this is emf?

You're right in thinking that the emf is caused by a difference in charge at either end of the battery.

Current flows around the circuit carrying charge from one end of the battery to the other, this eventually equalises the charge at either end of the battery and the battery is 'dead.'
Charge is not lost in a proper circuit, neither is current.

Less current will flow if you add some resistance, like a light bulb, to the circuit. This doesn't mean its been 'lost' at the bulb, it just means less current flows through the whole of the circuit.
5. A really simple analogy:

Consider a ramp ("positive" being the top of the ramp and "negative" being the bottom of the ramp). Balls roll down from the top of the ramp to the bottom (this is the 'current') and once they've reached the bottom, they're going to need something to bring it back to the top of the ramp so they balls can start rolling down again (i.e. if the current is going to keep going, it needs something to get it back through the battery). In a situation like this, the potential (not potential energy!) required to do this would be the change in gravitational potential energy / mass - in a circuit, it is the change in electric potential energy per unit charge.

The definitions above are correct as well, but I find the analogy to be really helpful in understand what exactly the purpose of emf is

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