Electricity voltage and current?

Question

Okay, I really need help here because I am having my Physics prelim practical tomorrow at eight am.

When the resistance increases in a circuit, what happens to the voltage? Would the voltage increase or decrease?

Also, there are two bulbs in a series circuit and another two bulbs in a parallel circuit. Which bulbs will light up the brightest? The bulbs in the series or parallel circuit? Because if I were to put 4A and 6V, each bulb for series and parallel circuit would still receive 12V each.

Thank you.

Answer 1

1) It depends on the circuit. If it is just one resistor across a battery, then increasing the resistance will cause the current to decrease, because the voltage is fixed. I = E/R decrease R causes I to increase.

If you have a bunch of resistors in series across a battery and increase the value of one, then the voltage across it will increase. The current is relatively fixed by the values of the other resistors, so E = IR.

2) Depends on the bulbs and te voltage. If the bulbs are all identical, and the battery voltage is the same, then the ones in the parallel circuit will be brighter because they get the most voltage. They each get the full battery voltage. The ones in the series circuit divide the battery voltage, and each bulb gets only half.

3) "Because if I were to put 4A and 6V, each bulb for series and parallel circuit would still receive 12V each"

What i think you are saying, is that if you increased the voltage on the series bulbs, they would get brighter. Of course, and you could increase it more and they would be brighter than the parallel ones.

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Answer 2

Just a correction.. Voltage increase will depend on the circuit. It might happen that voltage remains constant and current decreases.

Answer 3

It may help to think that current and voltage are analogous to water flow and hydraulic pressure.  A battery (etc.) supplies electrical pressure (measured as volts at the terminals even if there is no circuit and current).  Electrical current is a flow of electrons (similar to a flow of water molecules).  In a series circuit all electrons follow in a single path and there is a voltage drop (pressure drop) across each load (bulb, etc.) and the wire is supplied to provide nil resistance and can usually be neglected.  In a parallel circuit there are two possible paths (reducing overall resistance) that divide the current and there is a voltage drop across the combined resistance of the paths as well as across each load (bulb, etc.) in each path.  In a series circuit the resistances are added to get an equivalent resistance that can be used to calculate current (amps) for any given voltage (pressure).  For parallel circuits, determine the added resistance of loads in each separate leg then calculate the equivalent resistor for the parallel legs using formula for two (or more parallel paths).

Answer 4

Voltage = V

Current = I

Resistance = R

V = IR

Therefore, voltage is inversely proportional to resistance (greater R = lower V, lower R = greater V).

You cannot force current through a circuit, you can only provide a potential difference (voltage), and this determines the current based upon the resistance. Assume each bulb has equal resistance "r."

In series, resistance adds, so you have 12V = I*2r and I = 6A.

In parallel, resistance adds like so: 1/R =(1/first resistance) + (1/next resistance) etc., so you end up with a total resistance of r/2. Plug this into your equation and get: 12 = I * r/2 so I = 24A.

Power determines the brightness of your bulbs, and is given by P = IV, so you see that the bulbs in parallel light more brightly because 12V*24A > 12V * 6A.

See source for more information.