How does a resistor reduce current in a circuit? I'm in year 10 and I just have a question about resistors. How exactly does a resistor reduce current?
From what I've read the current before a resistor is the same as the current after a resistor so how does the resistor reduce current?
I know that the electrons bump into atoms as they move through a resistor but how does that reduce the current before  the resistor?
( Sorry if I'm being stupid and not seeing an obvious answer :) )
 A: As the the other answers already said:
When you read "a resistor reduces the current",
this does not mean, that the current after the resistor is smaller than before the resistor.
Instead it means, that the current with resistor is smaller than without resistor.
Instead of explaining a resistor with formulas and equations,
using the electric-hydraulic analogy will give a much better intuition
what is going on, especially for beginners.
Here is a table of some corresponding devices and quantities from
hydraulics (flowing water) and electrics (flowing electricity):
$$\begin{matrix}
\text{Hydraulics} & \text{Electrics} \\
\text{volume flow rate (measured in m$^3$/sec)} & \text{current (measured in Ampere)} \\
\text{pressure difference (measured in bar)} & \text{voltage (measured in Volt)} \\
\text{water pump} & \text{battery} \\
\text{wide pipe} & \text{conducting wire} \\
\text{narrow pipe} & \text{resistor}
\end{matrix}$$
With the above translation table in mind
consider these hydraulic and electric circuits:

(image from Hyperphysics - Current law and flow rate)
The resistor reduces the current, just like the narrow pipe reduces the flow rate.
The current before and after the resistor is the same,
just like the flow rate before and after the narrow pipe is the same.
A: 
From what I've read the current before a resistor is the same as the
current after a resistor so how does the resistor reduce current?

It is a common misconception that resistors "slow down" (reduce the speed) of electrons. If that were the case then the current leaving each resistor in a series circuit would be less than the current entering the resistor, so that the speed of the electrons coming out of the last resistor in series would be less than the speed of the electrons entering the first resistor. If that happened then electrons would be "piling up" in the resistors in between the first and last. That does not occur. The reason is conservation of charge.
To put it in a somewhat simplified manner, what the resistors do is to dissipate the electrical potential energy provided by the battery in the form of heat. As electrons move through the resistor they collide with the atoms and molecules of the resistor material. That briefly causes them to slow down a bit (losing kinetic energy), but then they speed up again (gain kinetic energy) due to the energy supplied by the electric field of the battery. The loss of kinetic energy shows up as heat. This alternately  slowing down and speeding up of the electrons produces a net constant average speed (current) going into and out of each resistor of a series circuit.
Hope this helps.
A: It reduces the current compared to a different circuit where the resistor is replaced with an ideal wire.
It doesn't change the current in a sequence of series-connected elements within a particular circuit --- that would violate KCL.
A: In a circuit, although electrons inside the conductors move haphazardly At high speeds on the order of 10^5 ms^-1, the force exerted by the applied electric field superimposes a slow average  “drift” velocity(~10^-3 m/s) on them, from the end with lower to the one with higher electric potential ( force always points in the direction of decreasing potential energy).

*

*When a power supply is connected to the ends of a metallic conductor, a potential difference is created across it. This sets up an electric field inside the conductor which exerts an electric force on the delocalised electrons and accelerates them opposite to the direction of the electric field, converting their electric potential energy into kinetic energy.


*As the delocalised electrons move through the conductor, they collide with the positive cations and lose their kinetic energy to the vibrational energy of the lattice, increasing its temperature. This is known as ohmic heating.


*After each collision, the electrons get repeatedly re-accelerated and collide. This “stop” and “go” motion results in an average drift velocity ( 10^-3 m/s), a slow average velocity against the direction of the applied electric field superimposed on the random high speed motion of charge carriers in a material by an electric field
Now the current, which is the rate at which electric charge passes through a point in the circuit, is directly proportional to this average drift velocity. What resistors do is that they increase the resistance, and thereby the loss of kinetic energy. This automatically decreases the average drift velocity of the electrons and accordingly, the rate of flow of electric charge( current)
A: Say we have a simple circuit consisting of a battery, a wire, and a resistor. The purpose of the resistor is to decrease the current in the circuit. How does this happen?
Firstly, a resistor is made of material that is less conductive than the wire. As a result, electrons aren’t able to move as quickly in the resistor as they are in the wire.

In the diagram above, the white circles represent the electrons in the wire at their initial state (before the battery is connected). The red circles represent the electrons a moment after the battery is connected to the wire.
When the electron closest to the negative end of the battery moves, it causes the next electron to move as well (because of the repulsive forces between electrons). This creates a cascading effect.
However, electron A in the above diagram cannot move as much (in the same timeframe) as all of the electrons behind it. As a result, electron A will be closer to all the electrons behind it than it was before. Hence, it repels these electrons more strongly. This slightly cancels the repulsive force on these electrons due the negative end of the battery, causing them to slow down.
On the other side of the resistor, electron B cannot move as much as the electrons in front of it. Hence, electron B will be farther from these electrons than it was before, so its repulsive force on these electrons decreases. Therefore, these electrons also slow down, since the force with which B is pushing them away is now smaller.
As a result, the resistor achieves its purpose, and the overall current in the circuit is reduced.
