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Unlock This CourseConnect a lamp to a cell and there is a current. But how big is it? In a series circuit the size of the current depends on two things only:
Think of it as a push against an opposition. The supply pushes the current round the loop. The components make it harder for the current to pass.
Key terms:
Picture a battery, a lamp and a buzzer in one loop. Now put an ammeter into the loop. Try it in three places: between the battery and the lamp, between the lamp and the buzzer, and between the buzzer and the battery.
The ammeter reads the same in all three places. If it reads 0.2 A before the lamp, it reads 0.2 A after the buzzer too.
This is because there is only one path. Whatever flows into a component must flow out of the other side, because it has nowhere else to go. Current is not used up as it goes round. The lamp and the buzzer transfer energy, but the current that leaves each one is just as big as the current that went in.
So in a series circuit we can talk about the current: one value for the whole loop. Anything that changes it changes it everywhere at once.
Start with one cell and one lamp. The lamp glows dimly. Add a second cell, facing the same way as the first, and the lamp is brighter. An ammeter in the loop shows a larger reading.
More cells give a larger applied voltage, and a larger applied voltage gives a larger current. A bigger push drives more current through the same components. Take a cell away, or turn a power supply down, and the current gets smaller.
The cells must all face the same way, with the positive terminal of one joined to the negative terminal of the next. A cell put in backwards pushes against the others, so the applied voltage is smaller, not larger.
Now keep the cells the same and change what is in the loop.
Add a second lamp in series with the first and both lamps are dim. The ammeter reading falls. Add a third and it falls again.
Each lamp or resistor you add puts more resistance into the one path. More components in series means more resistance, so a smaller current. Take a component out, and close the gap, and the current gets larger.
'Nature' means what kind of component it is. Different components have different amounts of resistance.
Swap a resistor for one of higher resistance and the current gets smaller, even though the number of components has not changed. A component of higher resistance means a smaller current. One of lower resistance means a larger current.
Here are some typical readings for a set of identical lamps and cells. Look at the pattern, not the exact numbers.
| Circuit | Ammeter reading | Lamps |
|---|---|---|
| One cell, one lamp | 0.20 A | Dim |
| Two cells, one lamp | 0.30 A | Bright |
| Two cells, two lamps | 0.20 A | Both dim |
| Two cells, three lamps | 0.15 A | All very dim |
More cells: the reading goes up. More lamps: the reading goes down.
Both ideas in the boxes above are really one rule:
For the same applied voltage, the greater the resistance in the circuit, the smaller the current. The smaller the resistance, the larger the current.
The exam asks for the qualitative effect. That means you describe what happens in words (increases, decreases, stays the same) without working out any numbers. Calculating with resistance comes in the lesson Voltage, Current and Resistance.
A variable resistor puts this rule to work. Its resistance can be changed by turning a knob or moving a slider. It is connected in series with the component it controls.
Put a variable resistor in series with a lamp. Turn its resistance up: the current in the loop gets smaller, so the lamp is dimmer. Turn its resistance down: the current gets larger, so the lamp is brighter.
Put a variable resistor in series with a motor. More resistance means a smaller current, so the motor turns more slowly. Less resistance means a larger current, so the motor turns faster.
| Change to the series circuit | Effect on the current |
|---|---|
| Add a cell (facing the same way) | Larger |
| Remove a cell | Smaller |
| Add a lamp or resistor in series | Smaller |
| Remove a lamp or resistor | Larger |
| Swap a component for one of higher resistance | Smaller |
| Increase the resistance of a variable resistor | Smaller |
| Move the ammeter to a different place in the loop | No change |
A series circuit has a battery, a buzzer and a resistor. The buzzer is too quiet because the current is too small. Describe two changes that would make the current larger, and explain why each one works.
Step 1: recall what the current depends on. It depends on the applied voltage and on the resistance of the components in the loop.
Step 2: change the applied voltage. Add another cell to the battery, facing the same way. The applied voltage is larger, so the current is larger.
Step 3: change the resistance. Swap the resistor for one of lower resistance (or take it out and close the gap). There is less resistance in the circuit, so the current is larger.
Each answer follows the same pattern: the change, what it does to the voltage or the resistance, then what that does to the current.
A student connects a battery of two cells, an ammeter, a lamp and a variable resistor in a series circuit. The ammeter is between the battery and the lamp. It reads 0.25 A.
(a) The student moves the ammeter so that it is between the lamp and the variable resistor. State the new reading on the ammeter. [1 mark]
(b) The student increases the resistance of the variable resistor. Describe and explain what happens to the lamp. [3 marks]
(c) The student sets the variable resistor back to where it started, then adds a third cell to the battery, facing the same way as the others. State and explain the effect on the current. [2 marks]
(d) Without changing the battery or the variable resistor, suggest one other change to the circuit that would make the current smaller. [1 mark]
(a) 0.25 A (1).
(b) There is more resistance in the circuit (1), so the current in the circuit gets smaller (1), so the lamp gets dimmer (1).
(c) The current gets larger (1) because the applied voltage is larger (1).
(d) Add another lamp (or a resistor) in series, or swap the lamp for one of higher resistance (1).
Part (b) is a chain of three links: resistance up, current down, lamp dimmer. Many students jump straight to 'the lamp gets dimmer' and get 1 mark out of 3. When a question says 'explain', write down every link, and always mention the current.