P6 - Circuits & Resistors, Voltage-Current graphs & Resistance

Question 1

What is current?

Answer 1

Current is the flow of charge around the circuit.

Question 2

What is required for current to flow?

Answer 2

• Current will only flow through a component if there is a voltage across that component, current is measured in amps, A.

Question 3

What is voltage?

Answer 3

Voltage is the driving force that pushes the current round. Voltage is measured in Volts, V.

Question 4

What is resistance?

Answer 4

• Resistance is anything in the circuit that reduces the current.
• Resistance is measured in Ohms, Ω

Question 5

What decides how big the current will be?

Answer 5

The relative sizes of the voltage and resistance decide how big the current will be.
• If you increase the voltage, then more current will flow.
• If you increase the resistance then less current will flow.

Question 6

What is charge carried by in a metal conductor?

Answer 6

In a metal conductor, the electric charge is carried by electrons.

Question 7

How do electrons cause resistance?

Answer 7

• When electrons flow through a conductor they collide with atoms in the metal, this causes resistance.
• Collisions between electrons and atoms cause the atoms to vibrate, the more atoms vibrate,the more they get in the way of electrons, so the more collisions there are.
• So an increase in collisions causes an increase in resistance.
• In increase in vibrations also increases the temperature of the conductor.

Question 8

What is a variable resistor?

Answer 8

A variable resistor is a resistor whose resistance can be changed by twiddling a knob or something.

Question 9

What are variable resistors useful for?

Answer 9

Variable resistors are useful for controlling the current flowing through a circuit.
• Turn the resistance up, current drops.
• Turn the resistance down, current rises.

Question 10

What happens if you move the slider, and how does it affect resistance?

Answer 10

• Older resistors were huge coils of wire with a slider on them.
• As you move the slider, the length of wire that has current flowing through it changes.
- Longer wires have more resistance, so less current flowing through them, this is because the longer the wire, the more atoms the electrons will collide with.

Question 11

What are variable resistors used for?

Answer 11

• Variable resistors are used for controlling the speed of motors and the brightness of bulbs.
- Turning the resistance down increases the speed of the moor, or the brightness of a bulb.

Question 12

What is the current through a resistor ( at a constant temperature ) proportional to?

Answer 12

• Current going through a resistor is proportional to voltage.

Question 13

What does a straight line graph for a resistor mean?

Answer 13

• Straight line graphs are used for ohmic resistors, which have a constant resistance.
• But there are different lines due to different resistances.

Question 14

What are non ohmic resistors?

Answer 14

Non Ohmic resistors are components whose resistance changes.

Question 15

For a filament lamp graph, what happens as the current increases?

Answer 15

• As the current increases, the temperature of the filament increases, and so the resistance increases, which is represented by a curve on a graph.

Question 16

What is the resistance of the component on a straight line graph?

Answer 16

• For a straight line graph, the resistance of the components is steady and is equal to the inverse of the gradient of the line, or 1/Gradient.
• The steeper the graph, the lower the resistance.

Question 17

What does it mean if the graph is a curve, and how can you find the resistance then?

Answer 17

• If the graph curves, it means the resistance is changing,
• In that case R can be found for any point by taking the pairs of values ( V, I ) and from the graph and placing them in the formula R = V/I
R = Voltage/Current.

Question 18

How does combining resistors affect the resistance of a circuit?

Answer 18

• Placing resistors in a parallel decreases resistance.

* Placing resistors in a series increases the resistance.

Question 19

What is the total resistance in a series circuit?

Answer 19

• The sum of the individual resistors.

Question 20

Why does connecting resistors in parallel decrease resistance?

Answer 20

• Resistors connected in parallel provide more paths for the current to travel down.
• So the total resistance of a parallel circuit is always less than that of the branch with the smallest resistance.
• Circuits with two resistors in parallel will have a lower resistance than just a circuit with either of the resistors by themselves, which means the parallel circuit has a higher current.