CP9 Resistance Flashcards
What is resistance measured in?
Ohm, Ω
How can a variable resistor be used to change the current in a circuit?
Increasing the resistance will decrease the current
What is the formula for the total resistance of multiple resistors in series?
R total = R1 + R2 + R3 …
Why does the total resistance of the circuit increase when two resistors are placed in series?
The total current has to flow through both resistors and so the total current is reduced by both resistors
What happens to the resistance of a circuit when a resistor is added in parallel?
The resistance of the circuit decreases
Why does the total resistance of the circuit decrease when two resistors are placed in parallel?
Providing a second route for current can only make it easier for current to flow. The current splits and goes some through wach resistor.
Think of having 1 corridor or 2 corridors to move a large numberofpeople
Describe the circuit you would use to investigate the relationship potential difference, current and resistance for a filament lamp
Build a series circuit with a power supply, a variable resistor, a filament bulb and an ammeter.
Connect a voltmeter in parallel with the bulb.
How would you use a circuit containing a variable resistor to make multiple measurements of potential difference and current?
Changing the resistance of the variable resistor will alter the values of current and p.d. which can be recorded in a table.
If you have a graph of current against potential difference how do you find the resistance?
The gradient gives the resistance.
How does current vary with potential difference in a filament lamp?
Initially current is proportional to p.d. the current continues to increase with p.d. but at a decreasing rate.
Explain the shape of the I-V graph for a filament lamp
As the P.D. increases the current increases. As the current increases there are more collisions between electrons and metal ions in the filament causing the metal ions to vibrate more (its temperature has increased) and so the resistance of the lamp increases with current.
How does current vary with potential difference in a diode and why?
0 for negative p.d. because diode only allows current to flow through in one direction
How does current vary with potential difference in a fixed resistor?
directly proportional
Increasing light intensity on an LDR …
…decreases the resistance of the LDR
Increasing temperature of a thermisstor…
…decreases the resistance of the thermistor
Resistance causes electrical energy to be transferred to …
… heat / thermal energy
Resistors get hot because …
… the electrons in the current collide with the ions in the lattice
