☰ Year plan · Science

Electric current (B): cells, resistance and symbols

Week 7 · Lesson 1 Electric Current (B): cells in series and in parallel, electric resistance, fixed and variable resistors, the factors that affect resistance, circuit symbols · 3 lessons
2

Resistance, resistors and the rheostat

Science Prep 3 Term 1, Unit 2: pp. 41–45
GoalDefine resistance, tell a fixed resistor from a rheostat, and say how a rheostat changes the current.
New words
electric resistance · the opposition of a conductor to the flow of charge; unit ohm (Ω)fixed resistor · a resistor with one set value of resistancerheostat · a variable resistor: a sliding contact changes how much wire is in the circuit
Explanation

Electric resistance is how much a conductor opposes the flow of charge. It is measured in ohms (Ω) with an ohmmeter. A fixed resistor has a set value. A variable resistor, or rheostat, can be changed by sliding a contact along a wire: more wire in the circuit means more resistance and so less current, and less wire means less resistance and more current. This is how the volume knob of a radio works: turning it changes the resistance, so it changes the current. A larger resistance for the same current also gives more heat. Careful: resistance does not depend on the current or on the voltage, at a given temperature. It belongs to the conductor itself. A rheostat does not “store” current: it only changes how hard it is for the current to flow.

ComponentResistanceHow it changes
fixed resistorset valueit does not
rheostatvariableslide the contact: more wire, more Ω
Worked examples
SliderResistanceCurrentLamp
less wirelowerlargerbrighter
more wirehighersmallerdimmer
A rheostat is in series with a lamp. The slider is moved so that more wire is in the circuit. The resistance goes up, the current goes down and the lamp dims. Moving the slider back makes the lamp brighter.
When Layla turns a radio’s volume knob up, the resistance in the circuit falls, so a larger current flows through the loudspeaker and the sound gets louder.
A fixed resistor of 10 Ω and one of 20 Ω are in the same circuit. Resistance is measured in ohms. 20 Ω opposes the current more than 10 Ω, so at the same voltage the 20 Ω resistor passes less current.
Class activity

Pencil-lead dimmer, with a grown-up. Connect one cell, a torch bulb and a soft pencil lead between two crocodile clips. Slide one clip along the lead: with the clips far apart the lamp is dim, near each other it is brighter. Switch off between tests. Draw what you saw and explain it with the idea of a longer path and more resistance. Keep each test short, because the wires can warm up.

Practice
1
What is the unit of resistance, and what instrument measures it?
2
How does a rheostat change the current?
3
The volume of a radio is made softer. Did the resistance go up or down?
4
True or false: “a resistor’s resistance doubles when the voltage doubles.” Explain.