# Resistance Questions: Car Battery, Starter Motor, Headlamps

• T7
In summary, the conversation discusses the calculation of terminal p.d., resistance, and power output in a circuit involving a car battery with an e.m.f. of 12V and internal resistance of 0.04 ohms, a starter motor drawing 100A, and headlamps rated at 12V and 36W. The conversation also raises questions about the relationship between voltage, power, and resistance in this circuit.
T7
Hi,

Here's the question:

"A car battery has an e.m.f. of 12V and an internal resistance of 0.04 ohms. The starter motor draws a current of 100A. a.) What is the terminal p.d. of the battery when the starter motor is in operation? b.) If the headlamps are rated at 12V, 36W, what is their resistance? c.) To what value will their power output decrease when the starter motor is in operation?"​

a.) I get the terminal p.d. for the first part as follows:

V = E - IR = 12 - 0.04 x 100 = 8 volts

b.) Now, as I understand the next part, the starter motor has not been engaged yet and the headlamps are presumably in parallel. But if the battery has an e.m.f of 12V, how can it power them? Moreover, how can their resistance be calculated as asked? Presumably the E.M.F. of 12 is equal to the lost volts (I x 0.04) plus the voltage across the lamps. But since the voltage across the lamps isn't going to be less than 12 (is it?), isn't one of the terms in the equation E = Ir + V going to have to be zero (?!)

[ c.) If I can get part b.) right, I don't think there will be any problems with c.) ]

I'm obviously misunderstanding something here. Can someone explain?

Cheers!

Last edited:
The headlamps are simultaneously rated for voltage and power. This assumes that both maxima are attained at the same current. This is independent of the rest of the circuit.

Is there a relation that involves voltage, power and resistance ?

http://www.technotetime.com/ohmslaw.html
If you are still confused, there is the DC Power Wheel on the right of that page

Last edited by a moderator:

## 1. What is resistance in a car battery?

Resistance in a car battery refers to the opposition of electric current flow within the battery. This can be caused by internal factors such as the material and design of the battery, as well as external factors like temperature and age.

## 2. How can I measure the resistance of a car battery?

The resistance of a car battery can be measured using a multimeter. Set the multimeter to the resistance mode and touch the probes to the positive and negative terminals of the battery. The reading will indicate the resistance in ohms.

## 3. What can cause the starter motor to have high resistance?

There are a few potential causes for high resistance in a starter motor. These can include worn out brushes, a corroded commutator, or dirty connections. A faulty solenoid or damaged wiring can also contribute to high resistance.

## 4. How do I know if the resistance in my starter motor is too high?

If your car is struggling to start or the starter motor is making a clicking sound, it could be a sign of high resistance. You can also use a multimeter to measure the resistance in the starter motor. A reading higher than the recommended range indicates high resistance.

## 5. Can resistance affect the performance of my car's headlamps?

Yes, resistance can affect the performance of your car's headlamps. If there is high resistance in the headlamp circuit, it can cause the lights to dim or flicker. This can be caused by faulty wiring, a weak battery, or a damaged alternator. It is important to address any resistance issues to ensure proper functioning of the headlamps.

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